US20100061882A1

US20100061882A1 - Magnesium alloy material and method for manufacturing the same

Info

Publication number: US20100061882A1
Application number: US12/517,134
Authority: US
Inventors: Mamoru NAKATA; Yuuichi Yamada; Koji ITAKURA; Yoshio Okada; Yoshihito Kawamura; Michiaki Yamasaki
Original assignee: Kobe Steel Ltd; Kumamoto University NUC
Current assignee: Kobe Steel Ltd; Kumamoto University NUC
Priority date: 2006-11-30
Filing date: 2007-11-27
Publication date: 2010-03-11
Also published as: WO2008069049A1; JP2008138249A; CN101622366A; JP5175470B2

Abstract

Disclosed is a magnesium alloy material which can be produced without the need of employing any specialized production facility or process and has excellent mechanical properties, particularly high elongation. Also disclosed is a process for producing the magnesium alloy material. The magnesium alloy material comprises a Mg—Gd—Zn alloy comprising 1 to 5 mass % of Zn and 5 to 15 mass % of Gd as essential ingredients, with the remainder being Mg and unavoidable impurities, wherein the Mg—Gd—Zn alloy has a long period stacking structure in its alloy structure and also has Mg₃Gd and/or Mg₃Zn₃Gd₂. The process for producing the magnesium alloy material comprises a melting/casting step and a forging processing step for subjecting the casted material to a hot forging processing at a predetermined processing rate.

Description

TECHNICAL FIELD

The present invention relates to a magnesium alloy material having excellent mechanical properties (tensile strength, 0.2% yield strength and elongation), particularly having high tensile strength and 0.2% yield strength while sustaining high elongation, and a process for producing thereof.

BACKGROUND ART

In general, since magnesium alloy materials have the lowest density and lightest weight among practically used alloys while also having high strength, they are being increasingly used in applications such as chassis of electrical products as well as automotive wheels, underbody parts and around-the-engine parts.
Since parts used in automobile-related applications are particularly required to have high mechanical properties, materials having specific shapes are produced by a single roll process or rapid solidification process using magnesium alloy materials to which elements such as Gd or Zn have been added (see, for example, Patent Documents 1, 2 and Non-Patent Documents 1 to 4) .
Patent Document 1: Japanese Patent Application Laid-open No. 06-041701
Patent Document 2: Japanese Patent Application Laid-open No. 2002-256370
Non-Patent Document 1: Michiaki Yamasaki and three others. “Novel Mg—Zn—Gd Alloys in which a long period stacking ordered Structure is Formed by High-Temperature Heat treatment”, Summary of Presentations at the 108th Spring Conference (2005) of the Japan Institute of Light Metals, Japan Institute of Light Metals, 2005, p. 43-44).
Non-Patent Document 2: Kim and two others, “Development of High Strength Mg—Zn—Gd Alloy using Rapid Solidification Process”, Summary of Presentations at the 109th Autumn Conference (2005) of the Japan Institute of Light Metals, Japan Institute of Light Metals, 2005, p. 9-10).
Non-Patent Document 3: Kim and two others, “Mechanical properties of Mg—Zn—Gd Rapid Solidification Ribbon Solidification Formation Material having a long period stacking ordered Structure”, Summary of Presentations at the 110th Spring Conference (2006) of the Japan Institute of Light Metals, Japan Institute of Light Metals, 2006, p. 355-356).
Non-Patent Document 4: Kim and two others, “Development of High Strength Mg—Zn—Gd Alloy using Rapid Solidification Process”, Summary of Presentations at the Autumn Conference of the Japan Institute of Metals, Japan Institute of Light Metals, 2005, p. 9-10).

DISCLOSURE OF THE INVENTION

Problems to be Solved by the Invention

However, although the above-mentioned magnesium alloy materials allow the obtaining of high mechanical properties in specific production processes, these production processes have a disadvantage of requiring special equipment and having low productivity, while also having a problem of the existence of limitations on those members that can be used practically. When the magnesium alloy materials are used for members such as engine pistons, the magnesium alloy materials require tensile strength and yield strength as well as high elongation. The conventional magnesium alloy materials have involved the problem of insufficient tensile strength and 0.2% yield strength when realizing high elongation.
With the foregoing in view, an object of the present invention is to provide a magnesium alloy material having superior mechanical properties, particularly having high tensile strength and 0.2% yield strength while sustaining high elongation without using special production equipment and processes, and a process for producing the same.

Means for Solving the Problems

In order to solve the above-mentioned problems, the present invention was configured as a magnesium alloy material in the manner described below.
Therefore, the magnesium alloy material of the present invention comprises an Mg—Gd—Zn alloy including 1 to 5% by mass of Zn and 5 to 15% by mass of Gd as essential components, with the remainder being Mg and unavoidable impurities,
wherein the Mg—Gd—Zn alloy has an alloy structure having a long period stacking ordered structure and having 4% or more of Mg₃Gd and/or Mg₃Zn₃Gd₂.
Since the alloy structure of the Mg—Gd—Zn alloy has the long period stacking ordered structure, the above-mentioned composition can enhance tensile strength and 0.2% yield strength of the magnesium alloy material. Since the alloy structure has Mg₃Gd and/or Mg₃Zn₃Gd₂elongation of the magnesium alloy material can be enhanced.
The magnesium alloy material, wherein an area ratio of Mg₃Gd and/or Mg₃Zn₃Gd₂in the alloy structure is preferably 53% or less.
Since the above-mentioned composition limits the area ratio of Mg₃Gd and/or Mg₃Zn₃Gd₂to a predetermined range, elongation of the magnesium alloy material is more appropriately set.
The magnesium alloy material, wherein provided that elongation (%) measured by JIS standard tensile test is defined as (x) and 0.2% yield strength (MPa) is defined as (y), the following formulae are preferably satisfied.
(−15.57x)+467<y<(−15.57x)+555, and x<20
Since elongation and 0.2% yield strength have a predetermined relationship, the above-mentioned composition can be applied to automobile parts such as engine pistons being required strict conditions on mechanical properties.
The process for producing a magnesium alloy material according to the present invention comprises a dissolving/casting step of dissolving and casting the Mg—Gd—Zn alloy according to claim 1 to provide a cast material and a plastic forming step of subjecting the cast material to hot plastic forming at a predetermined processing rate to produce a processed material.
In the procedure, the cast material is subjected to the hot plastic forming at the predetermined processing rate to locally (at the predetermined area ratio) fracture the long period stacking ordered structure formed in the dissolving/casting process, and Mg₃Gd and/or Mg₃Zn₃Gd₂precipitate/precipitates in the fractured grains.

ADVANTAGES OF THE INVENTION

Since the alloy structure of the Mg—Zn—Gd alloy has the long period stacking ordered structure and has Mg₃Gd and/or Mg₃Zn₃Gd₂, the magnesium alloy material according to the present invention can achieve high tensile strength and yield strength while sustaining high elongation. In addition, since the magnesium alloy material does not require special production equipment or processes, the magnesium alloy material has better productivity. Since the area ratio of Mg₃Gd and/or Mg₃Zn₃Gd₂is within the predetermined range, higher elongation can be achieved. Since high elongation can be achieved, the magnesium alloy material having excellent processability can be obtained. Furthermore, since elongation and 0.2% yield strength have the predetermined relationship, the magnesium alloy material according to the present invention can be applied to, for example, the automobile parts, particularly engine pistons being required strict conditions on mechanical properties.
The process for producing the magnesium alloy material according to the present invention can subject the cast material to the hot plastic forming at the predetermined processing rate to efficiently produce the magnesium alloy material having enhanced mechanical properties using general production equipment or processes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an optical micrograph showing the alloy structure of a magnesium alloy material (processed material) according to the present invention produced at an extruding rate of 2.5 mm/sec;

FIG. 2 is an optical micrograph showing the alloy structure of the magnesium alloy material (processed material) according to the present invention produced at the extruding rate of 5.0 mm/sec;

FIG. 3 is an optical micrograph showing the alloy structure of the magnesium alloy material (processed material) according to the present invention produced at the extruding rate of 7.5 mm/sec;

FIG. 4 is a graph showing the relationship between elongation and 0.2% yield strength of the magnesium alloy material (processed material);

FIG. 5 is a longitudinal sectional view showing the distribution of equivalent strain of the magnesium alloy material (processed material) according to the present invention;

FIG. 6 is an optical micrograph of a cross-section orthogonal to the processing direction of the magnesium alloy material after an extruding process (after tensile test);

FIG. 7 is an optical micrograph in subjecting precipitation regions of Mg₃Gd and/or Mg₃Zn₃Gd₂of the micrograph of FIG. 6 to black image processing; and

FIG. 8 is an optical micrograph in subjecting the micrograph of FIG. 7 to image processing for digitizing the micrograph to black or white.

DESCRIPTION OF THE SYMBOLS

- 10A: region having equivalent strain of 1.5 or more
- 10B: region having equivalent strain of less than 1.5 and 0.25 or more
- 10C: region having equivalent strain of less than 0.25

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, preferred embodiments for carrying out the present invention will be described with reference to the drawings.
FIGS. 1 to 3 are optical micrographs showing the alloy structure of a magnesium alloy material (processed material) in changing an extruding rate. In FIG. 1, the extruding rate is 2.5 mm/sec. In FIG. 2, the extruding rate is 5.0 mm/sec. In FIG. 3, the extruding rate is 7.5 mm/sec. FIG. 4 is a graph showing the relationship between elongation and 0.2% yield strength of the magnesium alloy material. FIG. 5 is a longitudinal sectional view showing the distribution of equivalent strain of the magnesium alloy material (processed material). FIGS. 6 to 8 are optical micrographs showing a process for calculating the area ratio of Mg₃Gd and/or Mg₃Zn₃Gd₂. FIG. 6 is an optical micrograph of a cross-section orthogonal to the processing direction of the magnesium alloy material after an extruding process (after tensile test). FIG. 7 is an optical micrograph in subjecting precipitation regions of Mg₃Gd and/or Mg₃Zn₃Gd₂to black image processing. FIG. 8 is an optical micrograph in subjecting the micrograph to image processing for digitizing the micrograph to black or white.
A magnesium alloy material according to the present invention is used in parts used in high-temperature environments such as the automobile parts, particularly pistons, valves, lifters, tappets and sprockets or the like for internal combustion engines. Examples of the shape of the magnesium alloy material include a plate shape and a rod shape, and the shape is suitably selected according to the shape of the part in which it is used.
The magnesium alloy material is composed of an Mg—Gd—Zn alloy comprising 1 to 5% by mass of Zn and 5 to 15% by mass of Gd as essential components, with the remainder being Mg and unavoidable impurities. The following provides a detailed description of each component.

[Alloy Components]

(Zn)

The Mg—Gd—Zn alloy contains 1 to 5% by mass of Zn as essential components. When the amount of Zn is less than 1% by mass, it is not possible to obtain Mg₃Gd, precluding the desired tensile strength and 0.2% yield strength (strength) of the magnesium alloy material. Even when the amount of Zn exceeds 5% by mass, increases in tensile strength and 0.2% yield strength corresponding to increase in the amount of Zn added are not obtained. In addition, Mg₃Gd and Mg₃Zn₃Gd₂or the like which precipitate in grain boundaries increase, and elongation decreases.

(Gd)

The Mg—Gd—Zn alloy contains 5 to 15% by mass of Gd as essential components. When the amount of Zn is less than 1% by mass, it is not possible to obtain Mg₃Gd, precluding the desired tensile strength and 0.2% yield strength (strength) of the magnesium alloy material. Even when the amount of Zn exceeds 5% by mass, increases in tensile strength and 0.2% yield strength corresponding to increase in the amount of Zn added are not obtained. In addition, Mg₃Gd and Mg₃Zn₃Gd₂or the like which precipitate in the grain boundaries increase, and elongation decreases.

(Selected Addition Elements)

The Mg—Gd—Zn alloy can contain 0.2 to 1.0% by mass of Zr contributing to fine granulation. When the amount of Zr is less than 0.2% by mass, fine granulating effect is apt to decrease. When the amount of Zr exceeds 1.0% by mass, it is hard to obtain increases in tensile strength and 0.2% yield strength corresponding to the fine granulating effect.

(Unavoidable Impurities)

Other components may be added to the Mg—Gd—Zn alloy excluding the components described above within the acceptable range as unavoidable impurities. For example, Fe, Ni, Cu, Si or the like may also be respectively contained at 0.2% by mass or less.
As shown in FIGS. 1 to 3, in the magnesium alloy material, the alloy structure of the Mg—Gd—Zn alloy has a long period stacking ordered structure (LPSO) and has Mg₃Gd and/or Mg₃Zn₃Gd₂. The area ratio of Mg₃Gd and/or Mg₃Zn₃Gd₂in crystal grains is preferably 53 or less.

(Long Period Stacking Ordered Structure)

As shown in FIGS. 1-3, the long period stacking ordered structure (abbreviated as LPSO) precipitates in the grains and grain boundaries of the magnesium alloy material in a dissolving/casting process. Particularly, the long period stacking ordered structure having a high concentration lamellarly exists in the grain boundaries. The precipitation of the long period stacking ordered structure enhances tensile strength and 0.2% yield strength of the magnesium alloy material. In the extruding process, the long period stacking ordered structure is locally fractured by process heating, and Mg₃Gd and/or Mg₃Zn₃Gd₂precipitate/precipitates in the fractured grains.
The long period stacking ordered structure refers to a structure in which an a plurality of ordered lattices are arranged, a plurality of ordered lattices are again arranged with a shift of an anti-phase, unit structures are formed that are several times to ten or more times the original lattice, and the period thereof is long. The long period stacking ordered structures appear within a slight temperature range between the ordered phase and disordered phase, the reflection of the ordered phase is split in electron diffraction patterns, and diffraction spots appear at locations corresponding to periods several times to ten or more times longer.
(Mg₃Gd and/or Mg₃Zn₃Gd₂)
Since slip system in Mg alloys are generally few, it is hard to subject the Mg alloys to plastic forming. On the other hand, the Mg alloy has a feature to be easily twinning-deformed. Since the Mg—Gd—Zn alloy of the present invention has a crystal face having the long period stacking ordered structure, it cannot be twinning-deformed. Therefore, when the Mg—Gd—Zn alloy is subjected to the extruding process, local plastic flow occurs in the alloy due to few slip system in Mg—Gd—Zn alloys. Heating caused by processing is increased by the local plastic flow, and this heating causes dynamic recrystallization. As shown in FIGS. 1 to 3, this dynamic recrystallization fractures the long period stacking ordered structure locally, and Mg₃Gd and/or Mg₃Zn₃Gd₂precipitate/precipitates in the fractured grains.
Although Mg₃Gd and Mg₃Zn₃Gd₂having a size of 100 to 400 nm, are very fine, Mg₃Gd and Mg₃Zn₃Gd₂have no consistency with the matrix. Therefore, Mg₃Gd and Mg₃Zn₃Gd₂do not contribute to enhancement in tensile strength and yield strength of the magnesium alloy material. However, Mg₃Gd and Mg₃Zn₃Gd₂contribute to enhancement in elongation. Increase in the precipitating amount of Mg₃Gd and/or Mg₃Zn₃Gd₂enhances considerable elongation.
The area ratio of Mg₃Gd and/or Mg₃Zn₃Gd₂is preferably 53% or less of the entire alloy structure, and more preferably 4 to 53%. When the area ratio exceeds 53%, tensile strength and 0.2% yield strength are largely reduced, and it is hard to obtain strength required as the automobile parts. When the area ratio is less than 4%, it is hard to obtain elongation required as automobile parts. The area ratio is controlled based on the processing rate in a plastic forming process in producing the magnesium alloy material. The faster the processing rate is, the larger the area ratio is (see FIGS. 1 to 3).
As shown in FIG. 4, when elongation (%) measured by JIS standard tensile tests is defined as (x) and 0.2% yield strength (MPa) is defined as (y) in the magnesium alloy material according to the present invention, the following formulae are more preferably satisfied.
(−15.57x)+467<y<(−15.57x)+555, and x<20
“The present invention examples” in FIG. 4 is data of results obtained by carrying out tests as described in the following Examples. A part of the data are shown in Table 1 of the following examples. “Existing materials” is extrusion materials made of an ordinary Mg practical alloy (AZ31).
If elongation and 0.2% yield strength satisfy the above-mentioned relationship, it becomes easier to apply the magnesium alloy material according to the present invention to automobile parts such as engine pistons being required strict conditions on mechanical properties. The above-mentioned relationship between elongation and 0.2% yield strength is achieved by adjusting the area ratio (precipitating amount) of Mg₃Gd and/or Mg₃Zn₃Gd₂while considering the component composition of the magnesium alloy.
Next, a process for producing the magnesium alloy material according to the present invention will be described.
The process for producing the magnesium alloy material includes a dissolving/casting step and a plastic forming step. The following provides a description of each step.
(Dissolving/Casting Step)
An Mg—Gd—Zn alloy containing 1 to 5% by mass of Zn and 5 to 15% by mass of Gd with the remainder being Mg and unavoidable impurities, is dissolved and cast to obtain a cast material. A long period stacking ordered structure lamellarly precipitates in the grains and grain boundaries of the magnesium alloy material in dissolving and casting. The long period stacking ordered structure having a high concentration precipitates in the grain boundaries. The precipitation of the long period stacking ordered structure enhances tensile strength and 0.2% yield strength of the magnesium alloy material. If the Mg—Gd—Zn alloy is cast, “the long period stacking ordered structure” is necessarily obtained regardless of casting conditions. Therefore, dissolving and casting are carried out in accordance with ordinary methods. In addition, dissolving is preferably flux refining in order to remove oxides from the molten metal.
The obtained cast material may be subjected to homogenization heat treatment. Among the above-mentioned lamellar structure (the long period stacking ordered structure having a high concentration), the structure precipitating in the grain boundaries of the casting structure by the homogenization heat treatment disappears, and tensile strength and elongation of the magnesium alloy material become higher. In this case, the temperature of the homogenization heat treatment is preferably 480° C. or higher, and the holding time thereof is preferably 1 hour or more. When the temperature of the homogenization heat treatment is less than 480° C. or the holding time is less than 1 hour, the solution of the lamellar structure is difficult to proceed, and the lamellar structure tends to remain in the grain boundaries of the casting structure. Therefore, the mechanical properties of the magnesium alloy material are difficult to be enhanced.

(Plastic Forming Step)

The cast material produced in the above-mentioned step or the cast material subjected to the homogenization heat treatment is subjected to hot plastic forming at a predetermined processing rate. Herein, the hot plastic forming is preferably the extruding process and/or the forging process. The long period stacking ordered structure produced by casting is locally fractured in process heating by subjecting the cast material to the hot plastic forming at the predetermined processing rate. That is, the long period stacking ordered structure is finely divided and is changed to a dot-like structure. Mg₃Gd and/or Mg₃Zn₃Gd₂precipitate/precipitates in the grains in which the long period stacking ordered structure is fractured. As shown in FIGS. 1 to 3, when the processing rate is fast, the long period stacking ordered structure is largely fractured, and the precipitating amount of Mg₃Gd and/or Mg₃Zn₃Gd₂, that is, the area ratio of Mg₃Gd and/or Mg₃Zn₃Gd₂in the alloy structure increases. The precipitation of Mg₃Gd and/or Mg₃Zn₃Gd₂enhances elongation of the magnesium alloy material.
The processing rate of the hot plastic forming is preferably 2.7 to 21 mm/sec in the extruding process. When the processing rate is less than 2.7 mm/sec, the area ratio decreases, and it is hard to obtain predetermined elongation required as the automobile parts. When the processing rate exceeds 21 mm/sec, the area ratio increases, and enhancement in elongation is observed. However, tensile strength and 0.2% yield strength decrease, and it is hard to obtain strength required as the automobile parts. Even in the forging process, the processing rate is preferably 2.7 to 21 mm/sec. The reason for setting the numerical value range is the same as that of the extruding process.
As shown in FIG. 5, in the process for producing the magnesium alloy material according to the present invention, it is preferable that a processed material 10 produced by the hot plastic forming has at least a portion 10A having equivalent strain of 1.5 or more. When the equivalent strain is less than 1.5, variation in the mechanical properties of the magnesium alloy material is apt to increase. When the processed material is used for the automobile parts or the like, a portion requiring high mechanical properties contains the portion 10A having equivalent strain of 1.5 or more. Therefore, it is preferable to subject the processed material 10 to the hot plastic forming so that portions 10B and 10C having equivalent strain of less than 1.5 are not formed, and the whole portion of the processed material 10 has equivalent strain of 1.5 or more. Herein, the processed material 10 is obtained by freely forging a cast material having a pillar shape. FIG. 5 shows the distribution of equivalent strain in a longitudinal cross-sectional view in the planar view of the processed material 10.
Equivalent strain refers to the equivalent strain corresponding to Von Mieses yield stress, and is calculated according to the following formula (1). In the formula (1), equivalent strain is represented by (ε), true strain in the direction of length by (ε₁), true strain in the direction of width by (ε₂), and true strain in the direction of thickness by (ε₃).
$\begin{matrix} [Formula 1] \\ ɛ = \sqrt{\frac{2}{3} (ɛ_{1}^{2} + ɛ_{2}^{2} + ɛ_{3}^{2})} & (1) \end{matrix}$
Although there are no particular limitations on the upper limit of equivalent strain, since the tensile strength, yield strength and elongation of the magnesium alloy material decrease if the applied equivalent strain is excessively high, it is preferably less than 2.3, and more preferably within the range of 1.5 to 2.0.
In the process for producing the magnesium alloy material according to the present invention, it is preferable that the processing temperature when carrying out the hot plastic forming is suitably selected corresponding to the working ratio of the cast material within a range of 300 to 500° C.
When an extruding process is used for the hot plastic forming, carrying out extrusion at an extrusion temperature of 300 to 500° C. and an extrusion ratio within the range of 5 to 9.9, and preferably 6 to 9, allows the obtaining of a magnesium alloy material having good equivalent strain and excellent mechanical properties.
When the hot plastic forming is a forging process, by carrying out forging under the condition represented by the following formula (2), good equivalent strain can be obtained and fine crystal grains can be attempted while preventing cracking of the cast material. When necessary equivalent strain is not obtained by only the forging process, the above-mentioned extruding process may be carried out prior to the forging process.
[Formula 2]
T≧2E+210 (2)
In the formula (2), T(° C.) represents the temperature at completion of forging, while E(%) represents the working ratio.
In the case of applying equivalent strain to a processed material in a forging process, the temperature at completion of forging and the working ratio become suitable and there is no occurrence of cracking during the forging process by carrying out forging so as to satisfy predetermined conditions. In other words, in the case of the temperature at completion of forging (T) not reaching a temperature having the value calculated by adding 210 to twice the working ratio (E), cracking tends to occur easily during forging, thereby making this unsuitable. In the case the temperature at completion of forging (T) is excessively high, fine crystal subgrains formed by plastic forming grow by dynamic treatment process and cause a decrease in the mechanical properties of the magnesium alloy material. Thus, the upper limit of the temperature at completion of forging (T) is preferably the temperature having the value calculated by adding 310 to twice the working ratio (E).
A stabilization treatment process in which the magnesium alloy material (processed material) is held at a temperature of 200 to 300° C. for 10 hours or more may be added to the process for producing the magnesium alloy material according to the present invention after carrying out the plastic forming process for the purpose of stabilizing the dimensions of the magnesium alloy material (processed material). Improvement in dimensional stability is preferable in terms of enabling the magnesium alloy material according to the present invention to be applied to products used while being subjected to the effects of heat, such as pistons, valves, lifters, tappets and sprockets for internal combustion engines.
When the plastic forming process is a forging process, a cutting process may be carried out for cutting a processed material to a predetermined shape such as that of a piston, valve, lifter, tappet or sprocket for an internal combustion engine as necessary following the stabilization treatment process for stabilizing dimensions as described above.

EXAMPLES

Examples of the present invention will be described hereinafter.

Examples 1 to 6

First, materials were weighed so that an alloy composition of Mg—Gd (12.9% by mass)-Zn (2.7% by mass)-Zr (0.6% by mass) is obtained. The materials were placed in a melting furnace and dissolved by flux refining. Continuing, the heated and dissolved material was cast in a metal mold (outer diameter φ: 150 mm) to produce an ingot. The ingot was subjected to homogenization heat treatment at 510° C. for 4 hours, and was subjected to a machining process to produce a cast material for an extruding process. Next, the cast material was placed in an extruding machine, and was subjected to the extruding process while changing an extruding rate to produce magnesium alloy materials (Examples 1 to 6, outer diameter φ: 6 mm). The magnesium alloy materials were produced while the extruding process temperature of 375° C. and the extrusion ratio of 9 were maintained constant.
Using liquid penetrant fluorescent test or the like, it was confirmed that no crack in the magnesium alloy materials (processed materials) after the extruding process. JIS No. 4 test pieces were cut out of the processed materials, and tensile strength, yield strength (0.2%) and elongation (%) thereof were measured based on JIS standard tensile tests. The results are shown in Table 1. When tensile strength is 250 MPa or more, when 0.2% yield strength is 150 MPa or more, or when elongation is 8% or more, the magnesium alloy materials can be applied as automobile parts.
After polishing the extruding cross-sections of test pieces after the tensile test with #120 to #1000 sandpaper, the extruding cross-sections were mirrored by buffing with alumina or the like, after which the mirrored surfaces were etched with aqueous glycol acetate or the like to prepare surfaces for observation of structure. These surfaces for observation of structure were observed with an optical microscope at a magnification of 400 times, and the area ratio in which Mg₃Gd and/or Mg₃Zn₃Gd₂occupy was calculated by image processing from the cross-sectional micrographs. The specific method will be described with reference to FIGS. 6 to 8. In the cross-section micrograph (FIG. 6) after the tensile test, a region where Mg₃Gd and/or Mg₃Zn₃Gd₂precipitate/precipitates was subjected to black image processing (FIG. 7). The micrograph (FIG. 7) subjected to image processing was subjected to image processing for digitizing the micrograph to black or white (FIG. 8), and the area ratio of the region where Mg₃Gd and/or Mg₃Zn₃Gd₂precipitate/precipitates to the whole alloy structure was calculated. The results are shown in Table 1. Mg₃Gd and Mg₃Zn₃Gd₂were confirmed by TEM.

TABLE 1

Cast article	Extruding	Area ratio of	Tensile	0.2% yield
diameter	rate	Mg₃Gd and/or	strength	strength	Elongation
(mm)	(mm/sec)	Mg₃Zn₃Gd₂(%)	(MPa)	(MPa)	(%)

Example 1	150	2.5	1.5	395	337	9.6
Example 2	150	2.5	1.5	396	340	9.5
Example 3	150	5	19.1	366	288	13.3
Example 4	150	5	19.1	367	290	13.6
Example 5	150	7.5	26.2	353	265	14.3
Example 6	150	7.5	26.2	352	264	14.6

From the results of Table 1, it was confirmed that the magnesium alloy materials (Examples 1 to 6) according to the present invention had high tensile strength and 0.2% yield strength, and had high elongation.

Claims

1. A magnesium alloy material comprising an Mg—Gd—Zn alloy including 1 to 5% by mass of Zn and 5 to 15% by mass of Gd as essential components, with the remainder being Mg and unavoidable impurities,

wherein the Mg—Gd—Zn alloy has an alloy structure having a long period stacking ordered structure and having 4% or more of Mg₃Gd and/or Mg₃Zn₃Gd₂.

2. The magnesium alloy material according to claim 1, wherein an area ratio of Mg₃Gd and/or Mg₃Zn₃Gd₂in the alloy structure is 53% or less.

3. The magnesium alloy material according to claim 1, wherein provided that elongation (%) measured by JIS standard tensile test is defined as (x) and 0.2% yield strength (MPa) is defined as (y), the following formulae are satisfied.

(−15.57x)+467<y<(−15.57x)+555, and x<20

4. A process for producing a magnesium alloy material comprising:

a dissolving/casting step of dissolving and casting the Mg—Gd—Zn alloy according to claim 1 to provide a cast material; and

a plastic forming step of subjecting the cast material to hot plastic forming at a predetermined processing rate to produce a processed material.