CN115210018A

CN115210018A - Molybdenum-containing alloys and related systems and methods

Info

Publication number: CN115210018A
Application number: CN202080097889.3A
Authority: CN
Inventors: 克里斯多佛·A·舒; 克里斯蒂安·奥利弗
Original assignee: Massachusetts Institute of Technology
Current assignee: Massachusetts Institute of Technology
Priority date: 2020-01-31
Filing date: 2020-11-25
Publication date: 2022-10-18
Also published as: AU2020429466A1; EP4096854A1; US20230383380A1; US20210238711A1; JP2023511720A; WO2021167671A1

Abstract

Molybdenum-containing alloys, and related systems and methods, are generally described. In certain embodiments, a second element and/or a third element may be included in addition to molybdenum for providing beneficial properties during sintering of the molybdenum-containing alloy. According to certain embodiments, the molybdenum-containing alloy is nanocrystalline. According to certain embodiments, the molybdenum-containing alloy has a high relative density. According to certain embodiments, the molybdenum-containing alloy may be relatively stable. Methods of the present invention for making molybdenum-containing alloys are also described herein.

Description

Molybdenum-containing alloys and related systems and methods

RELATED APPLICATIONS

Priority of U.S. provisional application No. 62/968,233, entitled "mobile-content Alloys and Associated Systems and Methods", filed 1, 31/2020 as required by 35u.s.c. § 119 (e), which is incorporated herein by reference in its entirety for all purposes.

Government sponsorship

The invention was made with government support in accordance with the Foundation No. 80NSSC19K1055 of the American national aeronautics and astronautics Marshall Space Flight Center (NASA) under government support. The government has certain rights in this invention.

Technical Field

Molybdenum-containing alloys and related systems and methods are generally described.

Disclosure of Invention

Molybdenum-containing alloys, and related systems and methods, are generally described. In certain embodiments, a second (and, optionally, a third) element may be included in addition to molybdenum for providing beneficial properties during sintering of the molybdenum-containing alloy. According to certain embodiments, the molybdenum-containing alloy is nanocrystalline. According to certain embodiments, the molybdenum-containing alloy has a high relative density. According to certain embodiments, the molybdenum-containing alloy may be relatively stable. Also described herein are methods of the present invention for making molybdenum-containing alloys. In some cases, the inventive subject matter relates to related products, alternative solutions to specific problems, and/or a plurality of different uses for one or more systems and/or articles.

Certain aspects relate to methods of forming metal alloys. In some embodiments, the method includes sintering particles comprising molybdenum (Mo) and a second element to produce a metal alloy, wherein Mo is the most abundant element in the metal alloy by atomic percent, and the relative density of the metal alloy is at least 80%.

In some embodiments, the method includes sintering particles comprising molybdenum (Mo) and chromium (Cr) to produce a metal alloy.

Also disclosed herein are metal alloys. In some embodiments, the metal alloy comprises molybdenum (Mo) and a second element, wherein Mo is the most abundant element in the metal alloy by atomic percent, and the relative density of the metal alloy is at least 80%.

In some embodiments, the metal alloy comprises molybdenum (Mo) and chromium (Cr), wherein Mo is the most abundant element in the metal alloy by atomic percentage.

Other advantages and novel features of the invention will become apparent from the following detailed description of various non-limiting embodiments of the invention when considered in conjunction with the drawings. In the event that the present specification and a document incorporated by reference contain conflicting and/or inconsistent disclosure, the present specification shall control.

Drawings

Some non-limiting embodiments of the present invention will be described by way of example with reference to the accompanying drawings, which are schematic and are not intended to be drawn to scale. In the drawings, each identical or nearly identical component that is illustrated is typically represented by a single numeral. For purposes of clarity, not every component may be labeled in every drawing, nor is every component of each embodiment of the invention shown where illustration is not necessary to allow those of ordinary skill in the art to understand the invention.

Fig. 1A-1C are exemplary schematic diagrams illustrating a sintering process according to certain embodiments.

Fig. 2A-2B show SEM images of exemplary molybdenum-chromium alloys according to one set of embodiments.

Fig. 3A-3B show SEM images of exemplary molybdenum-chromium-tungsten alloys according to one set of embodiments.

Fig. 4 is a diagram depicting a process of producing and sintering an alloy exhibiting nanophase separation sintering, according to some embodiments.

Fig. 5 is an SEM image of a powder after mechanically alloying it but before sintering it according to certain embodiments.

Fig. 6 is an SEM image of Mo15Cr that has been sintered up to 1450 ℃ according to some embodiments. The relative density of this sample was >98%. The darker phase present is a chromium rich phase that contributes to sintering.

Fig. 7 is an SEM image of Mo15Cr sintered to 1200 ℃ and quenched before reaching full density according to some embodiments. In this figure, the chromium rich phase (darker material) forming the necks (necks) between the particles is more easily seen.

Fig. 8 is an SEM image of Mo25W15Cr that has been sintered to 1450 ℃ and achieved a relative density of greater than 98%, according to certain embodiments. Similar to the Mo15Cr samples, the darker phases represent the chromium phase formed for accelerated sintering.

Fig. 9 shows a densification curve for a Mo15Cr alloy sintered to 1450 ℃ at a heating rate of 10 ℃/minute, in accordance with some embodiments, compared to pure Mo.

Fig. 10 shows a densification curve for a Mo25W15Cr alloy heated to 1450 ℃ at a rate of 10 ℃/minute, according to some embodiments.

Detailed Description

The present disclosure relates generally to metal alloys comprising molybdenum and methods of making molybdenum-containing alloys. Certain embodiments relate to the manufacture of molybdenum-containing alloys by sintering. In certain embodiments, a second element and/or a third element may be included in addition to molybdenum for providing beneficial properties during sintering of the molybdenum-containing alloy. In certain instances, the molybdenum-containing alloys described herein comprise an additional element other than molybdenum, such as chromium (Cr) and/or tungsten (W). Other elements may also be present. According to certain embodiments, the molybdenum-containing alloys described herein may comprise at least three elements (e.g., at least three metallic elements). However, it is not strictly required that three elements be present simultaneously, and in other embodiments, the molybdenum-containing alloy may contain only two elements.

As noted above, the present disclosure includes the method of the present invention for making a molybdenum-containing alloy. For example, certain embodiments relate to such sintering methods: wherein sintering is achieved at a relatively low temperature and/or within a relatively short period of time. In some embodiments, sintering is performed with little or no pressure applied during the sintering process. According to some embodiments, and as described in more detail below, sintering may be performed such that undesirable grain growth is limited or eliminated (e.g., by selection of materials and/or sintering conditions). Certain embodiments relate to the recognition that: the molybdenum-containing material may be sintered in a relatively short time, at a relatively low temperature, and/or with relatively low (or no) applied pressure, while maintaining high temperature stability, high relative density, and/or in some cases maintaining nanocrystallinity.

Certain embodiments described herein may provide advantages over existing articles, systems, and methods. For example, according to certain (but not necessarily all) embodiments, the molybdenum-containing metal alloy may have high strength, high hardness, and/or high resistance to grain growth. According to some (but not necessarily all) embodiments, the methods of forming metal alloys described herein may utilize relatively small amounts of energy, for example, due to the relatively short sintering times, relatively low sintering temperatures, and/or relatively low applied pressures employed.

In some embodiments, the metal alloy is formed by sintering a plurality of particles. In some embodiments, the particles may be in powder form. The shape of the particles may be, for example, spherical, cubic, conical, cylindrical, acicular, irregular, or any other suitable geometric shape. In some embodiments, at least some of the particles (e.g., at least 50%, at least 75%, at least 90%, or at least 95%) are single crystals. In certain embodiments, at least some of the particles (e.g., at least 50%, at least 75%, at least 90%, or at least 95%) are polycrystalline.

The particles from which the metal alloy is formed may be of any of a variety of sizes. In some embodiments, at least 50% (or at least 75%, at least 90%, at least 95%, or at least 99%) of the total particle volume is comprised of particles having a maximum cross-sectional dimension of less than 1 millimeter (or less than 500 microns, less than 100 microns, or less than 10 microns).

Fig. 1A-1C are exemplary schematic diagrams illustrating a sintering process according to some embodiments. In fig. 1A, the plurality of particles 100 are shown in spherical form (although other shapes may be used as described elsewhere). As shown in fig. 1B, the particles 100 may be arranged such that they contact each other. As shown in fig. 1C, when the particles are heated, they aggregate to form a single solid material 110. During the sintering process, according to certain embodiments, the gaps 105 (shown in fig. 1B) between the particles 100 may be substantially reduced or eliminated such that a solid (shown in fig. 1C) is formed having a high relative density.

According to certain embodiments, the particles from which the alloy is formed comprise a relatively large amount of molybdenum (Mo). For example, in some embodiments, mo is the element most abundant in the particles (e.g., the most abundant metal) by atomic percentage. (atomic percent is abbreviated herein as "atomic% (at.%)", or "atomic% (at.%)") according to certain embodiments, mo is present in the particles in the following amounts: at least 50 atomic%, at least 55 atomic%, at least 60 atomic%, at least 65 atomic%, at least 70 atomic%, at least 80 atomic%, at least 90 atomic%, or at least 95 atomic%. In some embodiments, mo is present in the particles in the following amounts: up to 96 atomic%, up to 97 atomic%, up to 98 atomic%, up to 99 atomic%, up to 99.5 atomic%, or greater. Combinations of these ranges are also possible. Other values are also possible.

According to certain embodiments, at least some of the particles comprise Mo and/or a second element (e.g., a second metal). The phrase "second element" is used herein to describe any element that is not Mo. The phrase "second metal" is used herein to describe any metallic element that is not Mo. The term "element" is used herein to refer to an element present in the periodic table. "metal elements" are those present in groups 1 to 12 of the periodic table other than hydrogen (H); al, ga, in, tl and Nh In group 13 of the periodic Table; sn, pb, and Fl in group 14 of the periodic Table; bi and Mc in group 15 of the periodic Table; po and Lv in group 16 of the periodic table; a lanthanide element; and actinides.

In some embodiments, a portion of the particles are comprised of Mo and another portion of the particles are comprised of a second element (e.g., a second metal, such as chromium). In certain embodiments, at least some of the particles comprise both Mo and a second element (e.g., a second metal, such as chromium).

According to certain embodiments, the second element is selected from chromium (Cr) and palladium (Pd). In some embodiments, both Cr and Pd are present (e.g., where the particles comprise at least three elements). In other embodiments, only one of Cr and Pd is present. In some embodiments, the second element is Cr.

According to certain embodiments, the second element and Mo exhibit miscibility gaps (solubility gaps). Two elements are said to exhibit a "miscibility gap" when their phase diagrams include regions in which the mixture of the two elements exists as two or more phases. In some embodiments, where the second element and Mo exhibit miscible interstitial spaces, the second element and Mo may be present in a metal alloy in at least two phases.

In some embodiments, the melting point of the second element is lower than the melting point of molybdenum (Mo). As will be understood by one of ordinary skill in the art, the melting point of an element refers to the melting point of the element in its pure form. For example, in the case of a metal, the melting point of the metal refers to the melting point of the metal in its pure form.

In some embodiments, mo is at least partially soluble in the second element.

The second element (e.g., chromium, palladium) may be present in various suitable percentages in the particles from which the alloy is made. According to certain embodiments, the second element is present in the particle in an amount of: less than or equal to 40 atomic%, less than or equal to 35 atomic%, less than or equal to 32 atomic%, less than or equal to 30 atomic%, less than or equal to 25 atomic%, less than or equal to 22 atomic%, less than or equal to 20 atomic%, less than or equal to 18 atomic%, or less than or equal to 16 atomic%. In some embodiments, the second element is present in the metal alloy in an amount of: at least 0.5 atomic%, at least 1 atomic%, at least 2 atomic%, at least 3 atomic%, at least 4 atomic%, at least 5 atomic%, at least 6 atomic%, at least 7 atomic%, at least 8 atomic%, at least 9 atomic%, at least 10 atomic%, or greater. Combinations of these ranges are also possible. For example, in some embodiments, the second element is present in the metal alloy in an amount of 0.5 atomic% to 40 atomic% of the metal alloy. In some embodiments, the second element is present in the metal alloy in an amount of 1 atomic% to 40 atomic% of the metal alloy. In some embodiments, the second element is present in the metal alloy in an amount from 8 atomic% to 32 atomic% of the metal alloy. Other values are also possible.

In some embodiments, the second element may be an activator element relative to Mo. Activator elements are those elements that increase the sintering rate of the material relative to the sintering rate observed in the absence of the activator element but under otherwise identical conditions. The activator elements are described in more detail below.

According to certain embodiments, the second element may be selected (e.g., for alloying with Mo) based on one or more of the following conditions:

1. the crystal grain size of the nanocrystalline state is thermodynamically stable;

2. a phase separation zone extending above the sintering temperature;

3. a second (e.g., solute) element having a lower melting temperature; and/or

Solubility of mo in the precipitated second phase.

According to some embodiments, the second element (e.g., cr) forms precipitates within the Mo parent phase. For example, in some embodiments, the metal alloy comprises a structure consisting of Mo-rich grains and Cr-rich precipitates. In some embodiments, precipitates of the second element (e.g., cr) may fill grain boundaries between Mo grains. In some embodiments, the second metal is chromium. In the context of the present disclosure, the present inventors have recognized and appreciated that the addition of a second metal, such as chromium, may provide certain benefits when alloyed with molybdenum. Without wishing to be bound by my theory, it is believed that chromium may segregate into a second phase upon heating, and also has a lower surface energy than molybdenum, and may preferentially segregate to the surface of the alloy to form bridges or "necks" between the grains of molybdenum. That is, chromium can form a connecting linkage (connecting linker) within the alloy at the grain boundaries of the molybdenum particles. In addition, chromium also allows for rapid diffusion of molybdenum through the chromium neck to promote rapid densification. For example, in fig. 2A-2B, SEM images of exemplary Mo15Cr are shown taking different stages of neck formation. In fig. 2A, the Mo15Cr alloy is heated to 850 ℃ and quenched, and shows an early stage of neck formation. In fig. 2B, the Mo15Cr sample was heated to 1200 ℃ and quenched, and shows intermediate neck growth and densification. Also, fig. 3A-3B show SEM images of exemplary Mo25W15Cr alloys. In fig. 3A, mo25W15Cr was heated to 900 ℃ and quenched, and shows an early stage of neck formation. In fig. 3B, the Mo25W15Cr alloy was heated to 1200 ℃ and quenched, and shows intermediate neck growth and densification.

In some embodiments, the particles from which the metal alloy is formed comprise only Mo and the second element (i.e., mo and the second element without additional metal or other elements). In other embodiments, the particles comprise Mo, a second element, and a third element. For example, in some embodiments, the particles comprise a third element (other than Mo and the second element). In some embodiments, the third element may be a metal element. The phrase "third element" is used herein to describe an element that is not Mo and is not the second element. That is, when the third element is present, the third element is different from Mo and the second element. In some embodiments, the metal alloy comprises a third metal, in which case the alloy comprises Mo, the second metal, and the third metal.

In certain embodiments, the particles from which the alloy is formed (e.g., comprising molybdenum, the second metal, and optionally the third metal or additional metal) may comprise a relatively large amount of metallic material. In some embodiments, at least 10 atomic%, at least 20 atomic%, at least 40 atomic%, at least 50 atomic%, at least 70 atomic%, at least 90 atomic%, at least 95 atomic%, at least 99 atomic%, at least 99.9 atomic% or more of the particulate material is made up of metal atoms in their metallic form (i.e., in the zero oxidation state). In some embodiments, at least 10, at least 20, at least 40, at least 50, at least 70, at least 90, at least 95, at least 99, at least 99.9, or more of the molybdenum atoms within the particle are in their metallic form. In certain embodiments, at least 10 atomic%, at least 20 atomic%, at least 40 atomic%, at least 50 atomic%, at least 70 atomic%, at least 90 atomic%, at least 95 atomic%, at least 99 atomic%, at least 99.9 atomic% or more of the atoms of the second element (e.g., the second metal) within the particle are in their metallic form. In some embodiments, at least 10 atomic%, at least 20 atomic%, at least 40 atomic%, at least 50 atomic%, at least 70 atomic%, at least 90 atomic%, at least 95 atomic%, at least 99 atomic%, at least 99.9 atomic% or more of the atoms of the third element (e.g., the third metal) within the particle are in their metallic form. In some embodiments, the molybdenum atom may form a metallic bond with other adjacent atoms, such as additional molybdenum atoms, and/or atoms of a second element (e.g., a second metal) and/or a third element (e.g., a third metal).

According to certain embodiments, the third element is selected from tungsten (W) and tantalum (Ta). In some embodiments, the third element is W.

According to some embodiments, when the third element is present, the third element and the second element exhibit a miscibility gap. In some embodiments in which the third element and the second element exhibit a miscibility gap, the third element and the second element can be present in a metal alloy in at least two phases.

In some embodiments, the third element (e.g., W, ta) may increase the melting temperature of certain Mo-based alloys described herein. For example, tungsten has a high melting temperature and forms a solid solution with molybdenum. Thus, it is believed that the melting temperature of the alloy can be selectively adjusted by increasing the amount of tungsten in the molybdenum-based alloy. As a non-limiting example, an alloy comprising 60 atomic% molybdenum, 25 atomic% tungsten, and 15 atomic% chromium (Mo 25W15 Cr) may exhibit a melting temperature that is 100 degrees (. Degree. C.) higher than pure molybdenum.

In some embodiments, the melting point of the third element is lower than the melting point of molybdenum (Mo).

The third element (e.g., tungsten) may be present in the particle in a variety of suitable percentages. According to certain embodiments, the third element is present in the particle in an amount of: less than or equal to 40 atomic%, less than or equal to 35 atomic%, less than or equal to 30 atomic%, less than or equal to 28 atomic%, less than or equal to 26 atomic%, or less. In some embodiments, the third element is present in the metal alloy in an amount of: at least 0.5 atomic%, at least 1 atomic%, at least 2 atomic%, at least 3 atomic%, at least 4 atomic%, at least 5 atomic%, at least 10 atomic%, at least 15 atomic%, at least 20 atomic%, at least 22 atomic%, at least 24 atomic%, or greater. Combinations of these ranges are also possible. Other values are also possible.

According to certain embodiments, the total amount of all metallic elements (e.g., the second element, the third element, and any additional optional elements) in the particles that are not Mo comprises less than 50 atomic%, less than or equal to 40 atomic%, less than or equal to 35 atomic%, less than or equal to 32 atomic%, less than or equal to 30 atomic%, less than or equal to 25 atomic%, less than or equal to 22 atomic%, less than or equal to 20 atomic%, less than or equal to 18 atomic%, or less than or equal to 16 atomic% of the particles. In some embodiments, the total amount of all elements (e.g., the second element, the optional third element, and any additional optional elements) in the particle that are not Mo comprises at least 0.5 atomic%, at least 1 atomic%, at least 2 atomic%, at least 3 atomic%, at least 4 atomic%, at least 5 atomic%, at least 8 atomic%, at least 10 atomic%, at least 12 atomic%, at least 14 atomic%, or more of the particle. Combinations of these ranges are also possible. Other values are also possible.

In some embodiments, the total amount of chromium (Cr), palladium (Pd), tungsten (W), and tantalum (Ta) present in the particle is less than 50 atomic percent of the particle, less than or equal to 40 atomic percent of the particle, less than or equal to 35 atomic percent of the particle, less than or equal to 32 atomic percent of the particle, less than or equal to 30 atomic percent of the particle, less than or equal to 25 atomic percent of the particle, less than or equal to 22 atomic percent of the particle, less than or equal to 20 atomic percent of the particle, less than or equal to 18 atomic percent of the particle, or less than or equal to 16 atomic percent of the particle. In some embodiments, the total amount of chromium (Cr), palladium (Pd), tungsten (W), and tantalum (Ta) present in the particles is at least 0.5 atomic%, at least 1 atomic%, at least 2 atomic%, at least 3 atomic%, at least 4 atomic%, at least 5 atomic%, at least 8 atomic%, at least 10 atomic%, at least 12 atomic%, at least 14 atomic%, or greater. Combinations of these ranges are also possible. For example, in some embodiments, the total amount of chromium (Cr), palladium (Pd), tungsten (W), and tantalum (Ta) present in the particle is 0.5 atomic% to 50 atomic% of the particle. In some of these embodiments, at least 90 atomic% (or at least 95 atomic%, at least 98 atomic%, at least 99 atomic%, or at least 99.9 atomic%) of the balance of the particle is comprised of molybdenum.

One of ordinary skill in the art will appreciate that to determine the total amount of chromium (Cr), palladium (Pd), tungsten (W), and tantalum (Ta) present in a given set of particles, the atomic percentages of each of these elements are added. For example, if the particle comprises 60 at% Mo, 15 at% Cr, and 25 at% W, the total amount of chromium (Cr), palladium (Pd), tungsten (W), and tantalum (Ta) present will be 40 at% (i.e., 15 at% from Cr, 25 at% from W, and 0 at% of all other elements in the list). One of ordinary skill in the art will also appreciate that not all of the elements in the above list need be present in the particle when performing this calculation. In the above exemplary calculations, for example, palladium and tantalum are not present in the particles.

In some embodiments, the total amount of chromium (Cr) and tungsten (W) present in the particle is less than 50 atomic percent of the particle, less than or equal to 40 atomic percent of the particle, less than or equal to 35 atomic percent of the particle, less than or equal to 32 atomic percent of the particle, less than or equal to 30 atomic percent of the particle, less than or equal to 25 atomic percent of the particle, less than or equal to 22 atomic percent of the particle, less than or equal to 20 atomic percent of the particle, less than or equal to 18 atomic percent of the particle, or less than or equal to 16 atomic percent of the particle. In some embodiments, the total amount of chromium (Cr) and tungsten (W) present in the particles is at least 0.5 atomic%, at least 1 atomic%, at least 2 atomic%, at least 3 atomic%, at least 4 atomic%, at least 5 atomic%, at least 8 atomic%, at least 10 atomic%, at least 12 atomic%, at least 14 atomic%, or greater. Combinations of these ranges are also possible. For example, in some embodiments, the total amount of chromium (Cr) and tungsten (W) present in the particles is 0.5 atomic% to 50 atomic% of the particles. In some of these embodiments, at least 90 atomic% (or at least 95 atomic%, at least 98 atomic%, at least 99 atomic%, or at least 99.9 atomic%) of the balance of the particle consists of molybdenum.

In some embodiments, the particles comprise Mo, cr, and W. In some embodiments, mo is present in the particles in an amount of at least 50 atomic percent (e.g., 50 to 99 atomic percent), cr is present in the particles in an amount of 0.5 to 30 atomic percent; and W is present in the particles in an amount of 0.5 atomic% to 30 atomic%. In some embodiments, W is present in the particle in an amount of 20 atomic% to 30 atomic%; cr is present in the particles in an amount of 10 to 20 at%; and at least 90 atomic% (or at least 95 atomic%, at least 98 atomic%, at least 99 atomic%, or at least 99.9 atomic%) of the balance of the particles is Mo. In some embodiments, mo is present in the particle in an amount of 50 atomic% to 70 atomic%, W is present in the particle in an amount of 20 atomic% to 30 atomic%, and Cr is present in the particle in an amount of 10 atomic% to 20 atomic%.

According to certain embodiments, the sintered particles may be nanocrystalline particles. According to certain embodiments, the nanocrystalline particles may include grains having the following grain sizes: less than or equal to 1000nm, less than or equal to 900nm, less than or equal to 800nm, less than or equal to 700nm, less than or equal to 600nm, less than or equal to 500nm, less than or equal to 400nm, less than or equal to 300nm, less than or equal to 200nm, less than or equal to 150nm, less than or equal to 125nm, less than or equal to 100nm, less than or equal to 50nm, less than or equal to 40nm, less than or equal to 30nm, less than or equal to 20nm, or less than or equal to 10nm. According to certain embodiments, the grain size of at least some of the nanocrystalline particles is less than or equal to 10nm. In some embodiments, the grain size of at least some of the nanocrystalline particles is greater than or equal to 5nm and less than or equal to 25nm. In some embodiments, the grain size of at least some of the nanocrystalline particles is greater than or equal to 10nm and less than or equal to 20nm.

According to certain embodiments, at least some of the nanocrystalline particles comprise Mo, a second element (e.g., a second metal, such as chromium), and/or a third element (e.g., a third metal, such as tungsten). In some embodiments, a portion of the nanocrystalline particles consists of Mo, while another portion of the nanocrystalline particles consists of the second element, and yet another portion of the nanocrystalline particles consists of the third element. In certain embodiments, at least some of the nanocrystalline particles comprise both Mo and a second element. In certain embodiments, at least some of the nanocrystalline particles comprise both Mo and a third element. In certain embodiments, at least some of the nanocrystalline particles comprise Mo, a second element, and a third element.

In some embodiments, mo is the element that is most abundant in at least some nanocrystalline particles, in atomic percent. In some embodiments, mo is the metal that is most abundant in at least some nanocrystalline particles, in atomic percent. In some embodiments, mo is the metallic element that is most abundant in at least some of the nanocrystalline particles, by atomic percentage. In some embodiments, at least some of the particles comprise Mo in an amount of at least 50 atomic%, at least 55 atomic%, at least 60 atomic%, at least 70 atomic%, at least 80 atomic%, at least 90 atomic%, or at least 95 atomic%. In some embodiments, at least some of the particles comprise Mo in an amount of up to 96 atomic%, up to 97 atomic%, up to 98 atomic%, or greater. Combinations of these ranges are also possible. Other values are also possible.

According to certain embodiments, at least some of the particles are formed by machining a powder comprising Mo and a second element. For example, certain embodiments include producing particles at least in part by machining a powder comprising a plurality of Mo particles and a plurality of second element particles (e.g., particles comprising Cr). Certain embodiments include making particles at least in part by machining particles comprising both Mo and a second element.

According to certain embodiments, at least some of the particles are formed by machining a powder comprising Mo, a second element (e.g., chromium), and a third element (e.g., tungsten). For example, certain embodiments include producing particles (e.g., nanocrystalline particles) at least in part by machining a powder comprising a plurality of Mo particles, a plurality of second element particles (e.g., particles comprising Cr), and a plurality of third element particles (e.g., particles comprising W). Certain embodiments include at least in part by the incorporation of both Mo and a second element; both Mo and a third element; both the second element and the third element; and/or all of the particles of Mo, the second element, and the third element are machined to produce particles (e.g., nanocrystalline particles).

In embodiments utilizing machining, any suitable machining method may be employed to machine the powder and form the particles. According to certain embodiments, at least some of the particles are formed by ball milling a powder comprising Mo and a second element (and/or, when present, a third element). The ball milling process may be, for example, a high energy ball milling process. In one non-limiting exemplary ball milling process, a tungsten carbide or steel mill bottle may be employed, wherein the ratio of balls to powder is 2:1 to 20 (e.g., 5:1 to 12, e.g., 10. According to certain further embodiments, the machining is performed in the absence of a process control agent. Other types of machining may also be used including, but not limited to, vibratory milling and planetary milling. In some embodiments, the mechanical processing (e.g., by ball milling or another process) can be performed under conditions sufficient to produce particles comprising the supersaturated phase (e.g., nanocrystalline particles). The supersaturated phase is described in more detail below.

According to certain embodiments, the mechanical processing (e.g., ball milling) is performed at a relatively low temperature. For example, in some embodiments, the particles are mechanically processed (e.g., ball milled) while at the following temperatures: less than or equal to 150 ℃, less than or equal to 100 ℃, less than or equal to 75 ℃, less than or equal to 50 ℃, less than or equal to 40 ℃, less than or equal to 35 ℃, less than or equal to 30 ℃, less than or equal to 25 ℃, or less than or equal to 20 ℃. In some embodiments, the mechanical processing (e.g., ball milling) is performed while the particles are at a temperature of at least 0 ℃. In some embodiments, the mechanical processing (e.g., ball milling) is performed at ambient temperature.

In certain embodiments, the mechanical processing (e.g., ball milling) can be performed for a time of greater than or equal to 6 hours (e.g., greater than or equal to 8 hours, greater than or equal to 10 hours, greater than or equal to 12 hours, or greater than or equal to 15 hours). In certain embodiments, the mechanical processing (e.g., ball milling) can be performed for a time of less than or equal to 18 hours. In some embodiments, the mechanical processing (e.g., ball milling) can be performed for a period of 6 hours to 18 hours. In some cases, mo and/or the second element (and/or the third element, if present) may be contaminated by the material used to perform the machining (e.g., the mill bottle material) if the machining time is too long. In some cases, the amount of the second element (and/or third element, if present) dissolved in Mo may increase with increasing machining (e.g., milling) time. In some embodiments, after the mechanical processing step (e.g., ball milling step), a phase rich in the second elemental material may be present.

According to certain embodiments, mo and the second element (and/or the third element, if present) are present in the particles as an unbalanced phase. According to certain embodiments, the particles may comprise a non-equilibrium phase in which the second element (and/or the third element, if present) is dissolved in Mo. In some embodiments, the non-equilibrium phase comprises a solid solution. According to some embodiments, the non-equilibrium phase may be a supersaturated phase comprising the second element (and/or third element, if present) dissolved in Mo. As used herein, "supersaturated phase" refers to a phase in which a material is dissolved in another material in an amount that exceeds the solubility limit. In some embodiments, the supersaturated phase may contain the activator element and/or stabilizer element forcibly dissolved in the Mo in an amount in excess of the amount of activator element and/or stabilizer element that would otherwise be dissolved in the equilibrium phase of Mo. For example, in one set of embodiments, the supersaturated phase is a phase that contains the activator element forcibly dissolved in Mo in an amount in excess of the amount of activator element that would otherwise be soluble in the equilibrium Mo phase.

In some embodiments, the supersaturated phase may be the only phase present after the mechanical processing (e.g., ball milling) process.

According to certain embodiments, the non-equilibrium phase may undergo decomposition during sintering of the particles (which sintering is described in more detail below). Sintering of the particles may cause a third element-rich phase to form at least one of a surface and/or a grain boundary of the particles. In some such embodiments, mo may be soluble in the second element and/or third element rich phase. The formation of the second element and/or third element rich phase may be the result of decomposition of the non-equilibrium phase during sintering. According to certain embodiments, the second element and/or third element rich phase may act as a fast diffusion path for Mo, thereby increasing sintering kinetics and accelerating the sintering rate of the particles. According to some embodiments, the decomposition of the non-equilibrium phase during sintering of the particles accelerates the sintering rate of the particles.

Some (but not necessarily all) embodiments include cold pressing a plurality of particles during at least a portion of the time prior to sintering. According to certain embodiments, it has been found that metal alloys comprising Mo and a second element (e.g., mo and Cr) and/or metal alloys comprising Mo, a second element, and a third element (e.g., mo, cr, and W) can be compressed such that high relative densities are achieved without the need for simultaneous heating. In some embodiments, cold pressing comprises compressing the plurality of particles with a force greater than or equal to 300MPa, greater than or equal to 400MPa, greater than or equal to 500MPa, greater than or equal to 750MPa, greater than or equal to 1000MPa, or greater. In some embodiments, cold compressing comprises compressing the plurality of particles with a force of up to 1400MPa or more. Combinations of these ranges are also possible (e.g., greater than or equal to 300MPa and less than or equal to 1400 MPa). Other ranges are also possible.

According to certain embodiments, the cold compression is performed at a relatively low temperature. For example, in some embodiments, cold compression is performed while the particles are at the following temperatures: less than or equal to 150 ℃, less than or equal to 100 ℃, less than or equal to 75 ℃, less than or equal to 50 ℃, less than or equal to 40 ℃, less than or equal to 35 ℃, less than or equal to 30 ℃, less than or equal to 25 ℃, or less than or equal to 20 ℃. In some embodiments, the cold compression is performed at ambient temperature.

As described above, certain embodiments include sintering a plurality of particles to form a metal alloy. Those of ordinary skill in the art are familiar with sintering processes that involve applying heat to the material (e.g., particles) to be sintered so that the material becomes a single solid mass.

According to certain embodiments, sintering may be performed while the metal particles are at a relatively low temperature and/or for a relatively short period of time, while maintaining the ability to form metal alloys having high relative densities, small grain sizes, and/or equiaxed grains.

According to certain embodiments, sintering the plurality of particles comprises heating the particles to a sintering temperature of: less than or equal to 2200 ℃, less than or equal to 2000 ℃, less than or equal to 1900 ℃, less than or equal to 1800 ℃, less than or equal to 1700 ℃, less than or equal to 1600 ℃, less than or equal to 1500 ℃, less than or equal to 1400 ℃, less than or equal to 1300 ℃, less than or equal to 1200 ℃, less than or equal to 1100 ℃, less than or equal to 1000 ℃, less than or equal to 900 ℃, less than or equal to 850 ℃, less than or equal to 800 ℃, or less than or equal to 750 ℃. According to certain embodiments, sintering the plurality of particles comprises heating the particles to a sintering temperature of: greater than or equal to 750 ℃, greater than or equal to 850 ℃, greater than or equal to 1000 ℃, greater than or equal to 1200 ℃, greater than or equal to 1450 ℃, or greater than or equal to 1600 ℃. Combinations of these ranges are also possible. For example, in some embodiments, sintering the plurality of particles comprises heating the particles to a sintering temperature greater than or equal to 750 ℃ and less than or equal to 2200 ℃. In some embodiments, the temperature of the sintered material is within these ranges for at least 10%, at least 25%, at least 50%, at least 75%, at least 90%, or at least 99% of the sintering time.

According to certain embodiments, sintering the plurality of particles comprises maintaining the particles within a range of sintering temperatures for less than 72 hours, less than 48 hours, less than or equal to 24 hours, less than or equal to 12 hours, less than or equal to 6 hours, less than or equal to 4 hours, less than or equal to 3 hours, less than or equal to 2 hours, or less than or equal to 1 hour (and/or, in some embodiments, for at least 10 minutes, at least 20 minutes, at least 30 minutes, at least 50 minutes, at least 3 hours, or at least 6 hours). Combinations of these ranges are also possible. For example, in some embodiments, sintering the plurality of particles comprises heating the particles to a first sintering temperature of greater than or equal to 600 ℃ and less than or equal to 1100 ℃ for a sintering time of greater than or equal to 6 hours and less than or equal to 24 hours.

According to certain embodiments, sintering comprises heating the particles to a first sintering temperature that is lower than a second sintering temperature required to sinter the Mo in the absence of the second element. To determine whether such conditions are met, one of ordinary skill in the art would compare the temperature required to achieve sintering in a sample comprising Mo and a second element to the temperature required to achieve sintering in a sample comprising Mo without the second element but otherwise identical to a sample comprising Mo and the second element. In some embodiments, the first sintering temperature may be at least 25 ℃, at least 50 ℃, at least 100 ℃, or at least 200 ℃ lower than the second sintering temperature.

According to certain embodiments, the non-equilibrium phase present in the particles (e.g., any of the non-equilibrium phases described above or elsewhere herein) undergoes decomposition during sintering. In some such embodiments, the decomposition of the non-equilibrium phase accelerates the sintering rate of the particles.

In some embodiments, sintering further comprises forming a second phase at least one of a surface and a grain boundary of the particle during sintering. In some such embodiments, the second phase is enriched in the second element. The term "rich" with respect to the content of elements in a phase means that the content of elements in the phase is at least 50 atomic% (e.g., at least 60 atomic%, at least 70 atomic%, at least 80 atomic%, at least 90 atomic%, at least 99 atomic%, or greater). The term "phase" is used herein generally to refer to a state of matter. For example, a phase may refer to the phase shown on the phase diagram. Generally, when multiple phases are present, they can be distinguished from each other even if both are solid phases.

Sintering may be performed in a variety of suitable environments. In certain embodiments, the particles are in an inert atmosphere during the sintering process. For example, when a reactive metal is employed in the particles, it may be useful to use an inert atmosphere. For example, mo and Cr (alone and/or together) are reactive under oxygen.

In some embodiments, the sintering is performed in an atmosphere in which at least 90 volume percent, at least 95 volume percent, at least 99 volume percent, or substantially all of the atmosphere is comprised of an inert gas. The inert gas may be or include, for example, helium, argon, xenon, neon, krypton, combinations of two or more of these, or other inert gases.

In certain embodiments, an oxygen scavenger (e.g., getter) may be included in the sintering environment. The use of an oxygen scavenger may reduce the extent to which the metal is oxidized during the sintering process, which may be advantageous according to certain embodiments. In some embodiments, the sintering environment may be controlled such that oxygen is present in an amount less than 1 volume percent, less than 0.1 volume percent, less than 100 parts per million (ppm), less than 10ppm, or less than 1 ppm.

In certain embodiments, the sintering is performed in an atmosphere comprising: which is exposed to oxygen (i.e., O) under sintering conditions ₂ ) When it is used, it will react with oxygen. In some embodiments, the sintering is carried out in the presence of hydrogen (H) ₂ ) Is carried out in an atmosphere of (2). In some embodiments, the combination of hydrogen and inert gas comprises at least 90 volume percent, at least 95 volume percent, at least 99 volume percent, or substantially all of the atmosphere in which sintering is conducted. In some embodiments, the combination of hydrogen and argon comprises at least 90 volume percent, at least 95 volume percent, at least 99 volume percent, or substantially all of the atmosphere in which sintering is conducted.

According to certain embodiments, the sintering is performed in the substantial absence of externally applied pressure. For example, in some embodiments, the maximum external pressure applied to the nanocrystalline particles is less than or equal to 2MPa, less than or equal to 1MPa, less than or equal to 0.5MPa, or less than or equal to 0.1MPa for at least 20%, at least 50%, at least 75%, at least 90%, or at least 98% of the time during which sintering is performed. The maximum external pressure applied to the nanocrystalline particles refers to the maximum pressure applied due to the application of an external force to the nanocrystalline particles, and does not include the pressure caused by gravity during the sintering process and the pressure generated between the nanocrystalline particles and the surface on which the nanocrystalline particles are located. Certain sintering processes described herein may allow for the production of relatively highly dense sintered ultrafine and nanocrystalline materials even in the absence or substantial absence of applied external pressure during the sintering process. According to certain embodiments, the sintering may be a pressureless sintering process.

According to certain embodiments, at least one activator element may be present during the sintering process. The activator element may improve the sintering kinetics of Mo. According to certain embodiments, the activator element may provide a high diffusion path for Mo atoms. For example, in some embodiments, the activator element atoms may surround the Mo atoms and provide a relatively high transport diffusion path for the Mo atoms, thereby reducing the activation energy for Mo diffusion. In some embodiments, this technique is referred to as activated sintering. In some embodiments, the activator element may reduce the temperature required to sinter the nanocrystalline particles relative to the temperature required to sinter the nanocrystalline particles in the absence of the activator element but under otherwise identical conditions. Thus, according to certain embodiments, the sintering may include a first sintering temperature, and the first sintering temperature may be lower than a second sintering temperature required to sinter the Mo in the absence of the third element. To determine the sintering temperature required to sinter Mo in the absence of the third element, samples of Mo material that did not contain the third element but were otherwise identical to the nanocrystalline particulate material were prepared. Then, the minimum temperature required to sinter the sample that does not contain the third element will be determined. In some embodiments, the presence of the second element reduces the sintering temperature by at least 25 ℃, at least 50 ℃, at least 100 ℃, at least 200 ℃, or more.

According to certain embodiments, at least one stabilizer element may be present during the sintering process. The stabilizer element may be any element that is capable of reducing the amount of grain growth that occurs relative to the amount that occurs in the absence of the stabilizer element but under otherwise identical conditions. In some embodiments, the stabilizer element reduces grain growth by reducing the grain boundary energy of the sintered material and/or by reducing the driving force for grain growth. According to certain embodiments, the stabilizer element may exhibit a positive heat of mixing with the sintered material. The stabilizer element may stabilize the nanocrystalline Mo by segregation in grain boundaries. This segregation may reduce the grain boundary energy, and/or may reduce the driving force for grain growth in the alloy.

In some embodiments, the stabilizer element may also be an activator element. According to certain embodiments, the use of a single element as both a stabilizer element and an activator element has the additional benefit of not requiring consideration of the interaction between the activator and the stabilizer. In some embodiments, the elements that can be used as both activator elements and stabilizer elements can be metallic elements, which can be any of the foregoing metallic elements.

According to certain embodiments, when one element fails to act as both a stabilizer and an activator, two elements may be employed. According to some embodiments, the interaction between the two elements may be considered to ensure that the activator and stabilizer effects are properly achieved. For example, in some cases, when the activator and stabilizer form an intermetallic compound, each element may be prevented from achieving its specified effect. Thus, at least in some cases, activator and stabilizer combinations having the ability to form intermetallic compounds at the intended sintering temperature should be avoided. The phase diagram can be used to analyze the possibility of intermetallic compound formation between two elements.

According to one set of embodiments, molybdenum particles and chromium particles (e.g., 10 at% Cr, 20 at% Cr, or 30 at% Cr with the balance being molybdenum) may be mechanically alloyed by ball milling, cold compressed, and subsequently annealed (e.g., for several hours in a thermomechanical analyzer). In some embodiments, the Mo-Cr alloy system exhibits nanocrystalline grain size stabilization by Cr segregation to Mo grain boundaries and by Cr-rich precipitates that form pinning grain boundaries and further impede grain growth.

According to certain embodiments, powders of the elements Mo, cr and W are mixed and milled to achieve supersaturation and to reduce the grain size to the nanometer scale. In some embodiments, annealing the compressed powder results in a nano-dual phase structure consisting of Mo-rich grains and Cr-rich precipitates.

As noted above, certain embodiments relate to the metal alloys of the present invention. According to certain embodiments, the metal alloy comprises molybdenum and at least one additional metal.

According to certain embodiments, the metal alloy comprises a relatively large amount of molybdenum (Mo). For example, in some embodiments, mo is the most abundant element (e.g., the most abundant metal) in the metal alloy by atomic percentage. According to certain embodiments, mo is present in the metal alloy in the following amounts: at least 50 atomic%, at least 55 atomic%, at least 60 atomic%, at least 65 atomic%, at least 70 atomic%, at least 80 atomic%, at least 90 atomic%, or at least 95 atomic%. In some embodiments, mo is present in the metal alloy in the following amounts: up to 96 atomic%, up to 97 atomic%, up to 98 atomic%, up to 99 atomic%, up to 99.5 atomic%, or greater. Combinations of these ranges are also possible. Other values are also possible.

The metal alloys described herein may comprise a second element. For example, the metal alloys described herein may comprise a second metal.

According to certain embodiments, the second element is selected from chromium (Cr) and palladium (Pd). In some embodiments, both Cr and Pd are present (e.g., where the alloy includes at least three elements). In other embodiments, only one of Cr and Pd is present. In some embodiments, the second element is Cr.

In some embodiments, mo is at least partially soluble in the second element. For example, in some embodiments, mo and the second element are in solid solution.

The second element (e.g., chromium, palladium) may be present in the metal alloy in a variety of suitable percentages. According to certain embodiments, the second element is present in the metal alloy in an amount of: less than or equal to 40 atomic%, less than or equal to 35 atomic%, less than or equal to 32 atomic%, less than or equal to 30 atomic%, less than or equal to 25 atomic%, less than or equal to 22 atomic%, less than or equal to 20 atomic%, less than or equal to 18 atomic%, or less than or equal to 16 atomic%. In some embodiments, the second element is present in the metal alloy in an amount of: at least 0.5 atomic%, at least 1 atomic%, at least 2 atomic%, at least 3 atomic%, at least 4 atomic%, at least 5 atomic%, at least 6 atomic%, at least 7 atomic%, at least 8 atomic%, at least 9 atomic%, at least 10 atomic%, or greater. Combinations of these ranges are also possible. For example, in some embodiments, the second element is present in the metal alloy in an amount of 0.5 atomic% to 40 atomic% of the metal alloy. In some embodiments, the second element is present in the metal alloy in an amount of 1 atomic% to 40 atomic% of the metal alloy. In some embodiments, the second element is present in the metal alloy in an amount from 8 atomic% to 32 atomic% of the metal alloy. Other values are also possible.

In certain embodiments, the metal alloy (e.g., comprising molybdenum, a second metal, and optionally a third metal or additional metal) may comprise a relatively large amount of the metallic material. In some embodiments, at least 50 atomic%, at least 70 atomic%, at least 90 atomic%, at least 95 atomic%, at least 99 atomic%, at least 99.9 atomic% or more of the metal alloy is made up of metal atoms in their metallic form (i.e., in the zero oxidation state). In some embodiments, at least 50 atomic%, at least 70 atomic%, at least 90 atomic%, at least 95 atomic%, at least 99 atomic%, at least 99.9 atomic% or more of the molybdenum atoms within the metal alloy are in their metallic form. In certain embodiments, at least 50 atomic%, at least 70 atomic%, at least 90 atomic%, at least 95 atomic%, at least 99 atomic%, at least 99.9 atomic% or more of the atoms of the second element (e.g., the second metal) within the metal alloy are in their metallic form. In some embodiments, at least 50 atomic%, at least 70 atomic%, at least 90 atomic%, at least 95 atomic%, at least 99 atomic%, at least 99.9 atomic% or more of the atoms of the third element (e.g., the third metal) within the metal alloy are in their metallic form. In some embodiments, the molybdenum atom may form a metallic bond with other adjacent atoms, such as additional molybdenum atoms and/or atoms of a second element (e.g., a second metal) and/or atoms of a third element (e.g., a third metal).

In some embodiments, the metal alloy includes only Mo and the second element (i.e., mo and the second element without additional metal or other elements). In other embodiments, the metal alloy comprises Mo, a second element, and a third element. For example, in some embodiments, the metal alloy includes a third element (other than Mo and the second element). In some embodiments, the third element may be a metal element. In some embodiments, the metal alloy comprises a third metal, in which case the alloy comprises Mo, the second metal, and the third metal.

The third element (e.g., tungsten) may be present in the metal alloy in a variety of suitable percentages. According to certain embodiments, the third element is present in the metal alloy in an amount of: less than or equal to 40 atomic%, less than or equal to 35 atomic%, less than or equal to 30 atomic%, less than or equal to 28 atomic%, or less than or equal to 26 atomic%. In some embodiments, the third element is present in the metal alloy in an amount of: at least 0.5 atomic%, at least 1 atomic%, at least 2 atomic%, at least 3 atomic%, at least 4 atomic%, at least 5 atomic%, at least 6 atomic%, at least 7 atomic%, at least 8 atomic%, at least 9 atomic%, at least 10 atomic%, or greater. Combinations of these ranges are also possible. Other values are also possible.

According to certain embodiments, the total amount of all metal elements (e.g., the second element, the third element, and any additional optional elements) in the metal alloy that are not Mo comprises less than 50 atomic%, less than or equal to 40 atomic%, less than or equal to 35 atomic%, less than or equal to 32 atomic%, less than or equal to 30 atomic%, less than or equal to 25 atomic%, less than or equal to 22 atomic%, less than or equal to 20 atomic%, less than or equal to 18 atomic%, or less than or equal to 16 atomic% of the metal alloy. In some embodiments, the total amount of all elements (e.g., the second element, the optional third element, and any additional optional elements) in the metal alloy that are not Mo comprises at least 0.5 atomic%, at least 1 atomic%, at least 2 atomic%, at least 3 atomic%, at least 4 atomic%, at least 5 atomic%, at least 8 atomic%, at least 10 atomic%, at least 12 atomic%, at least 14 atomic%, or greater. Combinations of these ranges are also possible. Other values are also possible.

In some embodiments, the total amount of chromium (Cr), palladium (Pd), tungsten (W), and tantalum (Ta) present in the metal alloy is less than 50 atomic%, less than or equal to 40 atomic%, less than or equal to 35 atomic%, less than or equal to 32 atomic%, less than or equal to 30 atomic%, less than or equal to 25 atomic%, less than or equal to 22 atomic%, less than or equal to 20 atomic%, less than or equal to 18 atomic%, or less than or equal to 16 atomic% of the metal alloy. In some embodiments, the total amount of chromium (Cr), palladium (Pd), tungsten (W), and tantalum (Ta) present in the metal alloy is at least 0.5 atomic%, at least 1 atomic%, at least 2 atomic%, at least 3 atomic%, at least 4 atomic%, at least 5 atomic%, at least 8 atomic%, at least 10 atomic%, at least 12 atomic%, at least 14 atomic%, or greater. Combinations of these ranges are also possible. For example, in some embodiments, the total amount of chromium (Cr), palladium (Pd), tungsten (W), and tantalum (Ta) present in the metal alloy is 0.5 atomic% to 50 atomic% of the metal alloy. In some of these embodiments, at least 90 atomic% (or at least 95 atomic%, at least 98 atomic%, at least 99 atomic%, or at least 99.9 atomic%) of the balance of the metal alloy is molybdenum.

One of ordinary skill in the art will appreciate that to determine the total amount of chromium (Cr), palladium (Pd), tungsten (W), and tantalum (Ta) present in a given metal alloy, the atomic percentages of each of these elements will be added. For example, if the metal alloy comprises 60 at% Mo, 15 at% Cr, and 25 at% W, the total amount of chromium (Cr), palladium (Pd), tungsten (W), and tantalum (Ta) present will be 40 at% (i.e., 15 at% from Cr, 25 at% from W, and 0 at% of all other elements in the list). Those of ordinary skill in the art will also appreciate that not all of the elements in the above list are necessarily present in the metal alloy in making this calculation. In the above exemplary calculations, palladium and tantalum, for example, are not present in the Mo-W-Cr alloy.

In some embodiments, the total amount of chromium (Cr) and tungsten (W) present in the metal alloy is less than 50 atomic%, less than or equal to 40 atomic%, less than or equal to 35 atomic%, less than or equal to 32 atomic%, less than or equal to 30 atomic%, less than or equal to 25 atomic%, less than or equal to 22 atomic%, less than or equal to 20 atomic%, less than or equal to 18 atomic%, or less than or equal to 16 atomic% of the metal alloy. In some embodiments, the total amount of chromium (Cr) and tungsten (W) present in the metal alloy is at least 0.5 atomic%, at least 1 atomic%, at least 2 atomic%, at least 3 atomic%, at least 4 atomic%, at least 5 atomic%, at least 8 atomic%, at least 10 atomic%, at least 12 atomic%, at least 14 atomic%, or greater. Combinations of these ranges are also possible. For example, in some embodiments, the total amount of chromium (Cr) and tungsten (W) present in the metal alloy is 0.5 atomic% to 50 atomic% of the metal alloy. In some of these embodiments, at least 90 atomic% (or at least 95 atomic%, at least 98 atomic%, at least 99 atomic%, or at least 99.9 atomic%) of the balance of the metal alloy is molybdenum.

In some embodiments, the metal alloy comprises Mo, cr, and W. In some embodiments, mo is present in the metal alloy in an amount of at least 50 atomic% (e.g., 50 atomic% to 99 atomic%), cr is present in the metal alloy in an amount of 0.5 atomic% to 30 atomic%; and W is present in the metal alloy in an amount of 0.5 atomic% to 30 atomic%. In some embodiments, W is present in the metal alloy in an amount of 20 atomic% to 30 atomic%; cr is present in the metal alloy in an amount of 10 atomic% to 20 atomic%; and at least 90 atomic% (or at least 95 atomic%, at least 98 atomic%, at least 99 atomic%, or at least 99.9 atomic%) of the balance of the metal alloy is Mo. In some embodiments, mo is present in the metal alloy in an amount of 50 atomic% to 70 atomic%, W is present in the metal alloy in an amount of 20 atomic% to 30 atomic%; and Cr is present in the metal alloy in an amount of 10 atomic% to 20 atomic%.

According to certain embodiments, the metal alloy comprising molybdenum is a nanocrystalline metal alloy. Nanocrystalline metals have certain advantages over their microcrystalline counterparts due to the large volume fraction of grain boundaries. As an example, nanocrystalline alloys generally have significantly higher tensile strengths.

Nanocrystalline material generally refers to material that contains at least some grains having a grain size of 1000nm or less. In some embodiments, the nanocrystalline material comprises grains having the following grain sizes: less than or equal to 900nm, less than or equal to 800nm, less than or equal to 700nm, less than or equal to 600nm, less than or equal to 500nm, less than or equal to 400nm, less than or equal to 300nm, less than or equal to 200nm, less than or equal to 100nm, less than or equal to 50nm, less than or equal to 20nm, less than or equal to 10nm, or less than or equal to 5nm. In some embodiments, the nanocrystalline material comprises grains having a grain size of at least 1nm or at least 5nm. Thus, in the case of a metal alloy, a nanocrystalline metal alloy is a metal alloy that contains grains having a grain size of 1000nm or less. In some embodiments, the nanocrystalline metal alloy comprises grains having the following grain sizes: less than or equal to 900nm, less than or equal to 800nm, less than or equal to 700nm, less than or equal to 600nm, less than or equal to 500nm, less than or equal to 400nm, less than or equal to 300nm, less than or equal to 200nm, less than or equal to 150nm, less than or equal to 125nm, less than or equal to 100nm, less than or equal to 50nm, less than or equal to 20nm, or less than or equal to 10nm. In some embodiments, the nanocrystalline metal alloy comprises grains having a grain size of at least 1nm, at least 2nm, or at least 5nm. Other values are also possible.

The "grain size" of a grain generally refers to the largest dimension of the grain. Depending on the geometry of the grains, the maximum dimension may be the diameter, length, width or height of the grains. According to certain embodiments, the grains may be spherical, cubic, pyramidal, cylindrical, acicular, or any other suitable geometric shape.

According to certain embodiments, a relatively large percentage of the volume of the metal alloy is made up of small grains. For example, in some embodiments, at least 50%, at least 75%, at least 90%, at least 95%, at least 99%, or substantially all of the volume of the metal alloy consists of grains having the following grain sizes: less than or equal to 1000nm, less than or equal to 900nm, less than or equal to 800nm, less than or equal to 700nm, less than or equal to 600nm, less than or equal to 500nm, less than or equal to 400nm, less than or equal to 300nm, less than or equal to 200nm, less than or equal to 150nm, less than or equal to 125nm, less than or equal to 100nm, less than or equal to 50nm, less than or equal to 20nm, or less than or equal to 10nm (and/or, in some embodiments, as little as 5nm, as little as 2nm, or as little as 1 nm). Other values are also possible.

According to certain embodiments, the metal alloy may have a relatively small average grain size. The "average grain size" of a material (e.g., a metal alloy) refers to the number average of the grain sizes of the grains in the material. According to certain embodiments, the average grain size of the metal alloy (e.g., bulk and/or nanocrystalline metal alloy) is less than or equal to 1000nm, less than or equal to 900nm, less than or equal to 800nm, less than or equal to 700nm, less than or equal to 600nm, less than or equal to 500nm, less than or equal to 400nm, less than or equal to 300nm, less than or equal to 200nm, less than or equal to 150nm, less than or equal to 125nm, less than or equal to 100nm, less than or equal to 50nm, less than or equal to 20nm, or less than or equal to 10nm. In certain embodiments, the average grain size of the metal alloy is as small as 25nm, as small as 10nm, as small as 5nm, as small as 2nm, as small as 1nm, or less. Combinations of these ranges are also possible. Other values are also possible.

According to a certainIn some embodiments, at least one cross-section of the metal alloy through the geometric center of the metal alloy has a small volume average cross-sectional grain size. The "volume average cross-sectional grain size" for a given cross-section of a metal alloy is determined by: obtaining a target cross-section, delineating the periphery of each grain in an image of the target cross-section (which may be a magnified image, such as that obtained by transmission electron microscopy), and calculating the equivalent circular diameter D of each delineated grain cross-section _i . The "equivalent circle diameter" of the crystal grain cross section corresponds to the area (A, from A = π r ² Determined) is equal to the cross-sectional area of the crystal grain in the target cross-section. Volume average cross-sectional grain size (G) _{CS, average} ) The following calculations were made:

wherein n is the number of crystal grains in the cross section, D _i Is the equivalent circle diameter of the crystal grain i.

According to certain embodiments, the volume average cross-sectional grain size of at least one cross-section of the metal alloy through the geometric center of the metal alloy is less than or equal to 1000nm, less than or equal to 900nm, less than or equal to 800nm, less than or equal to 700nm, less than or equal to 600nm, less than or equal to 500nm, less than or equal to 400nm, less than or equal to 300nm, less than or equal to 200nm, less than or equal to 150nm, less than or equal to 125nm, less than or equal to 100nm, less than or equal to 50nm, less than or equal to 20nm, or less than or equal to 10nm. In certain embodiments, the volume average cross-sectional grain size of at least one cross-section of the metal alloy through the geometric center of the metal alloy is as small as 25nm, as small as 10nm, as small as 5nm, as small as 2nm, as small as 1nm, or less. Combinations of these ranges are also possible. Other values are also possible.

According to certain embodiments, at least one cross-section of the metal alloy (optionally, throughout the geometric center of the metal alloy) has a volume average cross-sectional grain size of less than or equal to 1000nm, less than or equal to 900nm, less than or equal to 800nm, less than or equal to 700nm, less than or equal to 600nm, less than or equal to 500nm, less than or equal to 400nm, less than or equal to 300nm, less than or equal to 200nm, less than or equal to 150nm, less than or equal to 125nm, less than or equal to 100nm, less than or equal to 50nm, less than or equal to 20nm, less than or equal to 10nm (and/or as small as 25nm, as small as 10nm, as small as 5nm, as small as 2nm, as small as 1nm, or less); and a volume average cross-sectional grain size of at least a second cross-section of the metal alloy orthogonal to the first cross-section (optionally, through a geometric center of the metal alloy) is less than or equal to 1000nm, less than or equal to 900nm, less than or equal to 800nm, less than or equal to 700nm, less than or equal to 600nm, less than or equal to 500nm, less than or equal to 400nm, less than or equal to 300nm, less than or equal to 200nm, less than or equal to 150nm, less than or equal to 125nm, less than or equal to 100nm, less than or equal to 50nm, less than or equal to 20nm, less than or equal to 10nm (and/or as small as 25nm, as small as 10nm, as small as 5nm, as small as 2nm, as small as 1nm, or less). Other values are also possible.

According to certain embodiments, at least one cross-section of the metal alloy (optionally, throughout the geometric center of the metal alloy) has a volume average cross-sectional grain size of less than or equal to 1000nm, less than or equal to 900nm, less than or equal to 800nm, less than or equal to 700nm, less than or equal to 600nm, less than or equal to 500nm, less than or equal to 400nm, less than or equal to 300nm, less than or equal to 200nm, less than or equal to 150nm, less than or equal to 125nm, less than or equal to 100nm, less than or equal to 50nm, less than or equal to 20nm, or less than or equal to 10nm (and/or as small as 25nm, as small as 10nm, as small as 5nm, as small as 2nm, as small as 1nm, or less); a volume average cross-sectional grain size of at least a second cross-section of the metal alloy orthogonal to the first cross-section (optionally, also throughout the geometric center of the metal alloy, or otherwise) of less than or equal to 1000nm, less than or equal to 900nm, less than or equal to 800nm, less than or equal to 700nm, less than or equal to 600nm, less than or equal to 500nm, less than or equal to 400nm, less than or equal to 300nm, less than or equal to 200nm, less than or equal to 150nm, less than or equal to 125nm, less than or equal to 100nm, less than or equal to 50nm, less than or equal to 20nm, or less than or equal to 10nm (and/or as small as 25nm, as small as 10nm, as small as 5nm, as small as 2nm, as small as 1nm, or less); and a volume average cross-sectional grain size of at least a third cross-section of the metal alloy orthogonal to the first cross-section and orthogonal to the second cross-section (optionally also through the geometric center of the metal alloy) is less than or equal to 1000nm, less than or equal to 900nm, less than or equal to 800nm, less than or equal to 700nm, less than or equal to 600nm, less than or equal to 500nm, less than or equal to 400nm, less than or equal to 300nm, less than or equal to 200nm, less than or equal to 150nm, less than or equal to 125nm, less than or equal to 100nm, less than or equal to 50nm, less than or equal to 20nm, or less than or equal to 10nm (and/or as small as 25nm, as small as 10nm, as small as 5nm, as small as 2nm, as small as 1nm, or less).

In some embodiments, the metal alloy comprises relatively equiaxed grains. In certain embodiments, at least a portion of the grains in the metal alloy have an aspect ratio of less than or equal to 2, less than or equal to 1.8, less than or equal to 1.6, less than or equal to 1.4, less than or equal to 1.3, less than or equal to 1.2, or less than or equal to 1.1 (and in some embodiments, as low as 1). The aspect ratio of a grain is calculated as the maximum cross-sectional dimension of the grain through the geometric center of the grain divided by the maximum dimension of the grain orthogonal to the maximum cross-sectional dimension of the grain. The aspect ratio of the grains is indicated as a single number, where 1 corresponds to an equiaxed grain. In some embodiments, the number average of the aspect ratios of the grains in the metal alloy is less than or equal to 2, less than or equal to 1.8, less than or equal to 1.6, less than or equal to 1.4, less than or equal to 1.3, less than or equal to 1.2, or less than or equal to 1.1 (and in some embodiments, as low as 1).

Without wishing to be bound by any particular theory, it is believed that relatively equiaxed grains may be present when the metal alloy is produced without (or substantially without) applied pressure (e.g., by a pressureless or substantially pressureless sintering process).

In certain embodiments, the metal alloy comprises a relatively low cross-sectional average grain aspect ratio. In some embodiments, the cross-sectional average grain aspect ratio in the metal alloy is less than or equal to 2, less than or equal to 1.8, less than or equal to 1.6, less than or equal to 1.4,Less than or equal to 1.3, less than or equal to 1.2, or less than or equal to 1.1 (and, in some embodiments, as low as 1). A "cross-sectional average grain aspect ratio" of a metal alloy is said to fall within a particular range if at least one cross-section of the metal alloy through the geometric center of the metal alloy consists of a grain cross-section having an average aspect ratio falling within that range. For example, if the metal alloy includes at least one cross-section through the geometric center of the metal alloy, and wherein the cross-section is comprised of a grain cross-section having an average aspect ratio of less than 2, the cross-sectional average grain aspect ratio of the metal alloy will be less than 2. To determine the average aspect ratio of the grain cross-section from which the cross-section of the metal alloy is composed (also referred to herein as "average aspect ratio of the grain cross-section"): a cross-section of the metal alloy is obtained, the periphery of each grain is delineated in a cross-sectional image (which may be a magnified image, such as an image obtained by transmission electron microscopy) of the metal alloy, and the aspect ratio of the cross-section of each delineated grain is calculated. The aspect ratio of a grain cross-section is calculated as the maximum cross-sectional dimension of the grain cross-section (which runs through the geometric center of the grain cross-section) divided by the maximum dimension of the grain cross-section that is orthogonal to the maximum cross-sectional dimension of the grain cross-section. The aspect ratio of the grain section is represented as a single number, where 1 corresponds to an equiaxed grain section. Average Aspect Ratio (AR) of cross section of crystal grains constituting cross section of metal alloy _{Average out} ) Calculated as the number average:

wherein n is the number of crystal grains in the cross section, AR _i Is the aspect ratio of the cross section of the crystal grain i.

According to certain embodiments, a metal alloy having a cross-sectional average grain aspect ratio falling within a particular range (e.g., any of the ranges described elsewhere herein) has a first cross-section through the geometric center of the metal alloy and having a grain cross-sectional average aspect ratio falling within that range, and at least a second cross-section orthogonal to the first cross-section through the geometric center of the metal alloy and having a grain cross-sectional average aspect ratio falling within that range. For example, in accordance with certain embodiments, a metal alloy having a cross-sectional average grain aspect ratio of less than 2 includes a cross-section having an average aspect ratio of grain cross-section of less than 2 through a geometric center of the metal alloy and at least a second cross-section orthogonal to the first cross-section having an average aspect ratio of grain cross-section of less than 2 through the geometric center of the metal alloy.

According to certain embodiments, a metal alloy having a cross-sectional average grain aspect ratio falling within a particular range (e.g., any range described elsewhere herein) has a first cross-section through the geometric center of the metal alloy and the average aspect ratio of the grain cross-section falls within that range; a second cross-section orthogonal to the first cross-section through the geometric center of the metal alloy and having an average aspect ratio of the cross-section of the crystal grain falling within the range; and at least a third cross section orthogonal to the first and second cross sections, which intersects the geometric center of the metal alloy and in which the average aspect ratio of the cross section of the crystal grain falls within the range. For example, in accordance with certain embodiments, a metal alloy having a cross-sectional average grain aspect ratio of less than 2 includes a first cross-section having a grain cross-section average aspect ratio of less than 2 through a geometric center of the metal alloy, a second cross-section orthogonal to the first cross-section having a grain cross-section average aspect ratio of less than 2 through a geometric center of the metal alloy, and at least a third cross-section orthogonal to the first cross-section and the second cross-section having a grain cross-section average aspect ratio of less than 2 through a geometric center of the metal alloy.

According to certain embodiments, the grains within the metal alloy may be relatively small and relatively equiaxed. For example, according to certain embodiments, at least one cross-section (and in some embodiments, at least a second cross-section orthogonal to the first cross-section and/or at least a third cross-section orthogonal to the first cross-section and the second cross-section) may have a volume average cross-sectional grain size and an average aspect ratio of the grain cross-section that fall within any of the ranges outlined above or elsewhere herein.

Certain metal alloys described herein may have a high relative density. In some such embodiments, the metal alloy has a high relative density while maintaining its nanocrystalline character.

The term "relative density" refers to a metal alloyThe experiment of (2) measures the ratio of the density to the maximum theoretical density of the metal alloy. "relative density" (ρ) _{Relative to each other} ) Expressed as a percentage, and calculated as follows:

where ρ is _Measuring Is the experimentally measured density of the metal alloy, and p _{Maximum of} Is the maximum theoretical density of an alloy having the same composition as the metal alloy.

In some embodiments, the relative density of the metal alloy (e.g., sintered metal alloy, nanocrystalline metal alloy, and/or bulk metal alloy) is at least 80%, at least 85%, at least 90%, at least 92%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% (and/or, in certain embodiments, up to 99.8%, up to 99.9%, or greater). In some embodiments, the relative density of the nanocrystalline alloy is 100%. Other values are also possible.

According to certain embodiments, the metal alloy is fully dense. As used herein, the term "fully dense" (or "full density") refers to a material having a relative density of at least 98%. According to certain embodiments, the relative density of the metal alloy may affect other material properties of the metal alloy. Thus, by controlling the relative density of the metal alloy, other material properties of the metal alloy may be controlled.

According to certain embodiments, the metal alloys described herein may be substantially stable at relatively high temperatures. A metal alloy is said to be "substantially stable" at a particular temperature when the metal alloy includes at least one cross-section through the geometric center of the alloy as follows: wherein the volume average cross-sectional grain size of the cross-section (described above) does not increase by more than 20% (relative to the original volume average cross-sectional grain size) when the metal alloy is heated to that temperature for 24 hours in an argon atmosphere. One of ordinary skill in the art can determine whether a metal alloy is substantially stable at a particular temperature by: taking a section of an article, determining a volume average section grain size of the section at 25 ℃, heating the section to a specified temperature for 24 hours in an argon atmosphere, cooling the section back to 25 ℃, and determining the volume average section grain size of the section after heating. A metal alloy is said to be substantially stable if the volume average cross-sectional grain size of the cross-section after the heating step is less than 120% of the volume average cross-sectional grain size of the cross-section before the heating step. According to certain embodiments, a metal alloy that is substantially stable at a particular temperature includes at least one cross-section through a geometric center of the metal alloy as follows: wherein the volume average cross-sectional grain size of the cross-section increases by no more than 15%, no more than 10%, no more than 5%, or no more than 2% (relative to the original volume average grain size) when the object is heated to that temperature in an argon atmosphere for 24 hours.

In some embodiments, the metal alloy is substantially stable at least one temperature greater than or equal to 100 degrees Celsius (C.). In certain embodiments, the metal alloy is substantially stable at least one of the following temperatures: greater than or equal to 700 ℃, greater than or equal to 800 ℃, greater than or equal to 900 ℃, greater than or equal to 1000 ℃, greater than or equal to 1100 ℃, greater than or equal to 1200 ℃, greater than or equal to 1300 ℃, greater than or equal to 1400 ℃, greater than or equal to 1500 ℃, greater than or equal to 1600 ℃, greater than or equal to 1700 ℃, greater than or equal to 1800 ℃, greater than or equal to 1900 ℃, greater than or equal to 2000 ℃, greater than or equal to 2100 ℃, greater than or equal to 2200 ℃, greater than or equal to 2300 ℃, greater than or equal to 2400 ℃, or greater than or equal to 2500 ℃. Other ranges are also possible.

Certain metal alloys described herein are sintered metal alloys. Exemplary sintering methods that may be used to produce metal alloys according to the present disclosure are described above.

Certain metal alloys described herein are stable to grain growth.

According to certain embodiments, the metal alloy may be a bulk metal alloy (e.g., a bulk nanocrystalline metal alloy). A "bulk metal alloy" is a metal alloy that is not in the form of a thin film. In certain embodiments, the minimum dimension of the bulk metal alloy is at least 1 micron. In some embodiments, the bulk metal alloy has a minimum dimension of at least 5 microns, at least 10 microns, at least 25 microns, at least 50 microns, at least 100 microns, at least 500 microns, at least 1 millimeter, at least 1 centimeter, at least 10 centimeters, at least 100 centimeters, or at least 1 meter. Other values are also possible. According to certain embodiments, the metal alloy is not in the form of a coating.

In certain embodiments, the metal alloy occupies a volume of at least 0.01mm ³ At least 0.1mm ³ At least 1mm ³ At least 5mm ³ At least 10mm ³ At least 0.1cm ³ At least 0.5cm ³ At least 1cm, of ³ At least 10cm ³ At least 100cm ³ Or at least 1m ³ . Other values are also possible.

According to certain embodiments, the metal alloy comprises a plurality of phases. For example, in some embodiments, the metal alloy is a dual phase metal alloy. In some cases, the metal alloy includes a first solid phase enriched in Mo and a second solid phase enriched in the second metal. In other embodiments, the metal alloy is a single phase metal alloy.

Certain embodiments relate to thermally stable molybdenum-based metal alloys having nanocrystalline microstructures. The alloy can be prepared from metal powders by mechanical alloying and then consolidating at high temperatures into a fully dense material while retaining its nanoscale grain size. According to certain embodiments, the dense nanocrystalline alloy is significantly stronger than a similar alloy that is not nanocrystalline.

According to certain embodiments, the alloy is molybdenum (Mo) based and typically comprises chromium (Cr) and/or tungsten (W) of different compositions. According to some embodiments, they are prepared by high energy ball milling of elemental powders, which results in mechanical alloying (resulting in alloys) and grain refinement (formation of nanocrystalline structures). In some embodiments, the alloy powder is then cold compressed and annealed in an inert atmosphere without any applied pressure. According to certain embodiments, it is believed that the addition of Cr stabilizes the grain boundaries such that the nanocrystalline structure is maintained during the annealing process. It is also believed that the addition of Cr helps to accelerate the sintering (densification) process by forming a second phase during annealing, according to some embodiments. In some embodiments, alloys comprising Mo and Cr may achieve full densification at temperatures approaching 1450 ℃. This is lower than most conventional sintering methods for producing molybdenum-based alloys. This may also provide the advantage of allowing conventional equipment to be used when producing the alloy according to the methods described herein, and may also reduce the energy required to produce the part.

Some, but not necessarily all, embodiments described herein may have one or more advantages and/or improvements over existing methods, apparatus, and/or materials. According to some embodiments, the methods described herein allow for the scalable fabrication of fully dense bulk nanocrystalline components with potentially complex shapes. Alternative methods, such as the large plastic deformation (SPD) of dense, coarse grained materials, are considered to be generally non-expandable and are considered to be generally limited to simple part shapes. In addition, certain methods described herein allow for sintering of the powder without applied pressure during heating, which greatly simplifies the process route.

Certain articles, systems, and/or methods described herein may have any of a variety of commercial applications and/or may be particularly economically attractive. For example, certain alloys described herein may be manufactured using much less energy (due to low temperature and low pressure processing) than would be required for other types of molybdenum-containing alloys. Furthermore, according to certain embodiments, bulk metal components (e.g., nanocrystalline metal components) may replace any structural metal components in commercial applications, as they may provide significantly improved mechanical properties. According to some embodiments, the molybdenum alloys described herein may replace conventional molybdenum alloy components in the construction, automotive, aerospace, and nuclear industries, among others. In some embodiments, they may be used to reduce weight when their increased strength is not required. For example, according to certain embodiments, thinner panels may provide the same mechanical properties as thicker panels made from conventional alloys. In some embodiments, the alloys described herein may be used to provide both increased strength and weight savings.

Certain alloys described herein may also be advantageous in high temperature structural materials, such as in nuclear thermal propulsion. In some embodiments, the alloys may have a sufficiently high melting temperature so that they can be operated at high temperatures (e.g., temperatures of at least about 2500 ℃) for at least short periods of time (e.g., at least 1 minute, at least 10 minutes, or longer). Furthermore, certain embodiments of the alloys described herein may have a low neutron absorption cross section, making them particularly suitable for use in nuclear reactors.

The following examples are intended to illustrate certain embodiments of the invention, but not to exemplify the full scope of the invention.

Examples

This example describes the enhanced sintering of molybdenum-based alloys. In certain embodiments, this combination of gold may be used as a structural material in nuclear thermal propulsion. Certain alloys described herein may be sintered at low temperatures, quickly, and/or without the need for applied pressure during the sintering process. In some such embodiments, the alloy may also have a melting temperature sufficiently high such that it can be operated at temperatures as high as 2500 ℃ (e.g., for at least a short period of time), and/or have an acceptable neutron absorption cross section for nuclear reactors.

Molybdenum is a viable candidate for structural materials in various nuclear reactor applications. Molybdenum has a relatively low neutron absorption cross-section, which means that neutrons released during the nuclear reaction remain contained in the reactor. These neutrons then react with the nuclear fuel to sustain the nuclear reaction that makes the reactor function. In addition, molybdenum has a high melting temperature, making it structurally stable in the high temperature environment of a nuclear reactor. Finally, molybdenum has a high thermal conductivity, which can be used to transfer heat to the working fluid. These factors make molybdenum alloys a promising candidate for use in nuclear reactor designs including nuclear thermal propulsion systems.

However, pure molybdenum is generally not a suitable material for producing the complex components required for advanced nuclear technologies such as nuclear thermal propulsion. Due to the high melting temperature of molybdenum, it is often difficult to produce parts from molybdenum. Sintering pure molybdenum also typically requires high applied pressures to achieve full density, which limits the complexity of components that can be produced from molybdenum. Therefore, it would be useful to design molybdenum alloys to facilitate pressureless sintering at lower temperatures. Furthermore, while molybdenum does have a high melting temperature, nuclear thermal propulsion requires operation at temperatures just above the practical operating range of pure molybdenum. Alloying molybdenum to increase its melting temperature will therefore further increase its usefulness.

In certain embodiments, the molybdenum alloy may be designed such that rapid sintering (which may include nano-phase separation sintering) is achieved. This example discloses certain molybdenum alloys that undergo rapid, low temperature, pressureless sintering by nano-phase separation sintering. In this example, the alloying element selected to promote the nanophase-separated sintering in molybdenum is chromium. Chromium has been observed to segregate into second phases upon heating. In addition, chromium has a lower surface energy than molybdenum and therefore preferentially segregates to the surface of the powder particles to form necks between them. In addition, chromium rapidly diffuses molybdenum through the chromium neck to promote rapid densification. These steps in the sintering process can be seen in fig. 4.

To make the alloy, pure molybdenum and chromium powders were combined using mechanical alloying. Most tests used a ratio of about 15 atomic% chromium to the remaining molybdenum (or Mo15 Cr). This produced the metal powder shown in fig. 5 (diameter of about 1 micron), which was supersaturated (molybdenum and chromium were uniformly dispersed) and was nanocrystalline with a grain size of about 10nm. The powder is then pressed and formed into the shape of the part (or green body) to ensure that the powder particles contact each other. When the green body is raised to a higher temperature, the chromium atoms diffuse to the surface of the powder particles. These atoms segregate from the powder particles and form a solid chromium phase on the surface of the powder particles. This separate phase formed between the particles forms a neck between the particles as shown in fig. 7. These necks act as fast diffusion channels between the particles, allowing the material to flow through the system and further fill the gaps between the particles. This allows the green body to sinter and reach a relative density of >98%. The resulting alloy is one that undergoes initial sintering at temperatures as low as 750 ℃ and reaches a final density at temperatures as low as about 1450 ℃, as seen in the microstructure of fig. 6 and the densification curve of fig. 9.

To produce a molybdenum-based alloy with a higher melting temperature, tungsten is included in the alloy. Tungsten has a high melting temperature and forms a solid solution with molybdenum. Thus, it is believed that the melting temperature of the alloy can be adjusted by increasing the amount of tungsten alloyed with the material. For example, an alloy that is 60 at% molybdenum, 25 at% tungsten, and 15 at% chromium (Mo 25W15 Cr) should exhibit a melting temperature that is 100 degrees higher than pure molybdenum. Similar to the Mo15Cr alloy tested, a relative density of greater than 98% was achieved in the sample heated to only 1450 ℃ without the application of pressure, as seen in the microstructure of fig. 8 and the densification curve of fig. 10.

The final melting temperature of the material remains high because the chromium phase formed to accelerate sintering redissolves into the bulk molybdenum-based or molybdenum-tungsten-based material rather than melting at a lower temperature. While the addition of chromium does affect the melting temperature, the addition of more tungsten can be used to compensate for this.

The following processing steps were used to make the alloy:

mechanical alloying of elemental molybdenum, chromium and tungsten powders by ball milling

Shaping the powder into the desired shape (in the laboratory, the powder is pressed into pellets, but the actual final part may have other shapes and sizes)

The pressed powder is placed in a furnace with a controlled atmosphere. In this set of embodiments, argon is used, but other gases may work.

The pressed powder is heated to a desired temperature range. An initial stage of sintering was observed near 750 ℃ to 800 ℃ and the final density was achieved at a temperature near 1450 ℃.

The alloys described in this embodiment may provide one or more of the following advantages. The alloy described in this example achieves full densification at temperatures approaching 1450 ℃. This is lower than most conventional sintering methods used to produce molybdenum-based alloys. This is beneficial because it allows more conventional equipment to be used to produce these alloys. Furthermore, this reduces the energy required to produce the component.

The alloy described in this example is not held at its sintering temperature for a long time. This is beneficial in reducing the amount of energy required to produce the part. In addition, faster sintering of parts is beneficial for having higher part throughput, which parts can be used or sold by the entity that manufactured them.

The alloy described in this example can be sintered without the application of external pressure. This means that articles can be manufactured from alloys of similar capabilities using simpler tools than the alloys. Furthermore, more complex geometries can be fabricated from these alloys, making them useful for creating new objects with very specific functions.

The alloy described in this example can withstand high temperatures while exhibiting rapid sintering. Some methods of promoting densification of a material result in the creation of a second low melting temperature phase that prevents the resulting material from being used at higher temperatures. In contrast, in the alloys described in this example, the formed second phase re-dissolves back into the base alloy and the melting temperature remains high.

For the alloys described in this example, the material remains solid throughout the sintering process, so the shape change during the process is very limited. This is important when manufacturing components with specific tolerances for the final component geometry. Some accelerated sintering techniques result in the part deforming during the sintering process (particularly those in which liquid is formed) before full density is reached.

For the alloys described in this example, the melting temperature and nuclear absorption characteristics can be adjusted for the desired operating conditions of the final product. Tungsten alone does not have the neutron absorption characteristics necessary for the structural materials used in nuclear reactors, particularly those intended to contain reactions. Molybdenum alone cannot withstand the high temperatures of some advanced nuclear reactor designs. The use of these two elements to produce a high melting temperature and an enhanced combination of acceptable neutron absorption capabilities will help to produce new nuclear systems.

For the alloys described in this example, the production of the powders is industrially scalable. Mechanical alloying by ball milling is a common industrial process that is easily extended from laboratory quantities (a few grams) to industrially relevant quantities (many kilograms). Other methods of accelerated sintering, such as the production of nanoscale powders, are often difficult to scale to commercially viable levels.

It is also believed that these are the only molybdenum-based alloys designed to exhibit nano-phase separation sintering. The molybdenum-based alloys described herein have a variety of potential commercial applications. For example, in some cases, a molybdenum-based alloy (e.g., also with chromium and/or tungsten) may be used for nuclear thermal propulsion. Nuclear thermal propulsion engines generally require structures that exhibit a sufficiently low neutron absorption cross section while being capable of operating at temperatures of about 2500 ℃. In addition, molybdenum-based alloys may be advantageous to create the complex geometries required to maximize surface area and create through channels to more efficiently transfer heat to the propellant. Certain molybdenum-based alloys described herein co-optimize these properties in a manner that no other alloy is believed to be capable of.

Certain molybdenum-based alloys described herein may be incorporated into nuclear thermal propulsion systems (e.g., for deep space missions and possibly to mars manned missions). Certain molybdenum-based alloys described herein facilitate extrusion of components by conventional manufacturing techniques. The molybdenum-based alloys described herein may also provide a way for new 3D printing components with complex geometries (e.g., for more specialized spacecraft). Certain molybdenum-based alloys described herein may also be used in high temperature applications in new nuclear reactors (e.g., fission and/or fusion nuclear reactors).

While several embodiments of the present invention have been described and illustrated herein, those of ordinary skill in the art will readily envision a variety of other means and/or structures for performing the functions and/or obtaining the results and/or one or more of the advantages described herein, and each of such variations and/or modifications is deemed to be within the scope of the present invention. More generally, those skilled in the art will readily appreciate that all parameters, dimensions, materials, and configurations described herein are meant to be exemplary and that the actual parameters, dimensions, materials, and/or configurations will depend upon the specific application or applications for which the teachings of the present invention is/are used. Those skilled in the art will recognize, or be able to ascertain using no more than routine experimentation, many equivalents to the specific embodiments of the invention described herein. It is, therefore, to be understood that the foregoing embodiments are presented by way of example only and that, within the scope of the appended claims and equivalents thereto, the invention may be practiced otherwise than as specifically described and claimed. The present invention is directed to each individual feature, system, article, material, and/or method described herein. In addition, any combination of two or more such features, systems, articles, materials, and/or methods, if such features, systems, articles, materials, and/or methods are not mutually inconsistent, is included within the scope of the present invention.

The indefinite articles "a" or "an", as used herein in the specification and in the claims, should be understood to mean "at least one", unless expressly specified to the contrary.

The phrase "and/or," as used herein in the specification and claims, should be understood to mean "either or both" of the elements so connected, i.e., the elements that are present together in some cases and separately in other cases. Unless explicitly stated to the contrary, other elements than those explicitly stated by the "and/or" clause may optionally be present, whether related or unrelated to those elements specifically stated. Thus, as a non-limiting example, when used in conjunction with open language such as "comprising," a reference to "a and/or B" may refer in one embodiment to a without B (optionally including elements other than B); in another embodiment, to B without a (optionally including elements other than a); in yet another embodiment, refers to both a and B (optionally including other elements); and so on.

As used herein in the specification and claims, "or" is understood to have the same meaning as "and/or" as defined above. For example, when separating items in a list, "or" and/or "should be interpreted as being inclusive, i.e., including at least one of the plurality of elements or list of elements, but also including more than one, and optionally including additional unrecited items. Only terms explicitly indicated to the contrary, such as "only one" or "exactly one," or "consisting of … …" when used in the claims, are meant to include a plurality of elements or exactly one element of a list of elements. In general, the term "or" when used herein with the preceding exclusive term such as "one of two", "one", "only one", or "exactly one" should only be construed to indicate an exclusive alternative (i.e., "one or the other, but not both"). "consisting essentially of … …" when used in the claims shall have its ordinary meaning as used in the patent law field.

As used herein in the specification and claims, the phrase "at least one," when referring to a list of one or more elements, should be understood to mean at least one element selected from any one or more of the elements in the list of elements, but not necessarily including at least one of each and every element explicitly listed in the list of elements, and not excluding any combinations of elements in the list of elements. The definition also allows that elements may optionally be present other than the elements specifically identified in the list of elements referred to by the phrase "at least one," whether related or unrelated to those elements specifically identified. Thus, as a non-limiting example, "at least one of a and B" (or, equivalently, "at least one of a or B," or, equivalently, "at least one of a and/or B") can refer in one embodiment to at least one a, optionally including more than one a, but not the presence of B (and optionally including elements other than B); in another embodiment, to at least one B, optionally including more than one B, but no a (and optionally including elements other than a); in yet another embodiment, to at least one, optionally including more than one, a, and at least one, optionally including more than one, B (and optionally including other elements); and so on.

In the claims, as well as in the specification above, all transitional phrases such as "comprising," "including," "carrying," "having," "containing," "involving," "holding," and the like are to be understood to be open-ended, i.e., to mean including but not limited to. The transitional phrases "consisting of … …" and "consisting essentially of … …" should be closed or semi-closed transitional phrases, respectively, as set forth in U.S. patent office patent examination program manual section 2111.03.

Claims

1. A method of forming a metal alloy, comprising:

sintering particles comprising molybdenum (Mo) and a second element to produce the metal alloy, wherein:

mo is the most abundant element in the metal alloy in atomic percent, and

the relative density of the metal alloy is at least 80%.

2. A method of forming a metal alloy, comprising:

sintering particles comprising molybdenum (Mo) and chromium (Cr) to produce the metal alloy.

3. The method of claim 2, wherein Mo is the most abundant element in the metal alloy by atomic percent.

4. The method of claim 3, wherein the metal alloy has a relative density of at least 80%.

5. The method of any one of claims 1 and 3-4, wherein the second element is chromium.

6. The method of any one of claims 1 and 3-4, wherein the second element is palladium (Pd).

7. The method of any one of claims 1 to 6, further comprising a third element.

8. The method of claim 7, wherein the third element is present in the metal alloy in an amount of 0.5 atomic% to 40 atomic% of the metal alloy.

9. The method of any one of claims 7 to 8, wherein the second element and the third element exhibit a miscibility gap.

10. The method according to any one of claims 7 to 9, wherein the third element is tungsten (W).

11. The method of any one of claims 7 to 9, wherein the third element is tantalum (Ta).

12. The method of any one of claims 1 to 11, wherein the melting point of the metal alloy is at least 2,500 ℃.

13. The method of any one of claims 1 to 12, wherein the neutron absorption cross-section of the metal alloy is no greater than 18 ryan.

14. The method of any one of claims 1 to 13, wherein Mo is present in the metal alloy in an amount of at least 50 at%.

15. The method of any one of claims 1 to 14, wherein the Mo and the second element exhibit miscible interstitials.

16. The method of any one of claims 1 to 15, wherein the metal alloy is nanocrystalline.

17. A method according to claim 16, wherein the average grain size of the nanocrystalline metal alloy is no greater than 300nm.

18. The method of any one of claims 1 to 17, wherein the metal alloy is a bulk metal alloy.

19. The method of any one of claims 1 to 18, wherein the metal alloy is substantially stable at a temperature of at least 2500 ℃.

20. The method of any one of claims 1 to 19, wherein the metal alloy has a first grain size and a sintered material comprising Mo but in the absence of the second element has a second grain size, the first grain size being smaller than the second grain size.

21. The method of any one of claims 1 to 20, wherein the metal alloy is enriched in the second element at grain boundaries of the metal alloy.

22. A metal alloy, comprising:

molybdenum (Mo); and

a second element;

wherein:

mo is the most abundant element in the metal alloy in atomic percent, and

the relative density of the metal alloy is at least 80%.

23. A metal alloy comprising:

molybdenum (Mo); and

chromium (Cr);

wherein Mo is the most abundant element in the metal alloy in atomic percent.

24. The metal alloy of any one of claims 22 to 23, wherein the metal alloy is sintered.

25. The metal alloy of any one of claims 22 to 24, wherein Mo is the most abundant element in the metal alloy in atomic percent.

26. The metal alloy of any one of claims 22 to 25, wherein the relative density of the metal alloy is at least 80%.

27. The metal alloy of any one of claims 22 and 24 to 26, wherein the second element is chromium.

28. The metal alloy of any one of claims 22 and 24 to 26, wherein the second element is palladium (Pd).

29. The metal alloy of any one of claims 22 to 28, further comprising a third element.

30. The metal alloy of claim 29, wherein the third element is present in the metal alloy in an amount of 0.5 atomic% to 40 atomic% of the metal alloy.

31. The metal alloy of any one of claims 29 to 30, wherein the second element and the third element exhibit a miscibility gap.

32. The metal alloy of any one of claims 29 to 31, wherein the third element is tungsten (W).

33. The metal alloy of any one of claims 29 to 31, wherein the third element is tantalum (Ta).

34. The metal alloy of any one of claims 22 to 33, wherein the melting point of the metal alloy is at least 2,500 ℃.

35. The metal alloy of any one of claims 22 to 34, wherein the neutron absorption cross-section of the metal alloy is no greater than 18 ryan.

36. The metal alloy of any one of claims 22 to 35, wherein Mo is present in the metal alloy in an amount of at least 50 at%.

37. The metal alloy of any one of claims 22 to 36, wherein the Mo and the second element exhibit miscible interstitial spaces.

38. The metal alloy of any one of claims 22 to 37, wherein the metal alloy is nanocrystalline.

39. The metal alloy of claim 38, wherein the average grain size of the nanocrystalline metal alloy is no greater than 300nm.

40. The metal alloy of any one of claims 22 to 39, wherein the metal alloy is a bulk metal alloy.

41. The metal alloy of any one of claims 22 to 40, wherein the metal alloy is substantially stable at a temperature of at least 2500 ℃.

42. The metal alloy of any one of claims 22 to 41, wherein the metal alloy has a first grain size and a sintered material comprising Mo but in the absence of the second element has a second grain size, the first grain size being smaller than the second grain size.

43. The metal alloy of any one of claims 22 to 42, wherein the metal alloy is enriched in the second element at grain boundaries of the metal alloy.