EP2191028B1

EP2191028B1 - Magnesium alloy material and method for manufacturing the same

Info

Publication number: EP2191028B1
Application number: EP08831622.9A
Authority: EP
Inventors: Mamoru Nakata; Yuuichi Yamada; Koji Itakura; Yoshihito Kawamura; Michiaki Yamasaki
Original assignee: Kobe Steel Ltd; Nissan Motor Co Ltd; Kumamoto University NUC
Current assignee: Kobe Steel Ltd; Nissan Motor Co Ltd; Kumamoto University NUC
Priority date: 2007-09-18
Filing date: 2008-09-18
Publication date: 2015-11-18
Anticipated expiration: 2028-09-18
Also published as: CN101802235B; WO2009038215A9; JP2009074112A; CN101802235A; EP2191028A1; JP5201500B2; WO2009038215A1; US20100206438A1

Description

TECHNICAL FIELD

The present invention relates to a magnesium alloy material and a method for manufacturing the same and particularly to a magnesium alloy material having high mechanical strength and a method for manufacturing the same.

BACKGROUND ART

In general, magnesium alloy materials have the lowest density among alloys in practical use, lightweight and high strength and accordingly have been promoted for applications to chassis of electric products, wheels of automobiles, underbody parts, peripheral parts for engines, and the like.
In particular, with respect to parts for uses relevant to automobiles, since high mechanical characteristics are required, as magnesium alloy materials containing an element such as Gd, Zn and the like, materials with specified configurations have been manufactured by a single roll process and a rapid solidification process (e.g. Patent Document 1, Patent Document 2, and Non-Patent Document 1).
However, in specified manufacturing methods, although providing the above-mentioned magnesium alloy materials with high mechanical characteristics, there are problems that special facilities are required, the productivity is low, and further applicable parts are limited.
Therefore, in the case of manufacturing magnesium alloy materials, even being manufactured by plastic processing (extrusion) from common melt casting at high productivity without using special facilities or processes described in the above-mentioned Patent Documents, those with mechanical characteristics useful for practical applications are proposed (e.g. Patent Document 3 and Patent Document 4). The magnesium alloy materials disclosed in Patent Documents 3 and 4 are known to have a long period stacking ordered structure (LPO) in a structure and to have high mechanical characteristics.

Patent Document 1: Japanese Patent Application Laid-Open (JP-A) No. 06-041701
Patent Document 2: JP-A No. 2002-256370
Patent Document 3: International Publication No. 2005/052204 Pamphlet
Patent Document 4: International Publication No. 2005/052203 Pamphlet
Non-Patent Document 1: Lecture Summary, the 108th Conference of Japan Institute of Light Metals, P42-45 (2005)

Document "Surface, interface and stacking fault energies of magnesium from first principles calculations" discloses magnesium-alloy material with Zn as essential component wherein the alloy structure contains stacking faults of a thickened atomic layer. Properties of magnesium have been calculated for bulk, surface and stacking faults, regarded as homogeneous interfaces, using density functional theory within a plane wave pseudopotential technique. Using a supercell approach, stacking fault energies have been determined for intrinsic, extrinsic and twin-like structures for the basal plane together with generalised stacking fault energies on basal, pyramidal and prismatic planes.
Document WO2007/108467 discloses a magnesium-alloy material used for housing of electrical products, and wheel of motor vehicles, which is magnesium-zinc-rare earth metal type alloy containing zinc as main component, and gadolinium, and/or thulium as rare earth metal and method for manufacturing such magnesium alloy material comprising a casting step, a solution treatment step, a heat treatment step and a plasticity processing step.

DISCLOSURE OF THE INVENTION

PROBLEMS TO BE SOLVED BY THE INVENTION

However, there is room for the following improvements for conventional magnesium alloy materials.
That is, it has been required for the conventional magnesium alloy materials to further improve the strength in order to promote their application for automobiles for the purpose of lightweight.
In view of the circumstances of the above-mentioned problems, the invention has been completed to provide a magnesium alloy material excellent in mechanical characteristics without using specific manufacturing facilities and processes and a method for manufacturing the same.

MEANS FOR SOLVING THE PROBLEMS

To solve the above-mentioned problems, the invention provides a magnesium alloy material having the following configuration. That is, the magnesium alloy material is an Mg-Zn-RE alloy containing, as an essential component, Zn and at least one of Gd, Tb, and Tm as RE, and balance of Mg and unavoidable impurities, and in the alloy structure of the Mg-Zn-RE alloy, stacking faults of a thickened two-atomic layer of Zn and RE are formed.
Due to such a configuration, the magnesium alloy material contains the stacking faults, so that the tensile strength, 0.2% proof strength, and elongation (elongation ratio) are improved as compared with those having a long period stacking ordered structure (LPO).
Further, in the above-mentioned magnesium alloy material, the alloy structure of the Mg-Zn-RE alloy contains recrystallized grains which have an average crystal grain diameter of 5 µm or less and a surface area ratio of 35% or more with respect to the above-mentioned alloy structure.
Due to such a configuration, the fine recrystallized grains in the metal structure (mother phase) improve the mechanical characteristics and the tensile strength, 0.2% proof strength, and elongation are improved.
Further, in the above-mentioned magnesium alloy material, Zn is preferably in a component range of 0.5 to 3% by atom and RE is preferably in a component range of 1 to 5% by atom.
Due to such a configuration, since Zn and RE (Gd, Tb, and Tm) are adjusted in the prescribed component ranges, so that the magnesium alloy material is made easy to form the stacking faults improving the strength.
Further, to solve the above-mentioned problems, a method for manufacturing the magnesium alloy material involves a casting step of forming a cast material by casting an Mg-Zn-RE alloy containing, as an essential component, Zn and at least one of Gd, Tb, and Tm as RE, and balance including Mg and unavoidable impurities, a solution treatment step of carrying out solution treatment for the cast material, and a heat treatment step of carrying out heat treatment in prescribed conditions for the cast material subjected to the solution treatment and the above-mentioned heat treatment step is carried out in a condition satisfying -14.58[ln(x)] + 532.32 < y < -54.164[ln(x)] + 674.05 and 0 < x ≤ 2, wherein y denotes the heat treatment temperature (K) and x denotes the heat treatment time (h).
In the method for manufacturing the magnesium alloy material according to the above-mentioned procedure, the precipitates of Mg and RE become in a solid-solution state by the solution treatment and further the heat treatment step is carried out in the heat treatment condition of the prescribed range, so that the stacking faults of the thickened two-atomic layer of Zn and RE can be formed in the alloy structure (mother phase) of the Mg-Zn-RE alloy and accordingly tensile strength, 0.2% proof strength and elongation can be improved.
Further, with respect to a method for manufacturing the magnesium alloy material, the method involves a casting step of forming a cast material by casting an Mg-Zn-RE alloy containing Zn as an essential component, at least one of Gd, Tb, and Tm as RE, and the rest including Mg and unavoidable impurities, a solution step of carrying out solution treatment for the above-mentioned cast material, a heat treatment step of carrying out heat treatment for the cast material subjected to the solution treatment in prescribed conditions, and a plasticity processing step of carrying out plastic processing of the above-mentioned heat-treated cast material and the above-mentioned heat treatment step is carried out in conditions satisfying -14.58[ln(x)] + 532.32 < y < -54.164[ln(x)] + 674.05 and 0 < x ≤ 2, wherein y denotes the heat treatment temperature (K) and x denotes the heat treatment time (h). In the above-mentioned method for manufacturing the magnesium alloy material, the plasticity processing step is an extrusion process or a forging process.
In the method for manufacturing the magnesium alloy material according to the above-mentioned procedure, the precipitates of Mg and RE are in a solid-solution state by the solution treatment and further the heat treatment condition is adjusted to be in the prescribed range, so that the stacking faults of the thickened two-atomic layer of Zn and RE can be formed in the alloy structure (mother phase) of the Mg-Zn-RE alloy and accordingly the tensile strength, 0.2% proof strength and elongation can be improved. Further, execution of the plastic processing generates a large number of fine recrystallized grains in the alloy structure and the tensile strength, 0.2% proof strength and elongation can be improved more.

EFFECT OF THE INVENTION

A magnesium alloy material and its manufacturing method according to the invention have the following excellent effects.
Since the magnesium alloy material contains stacking faults of the thickened two-atomic layer of Zn and RE in the alloy structure (mother phase), the tensile strength, elongation, and 0.2% proof strength at a prescribed elongation ratio can remarkably be improved as compared with those having a long period stacking ordered structure. Further, if extrusion (plasticity) processing is carried out, since fine crystal grains are generated in the alloy structure, mechanical characteristics too high to be achieved generally can be obtained. Therefore, the magnesium alloy material can be used as, for example, automotive parts, particularly, parts such as pistons or the like which are required to have very severe mechanical characteristics.
Since the method for manufacturing the magnesium alloy material involves heat treatment in condition of the prescribed range after the solution treatment, the magnesium alloy material contains the stacking faults of the thickened two-atomic layer of Zn and RE in the alloy structure (mother phase). Therefore, the magnesium alloy material provided with the tensile strength, elongation, and 0.2% proof strength at a prescribed elongation ratio improved as compared with those of a conventional material can be produced efficiently by common manufacturing facilities or processes.
Further, in the method for manufacturing the magnesium alloy material, the heat treatment temperature and the heat treatment time are adjusted in a condition satisfying -14.58[ln(x)] + 532.32 < y < -54.164[ln(x)] + 674.05 and 0 < x ≤ 2, wherein y denotes the heat treatment temperature (K) and x denotes the heat treatment time (h), so that the magnesium alloy material provided with improved tensile strength, elongation, and 0.2% proof strength at a prescribed elongation ratio in a widened range (as compared with those of a magnesium alloy material having the long period stacking ordered structure) can be manufactured.

BRIEF DESCRIPTION OF THE DRAWINGS

Figs. 1(a) and 1(b) are TEM photographs of the state that the stacking faults are formed in the metal structure of the magnesium alloy material of the invention observed by a low magnification transmission electron microscope.
Fig. 2 is a TEM photograph of the stacking faults observed in the magnesium alloy material of the invention by a high-resolution transmission electron microscope.
Fig. 3 is a STEM photograph of the stacking faults in the magnesium alloy material of the invention observed by a high-angle scattered annular dark field method.
Fig. 4 is a TEM photograph of the state that the long period stacking ordered structure is formed in the metal structure of a conventional magnesium alloy material observed by a low magnification transmission electron microscope.
Fig. 5 is a flow chart showing a method for manufacturing the magnesium alloy material of the invention.
Fig. 6 is a graph schematically showing the relations of temperature and time in the solution treatment and heat treatment of the magnesium alloy material of the invention.
Fig. 7 is a graph showing the region of stacking faults formed in the metal structure at the heat treatment temperature and heat treatment time in the condition of the invention.
Figs. 8(a) to 8(c) are TEM photographs showing the state of the metal structure by heat treatment at 673K for 0.5 hours and 1 hour and at 523K for 2 hours for the magnesium alloy material of the invention.
Figs. 9(a) to 9(c) are TEM photographs showing the state of the metal structure by heat treatment at 723K for 2 hours, at 673K for 10 hours, and at 773K for 10 hours for the magnesium alloy material of the invention and the conventional magnesium alloy material.
Fig. 10 is a TEM photograph for comparing the states of the metal structures by heat treatment at 673K for 0.5 hour, at 673K for 10 hours, and at 773K for 10 hours for the magnesium alloy material of the invention and the conventional magnesium alloy material.
Figs. 11(a) to 11(c) are graphs showing the relation of 0.2% proof strength and elongation, the relation of tensile strength and elongation, and the relation of tensile strength and 0.2% proof strength before extrusion processing successively to the heat treatment step for the magnesium alloy material of the invention and the conventional magnesium alloy material.
Figs. 12(a) to 12(c) are graphs showing the relation of 0.2% proof strength and elongation, the relation of tensile strength and elongation, and the relation of tensile strength and 0.2% proof strength in the case of executing extrusion processing successively to the heat treatment step for the magnesium alloy material of the invention and the conventional magnesium alloy material.
Fig. 13 is a graph showing the correlation of the surface area ratio of the recrystallized grains in the metal structure and mechanical characteristics for the magnesium alloy material of the invention.
Fig. 14(a) is a TEM photograph showing the microstructure after the plastic processing in one example of conventional heat treatment conditions for the conventional magnesium alloy material and Fig. 14(b) is a TEM photograph showing the microstructure after the plastic processing in one example of heat treatment conditions of the invention for the magnesium alloy material of the invention.
Fig. 15(a) is a TEM photograph showing the microstructure after the plastic processing in the heat treatment at 773K for 10 hours for the conventional magnesium alloy material and Fig. 15(b) is a TEM photograph showing the microstructure after the plastic processing in heat treatment at 673K for 0.16 hour for the magnesium alloy material of the invention.
Fig. 16 (a) is a TEM photograph showing the microstructure after the plastic processing in the heat treatment at 673K for 0.5 hour for the conventional magnesium alloy material and 16 (b) is a TEM photograph showing the microstructure after the plastic processing in heat treatment at 673K for 1 hour for the magnesium alloy material of the invention.
Fig. 17 is a graph showing the relation of heat treatment temperature and the heat treatment time including the magnesium alloy material of the invention.
Fig. 18 is a block chart showing the respective steps for evaluating mechanical characteristics in the case of explaining Examples of the invention.
Figs. 19 (a) to 19(d) are TEM photographs in the case a cast ingot to be used in Examples of the invention is subjected to heat treatment at the respective temperatures for respective times.
Figs. 20 (a) to 20 (c) are TEM photographs in the case a cast ingot to be used in Examples of the invention is subjected to heat treatment at 673K for respective times.

EXPLANATION OF THE SYMBOLS

1: magnesium alloy material
2: stacking faults
3: long period stacking ordered structure (LPO)
4: recrystallized grains

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the best mode for carrying out the invention will be described with reference to drawings. Figs. 1(a) and 1(b) are TEM photographs of the state that the stacking faults are formed in the metal structure of a magnesium alloy material observed by a low magnification transmission electron microscope; Fig. 2 is a TEM photograph of the stacking faults observed in the magnesium alloy material by a high-resolution transmission electron microscope; Fig. 3 is a STEM photograph of the stacking faults in the magnesium alloy material observed by a high-angle scattered annular dark field method; Fig. 4 is a TEM photograph of the state that the long period stacking ordered structure is formed in the metal structure of a conventional magnesium alloy material observed by a low magnification transmission electron microscope.
A magnesium alloy material 1 is an Mg-Zn-RE alloy containing Zn as an essential component, at least one of Gd, Tb, and Tm as RE (rare earth metals), and the rest including Mg and unavoidable impurities, and herein an example containing Gd will be described. As shown in Figs. 1 to 3, the magnesium alloy material 1 contains stacking faults 2 of thickened two-atomic layer of Zn and RE in the alloy structure (mother phase). Substantially, the magnesium alloy material 1 contains stacking faults 2 including drawing type stacking faults in the two atomic layer where zinc (Zn) and rare earth (RE) elements are thicked (two atomic layer thickened) in the two atomic layer in the bottom face of the α-magnesium mother phase and the solute elements are thus thicked (the stacking faults will be described later).
Herein, the bottom face of the alloy structure (mother phase) means the alloy surface side in the mother phase, that is, both faces of the upper and lower side in the mother phase
In Fig. 1, the observation direction is in parallel to the a-axis of the mother phase crystals and in the electron diffraction pattern, streaks derived from the stacking faults but not from the long period stacking ordered structure can be observed in the c-axis direction. In Fig. 2, the observation direction is in parallel to the a-axis of the mother phase crystals and it can be understood that the stacking faults are drawing type stacking faults. In Fig. 3, the observation direction is in parallel to the a-axis of the mother phase crystals and it can be understood that the solute atoms are thicked in the two atomic layer.
Further, in the magnesium alloy material 1 in the case RE is Gd as the Mg-Zn-RE alloy, numberless stacking faults 2 are shown in the form of thin lines as shown in Fig. 1. When the thin linear stacking faults 2 are observed further at high resolution, as shown in Fig. 3, thicking by two atoms can be understood and it can be understood that it is the stacking faults 2. In the case where Gd is contained as RE, the two layer thick is thick by Zn atoms and Gd atoms. In the case where Gd is RE, the precipitates of Mg₃Gd are precipitated in the magnesium alloy material 1 (not-illustrated). The precipitates of Mg₃Gd form solid-solution (solution) by solution treatment and the stacking faults 2 are formed by this solid solution and heat treatment.

(Stacking faults and long period stacking ordered structure)

The stacking faults 2 in the magnesium alloy material 1 are drawing type stacking faults 2 by thick by the RE atom and Zn atom in the two atomic layer and the stacking direction is not particularly determined. On the other hand, the long period stacking ordered structure 3 shown in Fig. 4 is formed by stacking the RE atom and Zn atom in the c-axial direction of the magnesium mother phase crystal in certain cycles and thus the long period stacking ordered structure 3 and the stacking faults 2 can clearly classified in terms of the stacking direction and the cyclic property. In conventional investigations, it is made clear that an Mg-RE-Zn type alloy having the long period stacking ordered structure 3 has excellent mechanical characteristics (tensile strength, 0.2% proof strength, and elongation); however, with respect to the stacking faults 2, their existence, effects on the mechanical characteristics, and the like are not at all made clear. However, the investigations the inventors of the invention have made make the effects of the stacking faults 2 on the mechanical characteristics at first clear.

(Alloy composition)

[Zn: 0.5 to 3 (at.)%]

If Zn is less than 0.5 at.%, Mg₃Gd cannot be obtained in the cast state and even if solution treatment is carried out in the next step, the Gd element cannot sufficiently form solid solution with α-Mg. Therefore, the stacking faults 2 cannot be formed in the heat treatment step and the strength is lowered. Further, if Zn exceeds 3 at.%, not only the strength cannot be improved corresponding to the addition amount, but also Mg₃Gd precipitated in grain boundaries is increased and the elongation is lowered (resulting in brittleness). Accordingly, Zn is defined in a range of 0.5 to 3 at.% here.

[RE (one or more of Gd, Tb, and Tm)]

Although Gd, Tb, and Tm cannot develop the stacking faults 2 only by casting but form the stacking faults 2 by carrying out solid solution and heat treatment in prescribed conditions after casting. In the magnesium alloy material 1, the strength can be improved by precipitating the long period stacking ordered structure 3 in heat treatment condition; however, to obtain higher strength, the stacking faults 2 are formed by solid solution and heat treatment of Mg₃Gd (Mg₃Zn₃Tb₂ or Mg₂₄Tm₅) or the stacking faults 2 may be formed by solid solution and heat treatment of Mg₃Gd (Mg₃Zn₃Tb₂ or Mg₂₄Tm₅) and at the same time the long period stacking ordered structure 3 may be mixed.
Therefore, a prescribed amount of RE consisting of at least Gd, Tb, and Tm in the magnesium alloy material 1 is required. If the total amount of at least one of Gd, Tb, and Tm in the magnesium alloy material 1 is less than 1 at.%, Mg₃Gd (Mg₃Zn₃Tb₂ or Mg₂₄Tm₅) and the stacking faults 2 cannot be formed and if the total amount exceeds 5 at.%, not only the strength cannot be improved corresponding to the addition amount, but also Mg₃Gd precipitated in grain boundaries is increased and the elongation is lowered. Accordingly, the total content of RE, at least one of Gd, Tb, and Tm, in the magnesium alloy material 1 is defined in a range of 1 to 5 at.%.
Consequently, with respect to the alloy composition, the magnesium alloy material 1 has a composition on the basis of % by atom, defined by a composition formula Mg_100-a-bZn_aRE_b (in the composition formula, 0.5 ≤ a ≤ 3; 1 ≤ b ≤ 5). In the invention, components other than the above-described components may be added within a range of unavoidable impurities in a range that the effect of the magnesium alloy of the invention is not affected and for example, Zr, which contributes to fineness, in an amount of 0.1 to 0.5 at.% may be added.
Next, a method for manufacturing the magnesium alloy material will be described.
Fig. 5 is a flow chart showing a method for manufacturing a magnesium alloy and Fig. 6 is a graph schematically showing the relation of temperature and time of solution treatment and heat treatment of a magnesium alloy.
A magnesium alloy material 1 is first cast in a casting step S1 Herein, the magnesium alloy material 1 has a composition formula Mg_100-a-bZn_aRE_b and contains Gd as RE. Next, the cast material is subjected to solution treatment (solid solution formation of RE) in a solution treatment S2. In Fig. 6, the temperature of the solution treatment at that time is, as an example, 793K, and the solution treatment is carried out for 2 hours. In the cast material, a compound of Mg and Gd (Tb, Tm) formed by the casting is dissolved in a matrix and forms a solid solution by the solution treatment. The solution treatment is preferably carried out at 773K or higher for 2 hours or longer.
Further, the heat treatment step S3 of carrying out heat treatment for the cast material subjected to the solution treatment is carried out in prescribed conditions. Execution of the heat treatment step S3 forms the stacking faults 2 and at the same time precipitation of the long period stacking ordered structure 3 and precipitates of Mg₃Gd (Mg₃Zn₃Tb₂ or Mg₂₄Tm₅) and Mg₃Zn₃Gd₂ may sometimes coexist.
The heat treatment step S3 is carried out in condition of the range satisfying -14.58 [ln(x)] + 532.32 < y < -54.164 [ln(x)] + 674.05 and 0 < x ≤ 2, wherein y denotes the heat treatment temperature (K) and x denotes the heat treatment time (h).
When the heat treatment step S3 is carried out under the prescribed condition, as the magnesium alloy material 1, a structure of the phase region in which the stacking faults 2 capable of improving particularly the strength can be formed is provided. Fig. 7 is a graph showing the regions of the stacking faults to be formed in the metal structure at the heat treatment temperature and heat treatment time and Figs. 8(a) to 8(c) are TEM photographs showing the state of the metal structure of the magnesium alloy material obtained by heat treatment at 673K for 0.5 hour and 1 hour and at 523K for 2 hours. Figs. 9(a) to 9(c) are TEM photographs showing the state of the metal structure of the magnesium alloy material obtained by heat treatment at 723K for 2 hours, at 673K for 10 hours, and at 773K for 10 hours. Fig. 10 is a TEM photograph for comparing the states of the metal structures for magnesium alloy materials obtained by heat treatment at 673K for 0.5 hour, at 673K for 10 hours, and at 773K for 10 hours. Figs. 8, 9 and 10 are all photographed with the same scale and correspond to a part of the plot of Fig. 7.
As shown in Fig. 7, the range in which the stacking faults 2 are mainly formed is the range of the above-mentioned prescribed heat treatment condition. The range of the heat treatment condition is defined by calculating the curve equation approximating the range surrounded with the solid line of Fig. 7, based on the calculated curve equation. That is, the range surrounded with the solid line is approximately the range of the heat treatment condition. Further, formation of the long period stacking ordered structure 3 or precipitation of Mg₃Gd precipitates may occur in combination with the stacking faults 2. It is made possible to entirely improve the tensile strength, 0.2% proof strength, and elongation for the magnesium alloy material 1 by forming mainly the stacking faults 2 (reference to Examples)
Further, as shown in Fig. 8, it is found that stacking faults 2 are mainly formed in the case where the heat treatment temperature is 673K and the heat treatment time is set to be respectively 0.5 hour and 1 hour and the heat treatment temperature is 523K and the heat treatment time is set to be 2 hours. Further, as shown in Fig. 9, formation of the stacking faults 2 is not observed in the case where the heat treatment is carried out at a heat treatment temperature of 723K and for a heat treatment time of 2 hours, at 673K for 10 hours, and at 773K for 10 hours. Furthermore, as shown in Fig. 10, stacking faults are formed in the case where the heat treatment temperature is 673K and the heat treatment time is 0.5 hour, and no stacking fault is formed at 673K for 10 hours and at 773K for 10 hours.
The cast product subjected to the heat treatment is next subjected to the plastic processing step S4 for plastic processing, based on the necessity. The plastic processing of the plastic processing step S4 may be extrusion processing or forging processing. The plastically processed plastic-processing product is provided with remarkably improved tensile strength, 0.2% proof strength, and elongation (elongation ratio). Figs. 11(a) to 11 (c) are graphs showing the relation of 0.2% proof strength and elongation, the relation of tensile strength and elongation, and the relation of tensile strength and 0.2% proof strength before extrusion processing successively to the heat treatment step for the magnesium alloy materials. Figs. 12 (a) to 12 (c) are graphs showing the relation of 0.2% proof strength and elongation, the relation of tensile strength and elongation, and the relation of tensile strength and 0.2% proof strength in the case of executing extrusion processing successively to the heat treatment step for magnesium alloy materials (extruded materials). As shown in Figs. 11 and 12, as compared with the magnesium alloy material having the long period stacking ordered structure (LPO) 3, the magnesium alloy material 1 having the stacking faults 2 has stable data in the condition and is excellent in balance between the 0.2% proof strength and elongation, balance between the tensile strength and elongation, and balance between the relation of tensile strength and 0.2% proof strength. Further, the mechanical properties are high as a whole. Further, after the heat treatment step S3, the magnesium alloy material 1 subjected to the extrusion processing, which is the plastic processing step S4, shows high tensile strength., 0.2% proof strength, and elongation values as compared with those which is not subjected to the extrusion processing.
It is important that the magnesium alloy material 1 provides the formation of the stacking faults 2 in the case where the tensile strength, 0.2% proof strength, and elongation are improved in the heat treatment step S3 and the plastic processing step S4 and also in the case where precipitates of Mg₃Gd (Mg₃Zn₃Tb₂ or Mg₂₄Tm₅) and the long period stacking ordered structure 3 are precipitated in addition, if the stacking faults 2 are formed, the tensile strength, 0.2% proof strength, and elongation are improved.
Herein, if an alloy containing the stacking faults 2 is extrusion-processed, a large number of fine recrystallized grains with an average crystal grain diameter of 5 µm or less are generated in the alloy structure (mainly in the matrix part). The recrystallized grains are a main cause to further improve the mechanical properties after the extrusion. Fig. 13 shows the correlation between the surface area ratio of the recrystallized grains in the metal structure and mechanical properties. As shown in Fig. 13, as the surface area ratio of the recrystallized grains 4 is higher, the 0.2% proof strength tends to be improved more. It is preferable to have the strength at 35% surface area ratio or higher. Further, the average crystal grain diameter can be measured by observation with an optical microscope and calculation by an average crystal grain surface area method standardized in ASTM.
Further, Fig. 14(a) is a TEM photograph showing the microstructure after the plastic processing in one example of conventional heat treatment conditions for the conventional magnesium alloy material and Fig. 14(b) is a TEM photograph showing the microstructure after the plastic processing iri one example of heat treatment conditions of the invention for the magnesium alloy material of the invention. Fig. 15(a) is a TEM photograph showing the microstructure after the plastic processing in the heat treatment at 773K for 10 hours and Fig. 15(b) is a TEM photograph showing the microstructure after the plastic processing in heat treatment at 673K for 0.16 hour. Fig. 16(a) is a TEM photograph showing the microstructure after the plastic processing in the heat treatment at 673K for 0.5 hour and Fig. 16(b) is a TEM photograph showing the microstructure after the plastic processing in heat treatment at 673K for 1 hour. The extrusion conditions of Figs. 15 and 16 are an extrusion ratio of 10 and an extrusion speed of 2.5 mm/sec. As shown in Figs. 14 to 16, in the heat treatment conditions of the invention, it can be understood that a large number of recrystallized grains 4 are formed in the alloys after the plastic processing (extrusion processing). In Fig. 14(a), no recrystallized grain is formed. Further, no recrystallized grain is formed in the microstructure before the plastic processing (reference to Figs. 8 to 10).
In addition, since plastic processing step S4 shown in Fig. 5 improves the strength by adding the plastic processing for the cast products (extrusion processing and forging processing) which are subjected to the heat treatment, the step may be carried out in accordance with the uses of the magnesium alloy material 1. The magnesium alloy material 1 after the plastic processing may be processed by cutting or the like into a prescribed shape to obtain a product. Further, although the method for manufacturing the magnesium alloy material 1 is described here as a method involving a series of steps from the casting step S1 to the plastic processing step S4, the series of steps may be only from the casting step S1 to the heat treatment step S3 and the plastic processing step S4 may be carried out in the purchaser side who purchases the product.

[Examples]

Next, Examples of the invention will be described. Herein, Examples merely exemplify the invention and do not at all limit the invention. Fig. 17 is a graph showing the relation of heat treatment temperature and the heat treatment time. Fig. 18 is a block chart showing the respective steps for evaluating mechanical properties. Figs. 19 (a) to 19(d) are TEM photographs in the case where each cast ingot is subjected to heat treatment at the respective temperatures for the respective times. Figs. 12(a) to 20(c) are TEM photographs in the case where each cast ingot is subjected to heat treatment at a temperature of 673K for the respective times.
An Mg-Zn-Gd alloy containing 1 at. % of Zn, 2 at.% of Gd, and the rest including Mg and unavoidable impurities as a magnesium alloy material was loaded to a melting furnace and melted by a flux refining. Successively, the thermally melted material was cast by a casting die as shown in Fig. 18 to produce an ingot of a size of ϕ29 mm × L60 mm (S1) and the cast ingot was subjected to solution treatment at 793K for 2 hours (S2) and thereafter, the heat treatment at the respective temperatures was carried out (S3) and specimens subjected to plastic processing (S4) at an extrusion ratio of 10 and an extrusion temperature of 673K and specimens not subjected to the plastic processing were produced and a tensile test was carried out for them at room temperature (the tensile test at a high temperature was carried out for those which were not subjected to the plastic processing for reference). The strain velocity at the tensile test was ε = 5.0 × 10^-4 (s^-1). Further, the solution treatment and the heat treatment were carried out by a muffle furnace and the respective temperatures were shown in Fig. 17 for Examples: that is, the heat treatment was carried out for short times of 0.16 hour, 0.33 hour, 0.5 hour, 1 hour, and 2 hours. Fig. 18 collectively shows solution treatment and heat treatment as heat treatment. As shown in Fig. 17, a test was carried out for the magnesium alloy material in total of 24 types, as specimens, relevant to the above-mentioned respective temperatures and times.
TEM photographs of the microstructure by solution treatment and of those subjected to treatment of 773K × 0.16 hour, 523K × 2 hours, and 773K × 4 hours among specimens shown in Fig. 17 are shown in Fig. 19(a) to 19(d).
As shown in Fig. 19(a), it was found that only stacking faults appeared in the matrix together with the Mg₃Gd phase in the metal structure state in the case where merely solution treatment was carried out. The structure configuration was changed by heat treatment carried out thereafter and as shown in Fig. 19(b), the metal structure state was found containing precipitates of stacking faults at a high density and Mg₃Gd coexisting together in the case of a heat treatment at 773K for 0.16 hour. Further, in the case of heat treatment of 523K × 2 hours as shown in Fig. 19(c), the structure was found containing the stacking faults and LPO coexisting together. In addition, in other temperature ranges, it was found that the stacking faults were mainly precipitated and the Mg₃Gd phase and 14H-LPO phase (long period stacking ordered structure) were partially precipitated. On the other hand, as shown in Fig. 19(d), it was found that no stacking fault was observed and LPO was the main structure in the case of a conventional heat treatment of 773K × 4 hours.
Further, among those shown in Fig. 17, TEM photographs of microstructures of specimens of 673K × 0.16 hour, 673K × 0.5 hour and 673K × 1 hour are shown in Figs. 20(a) to 20(c). As shown in Fig. 20(a) to 20(c), it was found that the stacking faults were precipitated at high density and Mg₃Gd coexisting together in metal structures in the case of the heat treatment conditions.
As described, in the case of conventional long time heat treatment, the stacking faults formed at the time of solution treatment were found changing to LPO. Therefore, it was found that the 14H-LPO phase was precipitated under the conventional heat treatment conditions but precipitation of the stacking faults was not confirmed.

Further, Tables 1 and 2 shows those treated in conditions within the scope of the invention defined as Examples 1 to 7 among the specimens shown in Fig. 17 and those treated in the representative conditions out of the scope of the invention defined as Comparative Examples 1 to 6 among the specimens shown in Fig. 17 together with the conditions of the respective steps, structure states, 0.2% proof strength, tensile strength, and elongation. Table 1 shows those before the plastic processing (S4) was carried out and Table 2 shows those after the plastic processing (S4) was carried out.

[Table 1]

	Structure condition (main precipitates)	Composition	Heat treatment condtion before extrusion	Only heat treatment (room temperature)			Only heat treatment (high temperature (473K)
		Composition		0.2% yield	Tensile strength	Elongation	0.2% yield	Tensile strength	Elongation
		(at%)		(MPa)	(MPa)	(%)	(MPa)	(MPa)	(%)
Example 1	Mg₃Gd+ stacking faults	Mg₉₇Zn₁Gd₂	673 K × 0.5h	163	214	5.0	140	185	5.7
Example 2	Mg₃Gd+ stacking faults		673 K × 0.16h	170	220	4.9	146	190	5.6
Example 3	Mg₃Gd+ stacking faults		673 K × 1h	167	219	4.8	143	190	5.5
Example 4	Mg₃Gd+ stacking faults		626 K × 0.5h	167	217	4.8	143	188	5.5
Example 5	MG₃Gd+ stacking faults		648 K × 0.33h	165	215	5.0	142	186	5.9
Example 6	Mg₃Gd+ stacking faults		773 K × 0.16h	165	218	4.4	142	189	5.0
Example 7	Mg₃Gd+ stacking faults		523 K × 2h	161	216	4.5	141	186	5.0
Comparative Example 1	Mg₃Gd	Mg₉₇Zn₁Gd₂	Merely solution treatment	149	186	2.0	144	193	3.2
Comparative Example 2	Mg₃Gd+ LPO		713 K × 10 h	145	183	3.4	140	190	5.4
Comparative Example 3	Mg₃Gd+ LPO		723 K × 10 h	130	182	5.1	136	180	4.0
Comparative Example 4	Mg₃Gd+ LPO		473 K × 60 h	190	239	3.2	181	236	2.5
Comparative Example 5	Mg₃Gd+ LPO		523 K × 60 h	195	245	3.0	186	242	2.3
Comparative Example 6	LPO		773 K × 4h	146	183	3.0	142	191	4.0

*Solution treatment 793K × 2 hours

[Table 2]

	Structure condition (main precipitates)	Composition	Heat treatment condition before extrusion	Only heat treatment (room temperature)
		Composition		0.2% yield	Tensile strength	Elongation
		(at%)		(MPa)	(MPa)	(%)
Example 1	Mg₃Gd+ stacking faults	Mg₉₇Zn₁Gd₂	673 K × 0.5h	356	406	8.7
Example 2	Mg₃Gd+ stacking faults		673 K × 0.16h	370	416	8.6
Example 3	Mg₃Gd+ stacking faults		673K × 1h	365	414	8.4
Example 4	Mg₃Gd+ stacking faults		626 K × 0.5h	363	410	8.5
Example 5	Mg₃Gd+ stacking faults		648 K × 0.33h	359	407	9.0
Example 6	Mg₃Gd+ stacking faults		773 K × 0.16h	359	411	7.7
Example 7	Mg₃Gd+ stacking faults		523 K × 2h	351	392	10.2
Comparative Example 1	Mg₃Gd	Mg₉₇Zn₁Gd₂	Merely solution treatment	361	374	2.4
Comparative Example 2	Mg₃Gd+ LPO		773K × 10 h	350	394	8.5
Comparative Example 3	Mg₃Gd+ LPO		123 K × 10 h	325	382	11.8
Comparative Example 4	Mg₃Gd+ LPO		473 K × 60 h	360	392	6.9
Comparative Example 5	Mg₃Gd+ LPO		523 K × 60 h	380	400	7.6
Comparative Example 6	LPO		773 K × 4h	352	396	8.0

*solution treatment 793K × 2 hours
*extrusion temperature: 623K, extrusion ratio 10

As shown in Tables 1 and 2, the magnesium alloy material specimens of Examples 1 to 7 were all found having precipitates of Mg₃Gd and stacking faults in the metal structures and as a whole had high 0.2% proof strength, tensile strength, and elongation (reference to Figs. 11 and 12).
On the other hand, the magnesium alloy material specimens of Comparative Examples 1 to 6 where found having as a whole lowered 0.2% proof strength, tensile strength, and elongation as compared with those containing the precipitates of stacking faults since they did not have stacking faults (reference to Figs. 11 and 12).
Further, as shown in Tables 1 and 2, it was found that those subjected to the plastic processing (extrusion processing) were improved in 0.2% proof strength, tensile strength, and elongation as compared with those which were not subjected to the plastic processing (extrusion processing).
As described above, it is made possible to use a magnesium alloy material as a material excellent in the mechanical properties even if the magnesium alloy material is an Mg-Zn-RE alloy by precipitating stacking faults.

Claims

A magnesium alloy material of an Mg-Zn-RE alloy containing, as an essential component, Zn and at least one of Gd, Tb, and Tm as RE, and the rest including Mg and unavoidable impurities, wherein the alloy structure of the Mg-Zn-RE alloy contains stacking faults of a thickened two-atomic layer of Zn and RE, wherein Zn is contained in a component range of 0.5 to 3% by atom and RE is contained in a component range of 1 to 5% by atom.
The magnesium alloy material according to claim 1, wherein the alloy structure of the Mg-Zn-RE alloy contains recrystallized grains, the recrystallized grains have an average crystal grain diameter of 5 µm or less and a surface area ratio with respect to the alloy structure of 35% or more.
A method for manufacturing a magnesium alloy material comprising:
a casting step of producing a cast material by casting an Mg-Zn-RE alloy containing, as an essential component, Zn and at least one of Gd, Tb, and Tm as RE, and of the rest including Mg and unavoidable impurities;

a solution treatment step of carrying out solution treatment for the cast material; and

a heat treatment step of carrying out heat treatment in a prescribed condition for the cast material subjected to the solution treatment,

wherein the heat treatment step is carried out in a condition satisfying -14.58[ln(x)] + 532.32 < y < -54.164[ln(x)] + 674.05 and 0 < x ≤ 2, wherein y denotes the heat treatment temperature (K) and x denotes the heat treatment time (h),

wherein Zn is contained in a component range of 0.5 to 3% by atom and RE is contained in a component range of 1 to 5% by atom in the magnesium alloy material.
The method for manufacturing a magnesium alloy material according to claim 3, further comprising:
a plastic processing step of carrying out plastic processing for the cast material subjected to the heat treatment.
The method for manufacturing a magnesium alloy material according to claim 4, wherein the plastic processing in the plastic processing step is extrusion processing or forging processing.