WO2015111035A1

WO2015111035A1 - High performance creep resistant magnesium alloys

Info

Publication number: WO2015111035A1
Application number: PCT/IL2014/050649
Authority: WO
Inventors: Boris Bronfin; Nir Nagar; Vladimir KOTLOVSKY; Nir Moscovitch
Original assignee: Dead Sea Magnesium Ltd.
Priority date: 2014-01-23
Filing date: 2014-07-17
Publication date: 2015-07-30
Also published as: EP3097217B1; EP3097217A4; JP6590814B2; CN105934529A; IL230631A; IL230631A0; CN105934529B; EP3097217A1; US20160304996A1; JP2017507245A

Abstract

A magnesium based alloy consisting of, by weight: at least 94.8% magnesium, 2.5-4.6% neodymium, 0.05-0.40%) yttrium, and 0.03-0.65%) zirconium and incidental impurities. Optionally, the alloy further contains up to 0.02%>wt. calcium. The alloy is suitable for high pressure die casting (HPDC) as well as for sand casting, investment casting, permanent mold casting, twin roll casting and direct chill casting. The disclosed alloy exhibits good castability, high strength, high corrosion resistance and high creep resistance at room temperature, as well as in high temperatures.

Description

HIGH PERFORMANCE CREEP RESISTANT

MAGNESIUM ALLOYS

Field of the Invention

The present invention relates to creep -resistant magnesium-based alloys for applications at high temperatures which exhibit good castability, particularly suitable for high-pressure die casting, but advantageously used also in a processes comprising sand casting, investment casting, permanent mold casting, as well as direct chill casting or twin-roll casting. Background of the Invention

The magnesium industry is experiencing dramatic growth, in part due to the demands of the transportation industry to improve fuel economy and emissions. In addition, a great progress in weight reduction has been made in consumer applications of magnesium alloys such as power hand tools, lawn and garden equipment, electronic and optical equipment, etc. In order to significantly expand the above applications, new advanced alloys are required.

High pressure die casting (HPDC) is the dominant form of casting due to its productivity and suitability for mass production. Currently, most common and new magnesium alloys that are being used for HPDC process are Al containing alloys. However, these alloys cannot serve at temperatures higher than 150-170°C under high stresses of 60-100 MPa. US 6, 193,817 describes magnesium -based alloys containing 0.1-2.0 wt% Zn, 2.1-5.0 wt% RE elements (Ce based mishmetal) up to 0.4 wt% of a combination of at least two elements chosen from the group consisting of Zr, Hf and Ti, and optionally up to 0.5 wt% Mn and up to 0.5 wt% Ca. High -pressure die casting of the alloys results in low strength (TYS=120 MPa, UTS=165 MPa) and elongation (E= 2%). EP 1866452 discloses magnesium based alloys containing 1.5-4.0% RE elements, 0.3-0.8% Zn, 0.02-0.1% Al, 4-25 ppm Be and optionally up to 0.2% Zr, 0.3% Mn, 0.5%Y, and 0.1% Ca. The alloys, under die cast conditions, exhibit low tensile strength (TYS=130 MPa, UTS=160 MPa) and elongation (E= 1-3%).

WO 2009/086585 relates to magnesium based alloys containing 2-5% RE elements (primarily La and Ce, wherein La content is higher than Ce content) and 0.2-0.8% Zn. In addition, the alloys contain optionally Y, Gd, Zr, Mn, Ca, and Be. These alloys are also designated for high-pressure die casting but exhibit very low values of elongation, TYS, and UTS.

SU 1,360,223 discloses Mg-based alloys containing 0.1-2.5% Zn, 0.3- 1.0% Zr, 0.8-4.5% Nd, 0.5-5.0% Y, 0.8-4.5% Gd, and 0.01-0.05% Mn. The alloys are intended for sand casting process and exhibit optimal properties after full T6 treatment.

US 4, 116,731 describes heat treated and aged magnesium based alloys containing 0.8-6.0 wt% Y, 0.5-4.0 wt% Nd, 0.1-2.2 wt% Zn, 0.3- 1.1 wt% Zr, up to 0.05% Cu, and up to 0.2% Mn. Due to relatively wide concentration ranges claimed by the above patent, the alloys exhibit very diverse properties; in addition, they are designated only for sand casting process

EP 1329530 discloses magnesium-based casting alloys containing 0.2-0.8 wt% Zn, 0.2-0.8 wt% Zr, 2.7-3.3 wt% Nd, 0.0-2.6 wt% Y, and 0.03-0.25% Ca. The alloys exhibit high strength and high creep resistance after gravity casting and after full T6 heat treatment, as well as after extrusion and forging. However, the HPDC is not addressed. CN 1752251 describes magnesium alloys containing 0.35-0.8 wt% Zr, 2.5-3.6 wt% Nd, 0.0-0.4 wt% Zn, 0.0-0.5 wt% Ca, and 0.0-0.02 wt% impurities. The alloys are prepared by a two-stage process including a step of preparing intermediate master alloys Mg-Nd, Mg-Ca, and Mg-Zr, and a step of alloying said master alloys by Nd, Ca, and Zr. The complexity of the technology significantly increases the cost of the final alloy product. EP 1641954 describes creep resistant alloys containing 2.0-4.5% Nd, 0.2-1.0% Zr, 0.2%-7.0% HRE (Heavy Rare Earth Elements of atomic numbers 62-71), optionally up to 0.4% of other RE elements, up to 0.5% Y, up to 1.3% Zn, up to 0.5% Mn, and up to 0.4% Hf or Ti. The alloys are mainly designated for sand casting and, in addition, they are expensive due to the use of heavy rare earth elements, such as Gd in amounts of 1.0-1.6%.

US 2009/0081313 relates to biodegradable magnesium alloys containing 1.5- 5.0% Nd, 0.1-4.0% Y, 0.1-2.0% Ca, and 0.1-1.0 % Zr. The alloys are designated for manufacturing medical implants by extrusion. The high Ca content results in increased porosity, embrittlement and hot cracking in HPDC processes.

WO 2010/038016 relates to magnesium alloys containing 2.0-4.0% Y, 0.5- 4.0% Nd, 0.05-1.0% Zr, 0.0-5.5% Gd, 0.0-5.5% Dy, 0-5.5% Er, 0.0-0.2% Yb, and 0.0-0.04% Sm. In addition, the total content of Gd, Dy and Er is in the range of 0.3-12 wt.%. The alloy is dedicated for sand casting, and it can also be used as a wrought alloy. The alloy is unsuitable for HPDC process. Furthermore, the high content of heavy rare earth elements leads to high cost of these alloys.

WO 2011/117628 describes magnesium alloys containing 0.0- 10.0% Y, 0.0- 5.0% Nd, 0.00-1.2% Zr, 0.0-0.3% Gd, and 0.0-0.2% Sm, wherein the total content of Ho, Lu, Tm and Tb is in the range of 0.5-5.5 %. The alloy is dedicated for manufacturing medical implants. Due to very wide concentration ranges of Y, Nd, and heavy rare earth elements Ho, Lu and Tm, the alloys exhibit very diverse properties. The alloys are not suitable for HPDC process and are expensive. It is therefore an object of this invention to provide magnesium alloys suitable for high pressure die casting (HPDC) applications.

It a further object of this invention to provide magnesium-based alloys allowing crack-free castings at HPDC applications.

It is also an object of this invention to provide magnesium-based alloys having a superior combination of strength and ductility, as well as capability to operate at a temperature of 200°C for a long time.

It is another object of the present invention to provide alloys which are also suitable for sand casting, investment casting, and permanent mold casting, and which exhibit excellent combination of castability, creep performance, and corrosion resistance.

It is a still further object of this invention to provide alloys which are also suitable for direct chill casting and twin roll casting with subsequent plastic forming operations such as rolling, forging and extrusion. It is still another object of this invention to provide alloys which exhibit the aforesaid behavior and properties, and have an affordable cost.

Other objects and advantages of the present invention will appear as the description proceeds.

Summary of the Invention

The invention provides a lightweight alloy for high-pressure die casting (HPDC) process, consisting of at least 94.8 wt% magnesium, 2.5 to 4.6 wt% neodymium, 0.05 to 0.40 wt% yttrium, 0.03 to 0.65 wt.% zirconium, and incidental impurities. In one embodiment, the alloy according to the invention further contains up to 0.02 wt% calcium. The alloy according to the invention comprises essentially no heavy rare earth (HRE) elements with the atomic number from 61 to 70. The alloy according to the invention comprises essentially no cerium, lanthanum, and praseodymium. The alloy according to the invention comprising essentially no zinc. In one embodiment, the alloy according to the invention contains Nd and Y in an amount higher than 4.3 wt%. Said incidental impurities usually comprise Si, Fe, Cu, and Ni in an amount of up to 0.02 wt%. The lightweight alloy according to the invention is suitable for prolonged operations at high temperatures of up to 200°C. The alloys for HPDC and other applications according to the invention exhibit superior casting properties, high strength, high creep resistance, high corrosion resistance, and the articles manufactured from the alloys show superior performance at high temperatures. The alloy according to the invention is usable for high pressure dies casting (HPDC), but it may be also used for a process selected from the group consisting of sand casting, investment casting, and permanent mold casting. The alloy according to the invention are also usable for a process comprising either twin roll casting with subsequent rolling, or direct chill casting with subsequent forging, extrusion or rolling. In a preferred embodiment of the invention, the lightweight alloy is advantageously used for HPDC. In one embodiment, the alloy suitable for HPDC contains 2.8 to 4.3 wt% Nd, 0.06 to 0.25 wt% Y, 0.05 to 0.4 wt% Zr, and 0.0 to 0.02 wt% Ca. In a preferred embodiment of the invention, the alloy used for HPDC exhibits a tensile yield strength (TYS) at 200°C of at least 153 MPa, a compression yield strength (CYS) at 200°C of at least 152 MPa, a minimum creep rate of not more than 1.5x10 ¹⁰/s at 200°C under stress of 100 MPa, and a corrosion rate of not more than 2.65 mpy. When measured in relative units characterizing oxidation resistance, fluidity, and dies sticking, the alloy according to the invention preferably exhibits a castability of at least 96%. The invention relates to an article cast of the alloy containing 2.8 to 4.3 wt% Nd, 0.06 to 0.25 wt% Y, 0.05 to 0.4 wt% Zr, and 0.0 to 0.02 wt% Ca, the article exhibiting a superior combination of strength and ductility after T5 treatment which includes direct aging at 150"250°C for 1- 10 h. In one embodiment, said article exhibits a superior combination of strength and ductility after T5 treatment which includes direct aging at 175"225°C for 1-6 h.

The alloy according to the invention is also suitable for sand casting, investment casting, permanent mold casting, and low pressure modifications thereof; in one embodiment, the alloy contains 2.7 to 3.4 wt% Nd, 0.15 to 0.40 wt% Y, 0.3 to 0.6 wt% Zr, and 0.0 to 0.02 wt% Ca. The invention relates to an article cast of said alloy, the article exhibiting a superior combination of performance properties after full T6 heat treatment comprising solid solution heat treatment at 520-560°C for 1 to 16 hrs, followed by cooling in a quenching medium and by subsequent aging at 200"270°C for 1 to 16 h. In one embodiment, said article exhibits a superior combination of performance properties after full T6 heat treatment comprising solid solution heat treatment at 535-545°C for 3 to 5 hrs, followed by cooling in a quenching medium and by subsequent aging at 225"250°C for 3 to 6 h.

The alloy according to the invention may be advantageously used for forging, extrusion, and rolling; in one embodiment, the alloy contains 2.8 to 3.8 wt% Nd, 0.20 to 0.40 wt% Y, 0.35 to 0.60 wt% Zr, and 0.0-0.02 wt% Ca. The invention relates to an article cast in said alloy, the article exhibiting a superior combination of performance properties after T5 heat treatment comprising aging at 200"250°C for 1 to 16 h.

The present invention provides creep -resistant magnesium-based alloys designated for applications at temperatures as high as 200-250°C, which exhibit good castability and low susceptibility to hot tearing, which are strong and are corrosion resistant, and have excellent ductility. The invention provides a process for manufacturing a lightweight alloy for prolonged operation at high temperatures of up to 200°C, comprising steps of i) alloying magnesium with neodymium and zirconium at 765-785°C under intensive stirring; ii) settling the melt for 20-40 minutes to allow iron to settle! iii) adding yttrium, while avoiding intensive stirring to prevent the formation of Y-Fe intermetallic compounds; iv) optionally adding calcium prior to settling; v) settling the molten alloy for 30-60 minutes; and v) casting into desired form; wherein the steps are performed under a protective atmosphere of CO2 + 0.5% HFC 134a till solidification; the amount of magnesium in the alloy being at least 94.8 wt%, of neodymium from 2.5 to 4.6 wt%, of yttrium from 0.05 to 0.40 wt%, of zirconium from 0.03 to 0.65 wt.%, and of calcium 0.00 to 0.02%. The lightweight alloys thus manufactured are particularly suitable for high pressure die casting, but can be advantageously employed in sand casting, investment casting, and permanent mold casting.

The alloys according to the invention contain more than 94 wt% magnesium, from 2.5 to 4.6 wt% neodymium, from 0.05 to 0.40 wt% yttrium, from 0.03 to 0.65 wt% zirconium, optionally calcium up to 0.02 wt%, and incidental impurities. The alloys usually contain up to 0.007 wt% iron, up to 0.001 wt% nickel, up to 0.003 wt% copper, up to 0.015 wt% silicon, and eventually other incidental impurities. The alloys of the invention exhibit an excellent combination of high tensile and compressive yield strength, and high ductility. The great advantage of new alloys is related to their high creep rupture stress, creep strength and low minimum creep rate, as well as low corrosion rate measured under GM 9540 cyclic corrosion test. Thus, the alloys of the present invention combine superior performance properties, good castability, and relatively moderate cost. Articles according to the invention are preferably subjected to T5 or T6 heat treatments depending on preceding casting process and plastic forming operations. Brief Description of the Drawings

The above and other characteristics and advantages of the invention will be more readily apparent through the following examples, and with reference to the appended drawings, wherein:

Fig. 1. is Table 1, showing chemical compositions of alloys for HPDC according to the invention (Examples 1-7) and comparative alloys

(Comparative Examples 1-7);

Fig. 2. is Table 2, showing the results of die castability evaluation for alloys of Table l;

Fig. 3. is Table 3, showing mechanical and corrosion properties of the alloys of Table l;

Fig. 4. is Table 4, showing creep properties of the alloys of Table l;

Fig. 5. is Table 5, showing chemical compositions of alloys for sand casting according to the invention (Examples 8- 14) and comparative alloys (Comparative Examples 8-14);

Fig. 6. is Table 6, showing mechanical and corrosion properties of the alloys of Table 5;

Fig. 7. is Table 7, showing chemical compositions of alloys according to the invention after forging (Examples 15-18) and comparative alloys (Comparative Examples 15-18);

Fig. 8. is Table 8, showing mechanical properties of the alloys of Table 7; and Fig. 9. is a scheme showing GM 9540 cycling test procedure for the corrosion evaluation. Detailed Description of the Invention

It was found that certain combinations of elements in magnesium based alloys comprising neodymium, zirconium, yttrium, and optionally calcium, impart to the alloys superior properties, particularly for high pressure die casting. These properties include excellent combination of high tensile and compressive properties with high ductility, outstanding corrosion resistance, and creep properties allowing to achieve service temperatures up to 250°C. The above combination of properties can be realized at high-pressure die casting, at sand casting, as well as at direct-chill casting or twin roll casting followed by plastic forming processes such as forging, extrusion and rolling. Magnesium-based alloys of the instant invention contain 2.5 to 4.6 wt% neodymium. It was found by the inventors that if the Nd content is less than 2.5 wt%, the alloys have insufficient strength at ambient and elevated, and their creep resistance is not sufficient for serving at 250-300°C temperatures! Nd content higher than 4.6 wt% results in low ductility due to excessive amount of intermetallic compounds which are sources of cracks initiation and propagation. An alloy according to the present invention contains 0.05 to 0.40 wt % yttrium. It was found that yttrium content less than 0.05 wt% makes the alloy prone to oxidation and results in increased susceptibility to burning during molten metal handling at 700-780°C. On the other hand, increasing the yttrium content to more than 0.40 wt% leads to lower ductility, significantly deteriorated castability, while increasing the alloy cost. Zirconium is mainly used to remove iron in the case of high pressure die casting process. In the case of gravity casting (sand casting, investment casting, and permanent mold casting) it also serves as a grain refiner. It has been found that 0.03 wt.% of Zr is sufficient to ensure low iron content in the alloy, while at least 0.3 wt% of zirconium is required for grain refining. The upper limit for the zirconium content is about 0.65 wt% due to its limited solubility in Mg-Nd-Y alloys. The alloys of the present invention contain substantially no zinc! it would deteriorate creep resistance and corrosion performance due to the formation of Zn-Y-Nd-Zr coarse intermetallics. Furthermore, the alloys of the instant invention do not contain rare earth elements with low solubility in solid magnesium such as Ce and La. The presence of those elements results in deterioration of mechanical properties, and particularly of ductility, due to the formation of coarse intermetallic compounds. An admixture of calcium in the alloys of the invention of up to 0.02% may improve oxidation resistance. The Ca content is limited to 0.02% because higher Ca content leads to increased micro-porosity and embrittlement of the alloys.

The alloys of instant invention also do not contain heavy rare earth elements with atomic number higher than 60, they would increase the alloy price without remarkably improving the alloy performance.

Surprisingly simple alloys of the invention are suitable for HPDC and other applications, while exhibiting superior casting properties, high strength, high creep resistance, high corrosion resistance, and the articles manufactured from the alloys show superior performance at high temperatures.

The magnesium alloys according to the invention were examined along with comparative alloys. The results show that the new alloys exhibit better oxidation resistance and fluidity, as well as lower susceptibility to die sticking than comparative alloys. Neither burning nor oxidation was observed on the surface of ingots made of alloys according to this invention. In contrast, the preparation of comparative alloys was sometimes accompanied by significant oxidation and undesirable losses of alloying elements. The alloys according to the invention reached between 96 and 100% on the relative castability scale, when evaluating oxidation resistance, fluidity, and die sticking of the alloy on casting (see Examples below), in comparison with 73 and 83% of comparative alloys whose composition differed more or less from the composition of the invention.

Part of the ingots of both the new and the comparative alloys were then re- melted and high pressure die cast to produce different specimens for testing and examination. Other ingots were remelted, grain refined and sand cast into different specimens for testing. Tensile Yield Strength (TYS), Ultimate Tensile Strength (UTS), percent elongation (%E), compressive strength (CYS) and different creep characteristics such as Creep Strength, Creep Rupture Strength, and Minimum Creep Rate were then determined. Corrosion behavior was evaluated by the GM 9540 cyclic test. The alloys according to the invention surpassed the comparative alloys in creep stress to rupture, creep strength, and corrosion resistance. They also exhibit better combination of strength and ductility, characterized by elongation values, than comparative alloys.

The alloys according to the invention are very suitable for HPDC; it was found that they develop excellent properties after direct T5 aging at 150- 250°C for 1- 10 h, preferably at 175-225°C for 1-6 h. As for wrought alloys, it was found that the alloys according to the invention achieve very good properties after direct aging at 200-250°C for 1- 16 h. It was found that the alloys according to the invention also provide excellent mechanical properties on sand casting after full T6 heat treatment; particularly, good results were obtained when the heat treatment comprised solid solution heat treatment at 520"560°C for 1 to 16 hrs, followed by cooling in a quenching media and by subsequent aging at 200-270°C for 1 to 16 h, preferably after solid solution treatment at 535-545°C for 3 to 5 hrs, followed by cooling in a quenching media and by subsequent aging at 225-250°C for 3 to 6 h. The invention will be further described and illustrated in the following examples.

Examples

The alloys of the present invention were prepared in 150 1 crucible made of low carbon steel. The mixture of CO2 + 0.5% HFC 134a was used as a protective atmosphere. The raw materials used were as follows:

Magnesium (Mg) - pure magnesium, grade 9980A, containing at least 99.8% Mg.

Neodymium (Nd) - commercially pure Nd (less than 0.5% impurities).

Zirconium (Zr) - Zr95 Tablets, containing at least 95% Zr.

Yttrium (Y) - commercially pure Y (less than 1% impurities). Calcium (Ca) - pure Ca ( less than 0.1% impurities).

Contrary to the alloying procedure described in CN1752251, where intermediate Mg-Nd , Mg-Ca and Mg- Zr master alloys were used, the alloys of the present invention have been prepared using pure Nd and pure Ca that significantly simplifies the process, reduces its duration and markedly lowers the alloy cost. Neodymium and zirconium were added typically at 770-780°C with intensive stirring of the melt. After addition of zirconium, the melt was held for 20-40 minutes to allow iron to settle. Yttrium was added after the iron settling, without intensive stirring, to prevent the formation of Y-Fe intermetallic compounds, which leads to excessive loss of yttrium. A strict temperature control was provided during the alloying in order to insure that the melt temperature will not increase above 785°C, thus preventing an excessive contamination by iron from the crucible walls, and to ensure that the temperature will not decrease below 765°C, thus preventing an excessive loss of zirconium. Calcium was added prior to settling. After obtaining the required compositions, the alloys were held for 30-60 minutes for homogenization, and settling of iron and non-metallic inclusions, and then they were cast into the 15 kg ingots. The casting was performed with gas protection of the molten metal during solidification in the molds by CO2 + 0.5% HFC 134a mixture. The die casting trials were carried out using an IDRA OL-32O cold chamber die casting machine with a 345 ton locking force.

The castability was evaluated based on observed fluidity, oxidation resistance and die sticking or soldering. The casting temperature was 710°C. Each of the properties (fluidity, oxidation resistance, die sticking) was evaluated by assigning from 0 to 10 points on a relative scale, the higher the better (see Table 2). The sum of the points for an alloy divided by 30 and multiplied by 100 provides "castability coefficient", a relative assessment having a value between 0 and 100%, which characterizes the overall suitability of an alloy for die casting. The alloys according to the invention had castability coefficient between 96 and 100%, while comparative examples, even if differing only slightly from the new alloys of the invention, had castability coefficient between 73 and 83%.

Tensile and compression testing at ambient and elevated temperatures were performed using an Instron 4483 machine. Tensile Yield Strength (TYS), Ultimate Tensile Strength (UTS), percent elongation (%E) and Compression Yield Strength (CYS) were determined. The SATEC Model M-3 machine was used for creep testing. Creep tests were performed at 200°C and 250°C for 200h or until rupture under various stresses. Creep resistance was estimated by measuring rupture strength and creep strength. Creep strength is usually defined as the stress, which is required to produce a certain amount of creep at a specific time and temperature. It is a common practice to report creep strength as the stress, which produces 0.2% creep strain at a given temperature for 100 hours. This parameter is used by design engineers for evaluating the load-carrying ability of a material for limited creep strain in prolonged time periods. Creep rupture stress is the stress resulting in specimens rupture at a selected testing temperature for a certain time, usually 100 h. In addition, minimum creep rate at a steady state (MCR) was used to evaluate creep performance.

Corrosion behavior was evaluated as per GM9540 cyclic test for 40 days (Fig.9). The test procedure includes three main stages, combining both wet- dry transition and short sprays of light electrolyte solution. In this test a gradual increase of temperature is applied during the cycle. The die cast plates with dimensions of 140x100x3 mm were used. The plates were degreased in acetone and weighed prior to the test. Five replicates of each alloy were tested. At the end of the test the corrosion products were stripped in a chromic acid solution (180 g CrOs per liter solution) at 80°C about three minutes and the weight loss was determined. The weight loss was used to determine the average corrosion rate in mpy (milli-inch per year). Tables 1 to 4 illustrate chemical compositions, castability parameters, and properties of alloys for HPDC according to the invention and of comparative alloys. The new alloys of the invention demonstrate significantly better die castability evaluated by tendency to oxidation, fluidity and susceptibility to die sticking (Table 2), reflected by a castability coefficient of minimally 96%. As can be seen from Table 3, the new alloys are superior in tensile yield strength (TYS) and compressive yield strength(CYS) over the comparative alloys at both ambient and elevated temperatures. The same is true for UTS values. For example, TYS values of the new alloys according to the invention at 200°C are 150 MPa or more, usually 153 MPa or more, whereas the comparative alloys have lower values. Furthermore, new alloys exhibit much better combination of strength and elongation than comparative alloys. Corrosion resistance of the new alloys determined under GM 9540 cyclic test conditions (Fig. 9) also surpasses that property of the comparative alloys; the corrosion rate of the new alloys is less than 2.9 mpy, usually less than 2.7 mpy, such as 2.65 mpy or less (Tab. 3). In addition, new alloys also exhibit excellent creep resistance in the temperature range 200-250°C, outperforming the comparative examples (Table 4). The creep rupture strength of the new alloys for HPDC is typically about 200 MPa or more at 200°C, and about 105 MPa or more at 250°C. The MCR values of the new alloys are 1.5x10 ¹⁰/s or less at 200°C and 100 MPa, usually 1.0x10 ¹⁰/s or less! the comparative alloys have higher values even if differing only slightly in composition from the new alloys (Tab. 4). The excellent combination of these properties along with low susceptibility to hot tearing makes the alloys of the instant invention the most promising candidates for high pressure die casting of moving parts serving at high temperatures of 200-250°C where low moment inertia and correspondingly low vibration are required.

Tables 5-6 demonstrate chemical compositions and properties of alloys for sand casting according to the invention and of comparative alloys subjected to full T6 heat treatment. The alloys of the instant invention exhibit superior combination of TYS and Elongation in comparison with comparative examples. The compressive strength of new alloys is also higher both at ambient and elevated temperatures. Furthermore a great advantage of the alloys of this invention is that they combine excellent mechanical properties with outstanding corrosion resistance which outperforms corrosion resistance of comparative alloys.

Tables 7-8 illustrate chemical composition and mechanical properties of forged alloys of instant invention. The alloys of present inventions and comparative alloys were direct chilled cast, homogenized, forged and T5 heat treated. The forged alloys of the present invention exhibit higher TYS and UTS values than comparative alloys at both ambient temperature and 200°C. It is important that they the alloys according to the invention have also superior elongation and significantly higher compressive yield strength.

While this invention has been described in terms of some specific examples, many modifications and variations are possible. It is therefore understood that, within the scope of the appended claims, the invention may be realized otherwise than as specifically described.

Claims

1. A lightweight alloy for high-temperature applications, consisting of

i) at least 94.8 wt% magnesium,

ii) 2.5 to 4.6 wt% neodymium,

iii) 0.05 to 0.40 wt% yttrium ,

iv) 0.03 to 0.65 wt.% zirconium, and

v) incidental impurities.

2. An alloy according to claim 1, further containing up to 0.02 wt% calcium.

3. An alloy according to claim 1, comprising essentially no heavy rare earth (HRE) elements with the atomic number from 61 to 71.

4. An alloy according to claim 1, comprising essentially no cerium, lanthanum, and praseodymium.

5. An alloy according to claim 1, comprising essentially no zinc.

6. An alloy according to claim 1, wherein the total content of Nd and Y is higher than 4.3 wt%.

7. A lightweight alloy according to claim 1, for prolonged operation at a temperature of up to 200°C.

8. An alloy according to claim 1, usable for a process selected from the group consisting of high pressure die casting (HPDC), sand casting, investment casting, and permanent mold casting.

9. An alloy according to claim 1, usable for a process comprising either twin roll casting with subsequent rolling, or direct chill casting with subsequent forging, extrusion or rolling.

10. A lightweight alloy for high -temperature applications according to claim 8 usable for HPDC.

11. An alloy according to claim 10, which contains 2.8 to 4.3 wt% Nd, 0.06 to 0.25 wt% Y, 0.05 to 0.4 wt% Zr, and 0.0 to 0.02 wt% Ca.

12. An alloy according to claim 10, exhibiting a castability of at least 96%, when measured in relative units characterizing oxidation resistance, fluidity, and dies sticking.

13. An alloy according to claim 10, exhibiting a tensile yield strength (TYS) at 200°C of at least 153 MPa.

14. An alloy according to claim 10, exhibiting a compression yield strength (CYS) at 200°C of at least 152 MPa.

15. An alloy according to claim 10, exhibiting a minimum creep rate (MCR) of not more than 1.5x10 ¹⁰/s at 200°C under stress of 100 MPa.

16. An alloy according to claim 10, exhibiting a corrosion rate under GM 9540 of not more than 2.7 mpy.

17. An article cast of an alloy according to claim 10, exhibiting a superior combination of strength and ductility after T5 treatment which includes direct aging at 150-250°C for 1- 10 h.

18. An article according to claim 17, exhibiting a superior combination of strength and ductility after T5 treatment which includes direct aging at 175-225°C for 1-6 h.

19. An alloy according to claim 8, suitable for sand casting, investment casting, permanent mold casting, and low pressure modifications thereof, containing 2.7 to 3.4 wt% Nd, 0.15 to 0.40 wt% Y, 0.3 to 0.6 wt% Zr, and 0.0 to 0.02 wt% Ca.

20. An article cast of an alloy according to claim 19, exhibiting a superior combination of performance properties after full T6 heat treatment comprising solid solution heat treatment at 520"560°C for 1 to 16 hrs, followed by cooling in a quenching medium and by subsequent aging at 200-270°C for 1 to 16 h.

21. An article according to claim 20, exhibiting a superior combination of performance properties after full T6 heat treatment comprising solid solution heat treatment at 535-545°C for 3 to 5 hrs, followed by cooling in a quenching medium and by subsequent aging at 225"250°C for 3 to 6 h.

22. An alloy according to claim 9, suitable for forging, extrusion, and rolling, which contains 2.8 to 3.8 wt% Nd, 0.20 to 0.40 wt% Y, 0.35 to 0.60 wt% Zr, and 0.0-0.02 wt% Ca.

23. An article cast of an alloy according to claim 22, exhibiting a superior combination of performance properties after T5 heat treatment comprising aging at 200"250°C for 1 to 16 h.