CN111676432A

CN111676432A - Treatment method for improving aging hardness of Mg-Sn alloy

Info

Publication number: CN111676432A
Application number: CN202010730353.2A
Authority: CN
Inventors: 毕广利; 徐作成; 姜静; 李元东; 曹驰; 陈体军; 马颖
Original assignee: Lanzhou University of Technology
Current assignee: Lanzhou University of Technology
Priority date: 2020-07-27
Filing date: 2020-07-27
Publication date: 2020-09-18

Abstract

The invention discloses a treatment method for improving aging hardness of Mg-Sn alloy, and relates to the technical field of alloy aging treatment, wherein the alloy is Mg-7Sn alloy, and the treatment method comprises the step of treating the alloy for 96 hours or 120 hours under the stress aging action of 40MPa or 80MPa while carrying out isothermal aging treatment at 160 ℃. According to the invention, the Mg-Sn alloy is treated by stress aging of 40MPa or 80MPa, the peak hardness of the alloy reaches 65.1 HV when the stress aging is carried out at 40MPa, the required aging time is 120h, and the deformation is only 0.96%; the peak hardness of the alloy reaches 68.4 HV when the alloy is subjected to stress aging at 80MPa, the required aging treatment time is 96h, and the deformation is only 3.35%.

Description

Treatment method for improving aging hardness of Mg-Sn alloy

Technical Field

The invention relates to the technical field of alloy aging treatment, in particular to a treatment method for improving aging hardness of Mg-Sn alloy.

Background

Magnesium and magnesium alloy are low-density structural materials, have the advantages of good machinability, high specific strength and the like, and part of magnesium alloy is applied to the fields of aerospace, automobiles, biomedicine and the like. However, the development and application of magnesium alloys are limited by their disadvantages, such as low creep properties and poor corrosion resistance. Therefore, it is very important to develop a high strength heat resistant magnesium alloy.

The Mg-Sn alloy is a low-cost age-hardenable alloy, the maximum solid solubility of Sn in a magnesium matrix at the eutectic temperature is 14.48 wt%, and Mg with the melting point of 770 ℃ is precipitated after aging₂The Sn phase can effectively pin dislocation, inhibit grain boundary diffusion and improve the room temperature and high temperature performance of the alloy. However, Mg is precipitated in Mg-Sn alloy₂The coarse, unevenly distributed and low content of Sn phases results in Mg-Sn alloys with a lower age hardening behaviour. At present, alloying and heat treatment are methods to improve the age hardening behaviour of Mg-Sn alloys. The alloy elements of Zn, Al, Cu, Na, Ag, Mn, Ca, Hf, In, Li, Sb and the like can effectively refine Mg₂The Sn phase distributes to enhance the age hardening reaction. For example: huang et al investigated the age hardening behavior of Mg-1.5Sn-0.5Mn (at.%) alloys and found that the hardness of the Zn and Ag co-addition alloys increased to 83 HV and the aging time decreased from 120h to 100 h. Studies by Li et al indicate that consistent additions of Ca and Ag significantly enhance the age hardness of Mg-7Sn (wt.%) alloys, increasing from 61.1 HV to 80.4 HV, but alloying increases the production cost of magnesium alloys.

Disclosure of Invention

In order to solve the problems, the invention provides a treatment method for improving the aging hardness of Mg-Sn alloy, which is used for treating the Mg-Sn alloy by stress aging of 40MPa or 80MPa, wherein the peak hardness of the alloy reaches 65.1 HV when the stress aging is carried out at 40MPa, the required aging time is 120h, and compared with the alloy subjected to near isothermal aging treatment, the aging treatment time is shortened from 166 h to 120h, the treatment time is shortened by 27.7%, and the deformation is only 0.96%; the peak hardness of the alloy reaches 68.4 HV when the alloy is subjected to stress aging at 80MPa, the aging treatment time is shortened from 166 h to 96h compared with the alloy subjected to isothermal aging treatment only, the treatment time is shortened by 42.2 percent, and the deformation is only 3.35 percent.

In order to achieve the purpose, one of the technical schemes adopted by the invention is as follows: a treatment method for improving the age hardness of Mg-Sn alloy, wherein the alloy is Mg-7Sn alloy, and the treatment method comprises the step of treating the alloy for 96h or 120h under the stress aging action of 40MPa or 80 MPa.

Further, the method also comprises the step of carrying out isothermal aging treatment on the alloy while carrying out stress aging treatment, wherein the temperature of the isothermal aging treatment is 160 ℃.

Furthermore, the treatment method is to treat the alloy simultaneously by stress aging of 40MPa and isothermal aging of 160 ℃, and the treatment time is 120 h.

Furthermore, the adopted treatment method is to treat the alloy simultaneously by stress aging of 80MPa and isothermal aging of 160 ℃, and the treatment time is 96 h.

Furthermore, the solution treatment temperature of the alloy is 490 ℃, the solution treatment time is 10 hours, the quenching medium is room temperature tap water, and the quenching time is less than 10 s.

The invention has the beneficial effects that:

according to the invention, the Mg-Sn alloy is treated by stress aging of 40MPa or 80MPa, the peak hardness of the alloy reaches 65.1 HV when the stress aging is carried out at 40MPa, the required aging time is 120h, compared with the isothermal aging alloy at 0MPa, the aging treatment time is shortened from 166 h to 120h, the treatment time is shortened by 27.7%, and the deformation is only 0.96%; the peak hardness of the alloy reaches 68.4 HV when the alloy is subjected to stress aging at 80MPa, and compared with the alloy subjected to isothermal aging at 0MPa, the aging treatment time is shortened from 166 h to 96h, the treatment time is shortened by 42.2%, and the deformation is only 3.35%.

The grain sizes of the T6, 4ST6 and 8ST6 state alloys are 123.6 μm, 126.2 μm and 128.6 μm respectively; a great deal of twin crystals and Mg are formed in the alloy after stress aging₂Sn phase, and twins and Mg₂The number of Sn phases increases with increasing stress; most of Mg in 8ST6 alloy₂Sn phase is on basal plane, has plate-like and rod-like shapes, has average lengths of 30nm and 50nm respectively, and has the same phase with α -Mg

And

the orientation relationship of (1). Stress aging obviously improves the mechanical property of the alloy, namely the alloy in 4ST6 state

、

And

of alloys in the 8ST6 state at 134.4 MPa, 190.4 MPa and 6.7%, respectively

、

And

163.6 MPa, 217.2 MPa and 8.5 percent respectively.

In addition to the objects, features and advantages described above, other objects, features and advantages of the present invention are also provided. The present invention will be described in further detail below with reference to the drawings.

Drawings

The accompanying drawings, which are incorporated in and constitute a part of this application, illustrate embodiments of the invention and, together with the description, serve to explain the invention and not to limit the invention.

FIG. 1 is OM and SEM microstructures of Mg-7Sn alloys in as-cast and solid solution states according to embodiments of the present invention: (a, c) as-cast; (b, d) solid solution state;

FIG. 2 is a graph of the as-cast and solid solution scanning surface profiles of Mg-7Sn alloys in accordance with an embodiment of the present invention: (a) casting state; (b) solid solution state;

FIG. 3 is a 160 ℃ stress age hardening curve for an Mg-7Sn alloy according to an embodiment of the present invention;

FIG. 4 is a 160 ℃ stress aging deformation curve of an Mg-7Sn alloy of an embodiment of the present invention;

FIG. 5 is an XRD pattern of a Mg-7Sn alloy in a 160 ℃ stress aging peak state for an example of the invention: (a) t6; (b)4ST 6; (c) 8ST 6;

FIG. 6 shows the structural change of the Mg-7Sn alloy in the peak state of stress aging at 160 ℃ in the embodiment of the invention: (a-c) T6; (d-f)4ST 6; (g-i) 8ST 6;

FIG. 7 is a graph of room temperature tensile elongation of an Mg-7Sn alloy in accordance with an embodiment of the present invention;

FIG. 8 is a TEM result of an example 8ST6 alloy of the present invention: (a, b):

；(c，d)：

；(e)：Mg₂sn phase size distribution; (f) the method comprises the following steps EDS results;

FIG. 9 is a TEM topography and SAED results along different ribbon axes for example 8ST6 alloy of the present invention: (a, d): a topography map; (b, e): SAED; (c, f): schematic of SAED results;

FIG. 10 is an HRTEM topography of example 8ST6 alloy along different ribbon axes according to the invention;

FIG. 11 is a HRTEM analysis and Fourier transform of a typical morphology phase of the ST6 alloy of example 8 of the present invention.

Detailed Description

In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

Example 1

A treatment method for improving the aging hardness of Mg-Sn alloy comprises the steps of firstly carrying out solution treatment on the Mg-7Sn alloy for 10 hours at 490 ℃, then quenching, wherein a quenching medium is room-temperature tap water, the time from sampling to quenching is less than 10s, a uniform metallographic structure can be obtained, and an Optical (OM) microstructure and a Transmission Electron Microscope (TEM) observation are carried out on the Mg-7Sn alloy before and after the solution treatment, so that the results are shown in figures 1-2, in figure 1, a and c are as-cast alloys, b and d are as-solid-solution alloys, in figure 2, a is as-cast alloy, and b is as-solid-solution alloy;

and then treating the Mg-7Sn alloy for 96 hours under the stress aging action of 80MPa, and simultaneously carrying out isothermal aging treatment, wherein the treatment temperature is 160 ℃, the stress aging treatment is carried out in a loadable stress resistance furnace, simultaneously, a digital micrometer is used for testing the deformation of the sample, each sample is tested for 5 times, the statistical average value is taken as the total deformation of the sample, and the result is shown in a figure 3-4, and the hardness of the Mg-7Sn alloy is measured to be 68.4 HV, and the deformation is measured to be 3.35%.

Example 2

A treatment method for improving the aging hardness of Mg-Sn alloy comprises the steps of firstly carrying out solid solution treatment on the Mg-7Sn alloy for 10 hours at 490 ℃, then quenching, wherein a quenching medium is room-temperature tap water, the time from sampling to quenching is less than 10s, a uniform metallographic structure can be obtained, then treating the Mg-7Sn alloy for 120 hours under the stress aging action of 40MPa, simultaneously carrying out isothermal aging treatment, the treatment temperature is 160 ℃, the stress aging treatment is carried out in a loadable stress resistance furnace, simultaneously, a digital micrometer is used for testing the deformation of a sample, each sample is tested for 5 times, the statistical average value is used as the total deformation of the sample, and the result is shown in figures 3-4, and the measured hardness of the Mg-Sn alloy is 65.1 HV, and the deformation is 0.96%.

Comparative example 1

A treatment method for improving the aging hardness of Mg-Sn alloy comprises the steps of firstly carrying out solution treatment on the Mg-7Sn alloy at 490 ℃ for 10 hours, then quenching, wherein the quenching medium is room-temperature tap water, and the time from sampling to quenching is less than 10 seconds, so that the T4 alloy is obtained.

Comparative example 2

The treatment method comprises the steps of firstly carrying out solid solution treatment on the Mg-7Sn alloy for 10 hours at 490 ℃, then quenching, wherein a quenching medium is room-temperature tap water, the time from sampling to quenching is less than 10 seconds, a uniform metallographic structure can be obtained, and then carrying out isothermal aging treatment on the Mg-7Sn alloy for 96 hours at 160 ℃ to obtain the T6 alloy.

Examples of the experiments

FIG. 1 is an optical and scanning microstructure of the Mg-7Sn alloy treated in example 1 and comparative example 1 after 490 ℃ solution treatment, the results show that the microstructure of the as-cast Mg-7Sn alloy consists of α -Mg matrix and (α -Mg + Mg)₂Sn) eutectic structure and Mg₂Sn phase composition, after solution treatment (α -Mg + Mg)₂Sn) eutectic structure and Mg₂The Sn phase dissolves at high temperature, resulting in transformation of the alloy into a single phase α -Mg solid solution structure with an average grain size of 125.4 μm the purpose of the solution treatment is to homogenize the alloying elements to ensure the formation of a homogeneous supersaturated solid solution FIG. 2 is a corresponding EDS surface profile, where a is a scan surface profile of the as-cast alloy and b is a scan surface profile of the as-dissolved alloy, showing that the alloying elements are uniformly distributed to form a supersaturated solid solution after 10h of solution treatment of Mg-7Sn at 490 ℃.

FIG. 3 shows the age hardening curves of the alloys treated in examples 1 and 2 and comparative example 2 under different conditions of 0MPa, 40MPa and 80MPa of compressive stress at 160 ℃, wherein the hardness of the 166 h alloy reaches the maximum value of 64.6 HV and is increased by 37.4% in the 0MPa isothermal aging process; the peak hardness of the alloy reaches 65.1 HV at the stress aging of 40MPa, and the required aging time is 120 h; the alloy reaches 68.4 HV after aging for 96 hours at 80MPa stress aging. The comparison shows that the time of the alloy reaching the aging peak value is shortened by the compressive stress, and the aging peak value time of 40MPa stress aging is shortened from 166 h to 120h and is shortened by 27.7 percent compared with that of 0MPa isothermal aging alloy; compared with the aging peak time of the 0MPa isothermal aging alloy, the aging peak time of the 80MPa stress aging alloy is shortened from 166 h to 96h, and is shortened by 42.2%.

FIG. 5 shows that the phase compositions of the alloys after stress ageing at 40MPa and 80MPa are still C-Mg2Sn phase and alpha-Mg phase, indicating that the compressive stress has no influence on the structure of the precipitated phases during ageing.

Examples 1-2 and comparative example 2 were carried outOptical and scanning microstructural characterisation, the results are shown in fig. 6, where fig. 6a and 6b show that the average grain size of the isothermally aged peak (T6) alloy of comparative example 2 is 123.6 μm without significant micron-sized second phases forming; FIG. 6c is a high power SEM image and partial magnified view of T6 alloy, showing nano-sized Mg₂Sn phase precipitation, precipitated Mg₂The Sn phase has two types of continuous precipitation and discontinuous precipitation, and the discontinuous precipitation phase is densely distributed along the grain boundary, mainly because the grain boundary has higher energy than the inside of the crystal grain and can be used as the nucleation center of the precipitation phase, and the nucleation work required by the precipitation phase at the grain boundary is low, so the precipitation phase preferentially nucleates on the grain boundary, namely, the discontinuous nucleation. The smaller the size of the discontinuous precipitated phase on the grain boundary is, the more uniform the distribution is, and the more advantageous the improvement of the mechanical property is. The continuous precipitated phase is formed inside the crystal grains, and atoms are continuously aggregated and nucleated by diffusion to form the precipitated phase, and the formation of the precipitated phase requires high nucleation work, namely continuous nucleation. FIG. 6d shows that the average grain size of the 40MPa stress-aged peak (4 ST 6) alloy is 126.2 μm with a small amount of twinning formed, which effectively refines the original grains; FIG. 6e is an SEM of the 4ST6 alloy, and analysis reveals no significant micron-scale second phase formation, with the clear view of twins, which are long laths in morphology; FIG. 6f is a high power SEM and corresponding magnified view showing the presence of large and densely distributed Mg₂Compared with the T6 alloy, the volume fraction of the precipitated Mg2Sn phase is obviously improved, and the distribution is more uniform. FIG. 6g shows that the average grain size of the 80MPa stress aged peak (8 ST 6) alloy is 128.6 μm, with a significant amount of twinning observed; compared with the T6 alloy and the 4ST6 alloy, the number of twin crystals is obviously increased, the original crystal grains are favorably refined through the formation of a large number of twin crystals, and the crystal grains are refined under the interaction of twin crystal boundaries in the aging process; FIG. 6h shows the twin morphology is still long lath-like; i shows a high power SEM and corresponding magnification of the 8ST6 alloy, clearly showing dense uniform Mg₂Precipitating a Sn phase; similar to the T6 alloy, there are grain boundary precipitated phase and grain interior precipitated phase, but it is obvious that the 80MPa compressive stress significantly refines the grain boundary precipitated phase so that the distribution of the precipitated phase on the grain boundary is discontinuous, but the number of the precipitated phase is not constantStill more than the T6 alloy; compared with 4ST6 alloy, the precipitated phase is obviously refined after 80MPa stress aging; the fact that the number of the precipitated phases is increased and the size is obviously reduced along with the increase of the compressive stress is related to the nucleation and growth process of the precipitated phases, the deformation amount of the alloy is increased along with the increase of the compressive stress, namely the increase of dislocation and twin crystal boundary in the aging process is meant, and the defects can provide nucleation centers for the precipitated phases like the crystal boundary, so that the nucleation rate of the precipitated phases is improved, and the precipitated phases are refined. The alloy is continuously deformed during the stress aging process to accumulate strain, the deformation amounts of the 4ST6 and 8ST6 alloys are respectively accumulated to reach 0.96 percent and 3.35 percent, and the obvious twin crystal of the microstructure is observed. Research shows that the existence of dislocation and twin boundary is favorable for the nucleation of precipitated phase, so that the quantity of Mg in the alloy is increased due to the increase of the quantity of the dislocation and the twin boundary along with the increase of the compressive stress₂The nucleation rate of the Sn phase is also improved, so that the Mg2Sn phase is refined, and the uniform distribution of the Mg2Sn phase is promoted.

The alloys treated in examples 1-2 and comparative examples 1-2 were subjected to room temperature tensile tests, the tensile curves are shown in FIG. 7, and the detailed mechanical property parameters are shown in Table 1. the results show that the stress aging treatment significantly enhances the mechanical properties of the alloys, and the strengthening effect on the alloys is more significant with the increase of compressive stress.in comparative example 1, the alloys subjected to solution treatment have the maximum elongation of 32.4%, but the yield strength and tensile strength are lower, respectively 53.5 MPa and 168.9 MPa, coarse eutectic structures after solution treatment are completely dissolved in α -Mg matrix, the alloy elements are uniformly distributed, the grain boundaries are clear, and therefore the plasticity of the alloys in a solid solution state is excellent, in comparative example 2, the alloys subjected to isothermal aging at 160 ℃ are strengthened, the yield strength and tensile strength are respectively increased to 104.6 MPa and 178.9 MPa, 95.5% and 5.9% respectively higher than those of the alloys subjected to solution treatment, but the yield strength and tensile strength are greatly improved by the plasticity, and the plasticity is reduced to 10.9%, and the main reasons for strengthening Mg and Mg are improved₂The Sn phase is precipitated. In example 2, the tensile strength of the yield strength of the alloy after stress aging at 40MPa is increased to 134.4 MPa and 190.4 MPa respectively, and the plasticity is reduced by 6.7 percent; the comparison shows that the introduction of 40MPa compressive stress is compared with the introduction of the stress-free isothermal agingThe alloy can be effectively reinforced, and the yield strength and the tensile strength are respectively improved by 28.5 percent and 6.4 percent; this difference in mechanical properties is mainly due to the change in the precipitation behavior of the precipitated phase caused by the introduction of the compressive stress.

In example 1, the yield strength and tensile strength of the alloy after 80MPa stress aging are respectively improved to 163.6 MPa and 217.2 MPa, the mechanical properties of the alloy are respectively improved by 21.7 percent and 14.1 percent compared with 40MPa stress aging, the elongation of the alloy after 80MPa stress aging is 8.5 percent, and the plasticity of the alloy is improved compared with 40MPa stress aging; more defects such as dislocation, twin crystal and the like are accumulated in the alloy along with the increase of the compressive stress, and the crystal defects as heterogeneous nucleation nodes of the precipitated phase influence the size, the morphology, the quantity, the crystallographic orientation relation and the like of the precipitated phase, so that the mechanical property of the alloy is greatly influenced.

TABLE 1 statistical table of mechanical properties of T7 alloy at 160 ℃ compressive stress aging peak state

FIG. 8 shows a bright field TEM image, corresponding EDS spot analysis, and a homogeneous size distribution plot of the 8ST6 alloy. A large amount of uniformly distributed Mg can be seen₂Sn phases are formed, the size of these phases being mainly concentrated at 30nm and 50nm, with the average size of the platy phases being 30nm and the average size of the rod-like phases being 50 nm. FIG. 9 shows Mg in more detail₂Morphology and electron diffraction pattern of the Sn phase. FIGS. 9 a and 9 b are electron beam edges [0001 ]]_αAxial imaged precipitated phase topography and corresponding electron diffraction patterns, FIGS. 9 d and 9 e are electron beam profiles

The morphology of the precipitated phase of the axial imaging and the corresponding electron diffraction pattern can be seen to form a large number of precipitated phases with the size less than 100 nm, and the morphology of the precipitated phase mainly has a rod shape and a plate shape and is respectively indicated by a black arrow and a yellow arrow; the length of the rod-like precipitated phase is about 50nm, the thickness thereof is about 20nm, and the growth direction thereof is

(ii) a The diameter of the plate-like precipitated phase is about 10nm to 30nm, and the long axis direction of the precipitated phase

The directions are parallel; the majority of precipitated phases being located in

Basal planes, there is also a significant presence of non-basal precipitates, which as indicated by the white arrows in the figure are non-basal precipitates which are rod-like in morphology and which grow in a direction perpendicular to the direction of growth of the basal precipitates and have a dimension of about 30nm₂FIG. 9 c and FIG. 9 f are schematic diagrams of electron diffraction patterns, and it can be seen that the orientation relationship of the precipitated phase and the α -Mg matrix is:

and

。

FIG. 10 is a HRTEM photograph of the alloy at the stress aged peak at 80MPa along different crystallographic axes. FIGS. 10a, b and c, d are the axes of the ribbons

Direction and

direction, showing a large amount of Mg₂Sn phase formation of these Mg₂The Sn phase is uniformly distributed and dispersed, most of precipitated phases of the alloy are positioned on a base surface under different Mg matrix crystal band axes, the appearance is platy and rodlike, and the growth direction of most of the precipitated phases is

The size is 10 nm-40 nm, the morphology of a typical precipitated phase is marked by a white line, and high-power HRTEM photos clearly show lattice fringes of plate-shaped and rod-shaped precipitated phases. Fig. 11 more clearly analyzes the lattice parameters and size dimensions of typical precipitated phases, and the corresponding fourier transforms. FIG. 11 a shows a rod-like Mg₂The microstructure characteristics of Sn phase, the results show that rod-shaped Mg₂Long and short axes of Sn phase are 20nm and 15nm, respectively, rod-like Mg₂The Sn phase has a coherent relationship with the interface of the α -Mg matrix, FIG. 11b shows the platy Mg₂The lattice spacing of the Sn phase is about 0.32 nm. FIG. 11 d shows platy Mg₂The microstructure of Sn phase indicates rod-like Mg₂The length of the Sn phase is about 30 nm; FIG. 11e shows a rod-like Mg₂The lattice spacing of the Sn phase was 0.27 nm.

The above description is only for the purpose of illustrating the preferred embodiments of the present invention and is not to be construed as limiting the invention, and any modifications, equivalents, improvements and the like that fall within the spirit and principle of the present invention are intended to be included therein.

Claims

1. A treatment method for improving the aging hardness of Mg-Sn alloy is characterized in that the alloy is Mg-7Sn alloy, and the treatment method is that the alloy is treated for 96h or 120h under the stress aging action of 40MPa or 80 MPa.

2. The process for increasing the age hardness of a Mg — Sn alloy according to claim 1, further comprising subjecting the alloy to isothermal aging at 160 ℃ simultaneously with the stress aging.

3. The treatment method for increasing the age hardness of Mg-Sn alloys according to claim 2, characterized in that the treatment method is to treat the alloys simultaneously with stress aging of 40MPa and isothermal aging at 160 ℃ for 120 h.

4. The treatment method for increasing the age hardness of Mg-Sn alloys according to claim 2, characterized in that the treatment method is that the alloy is treated simultaneously by stress aging at 80MPa and isothermal aging at 160 ℃ for 96 h.

5. The treatment method for improving the age hardness of Mg-Sn alloy according to any of claims 1 to 4, wherein the solution treatment temperature of the alloy is 490 ℃, the solution treatment time is 10h, the quenching medium is room temperature tap water, and the quenching time is less than 10 s.