# Patent

## Title
High-strength and high-toughness magnesium alloy and method for manufacturing same

## Abstract

Provided is a high-strength and high-toughness magnesium alloy which has practical level of both the strength and the toughness for expanded applications of the magnesium alloys, and is a method for manufacturing thereof. The high-strength and high-toughness magnesium alloy of the present invention contains: a atom % in total of at least one metal of Cu, Ni, and Co; and b atom % in total of at least one element selected from the group consisting of Y, Dy, Er, Ho, Gd, Tb, and Tm, while a and b satisfying the following formulae (1) to (3),

0.2≦a≦10  (1)

0.2≦b≦10  (2)

2/3a−2/3<b.  (3)

## Background
TECHNICAL FIELD
The present invention relates to a high-strength and high-toughness magnesium alloy and a method for manufacturing thereof, more particularly to a high-strength and high-toughness magnesium alloy which attained the high strength and high toughness by containing a specific amount of specified rare-earth element thereto, and to a method for manufacturing thereof.
BACKGROUND ART
Magnesium alloys have begun to be rapidly in widespread use as casing of cell phones or laptop computers, and as automobile parts, along with the recycling performance thereof.
To be used for these uses, however, the magnesium alloys are required to have high strength and high toughness. For the manufacture of high-strength and high-toughness magnesium alloy, various studies have been conducted from the viewpoint of materials and the like.
According to a disclosure of the inventors of the present invention, an ingot of magnesium alloy having a composition of 97 atom % Mg-1 atom % Zn-2 atom % Y forms a long-period stacking ordered structure therein, and high strength and high toughness are obtained at room temperature by applying extrusion working to the ingot, (for example, refer to Patent Document 1).

    [Patent Document 1] WO 2005/052203


DISCLOSURE OF THE INVENTION
Above-described conventional high-strength and high-roughness magnesium alloys are essential requirements for containing Zn therein. To this point, the inventors of the present invention have studied whether a magnesium alloy, in which Zn is substituted with other metal, can provide high strength and high toughness.
The present invention has been perfected taking into account the above situations, and an object of the present invention is to provide a high-strength and high-toughness magnesium alloy which has practical level of both the strength and the toughness for expanded applications of the magnesium alloys, and to provide a method for manufacturing thereof.
To solve the above problems, the high-strength and high-toughness magnesium alloy in the present invention contains: a atom % in total of at least one metal of Cu, Ni, and Co; and b atom % in total of at least one element selected from the group consisting of Y, Dy, Er, Ho, Gd, Tb, and Tm, while a and b satisfying the following formulae (1) to (3), and more preferably a and b satisfying the following formulae (1′) to (3′),

0.2≦a≦10  (1)

0.2≦b≦10  (2)

2/3a−2/3<b  (3)

0.2≦a≦5  (1′)

0.2≦b≦5  (2′)

2/3a−1/6<b.  (3′)

The high-strength and high-toughness magnesium alloy in the present invention can also have a long-period stacking ordered structure phase.
The high-strength and high-toughness magnesium alloy in the present invention can also have an α-Mg phase, and the α-Mg phase can also have a lamellar structure.
The high-strength and high-toughness magnesium alloy in the present invention can also have a compound phase.
The high-strength and high-toughness magnesium alloy in the present invention is a magnesium alloy cast, and the magnesium alloy cast can also be heat-treated.
The high-strength and high-toughness magnesium alloy in the present invention can also be a plastic work product obtained by applying plastic-working of the magnesium alloy cast.
The high-strength and high-toughness magnesium alloy in the present invention is composed of a plastic work product having a long-period stacking ordered structure phase, which plastic work product is manufactured by preparing a magnesium alloy cast having a atom % in total of at least one metal of Cu, Ni, and Co, and b atom % in total of at least one element selected from the group consisting of Y, Dy, Er, Ho, Gd, Tb, and Tm, while a and b satisfying the following formulae (1) to (3), then by cutting the magnesium alloy cast into chip-shaped casts, and then by solidifying the casts by plastic-working, and preferably is manufactured thereby while a and b satisfying the following formulae (1′) to (3′),

0.2≦a≦10  (1)

0.2≦b≦10  (2)

2/3a−2/3<b  (3)

0.2≦a≦5  (1′)

0.2≦b≦5  (2′)

2/3a−1/6<b.  (3′)

The high-strength and high-toughness magnesium alloy in the present invention is composed of a plastic work product having a long-period stacking ordered structure phase, which plastic work product is manufactured by preparing a magnesium alloy cast having a atom % in total of at least one metal of Cu, Ni, and Co, and b atom % in total of at least one element selected from the group consisting of Y, Dy, Er, Ho, Gd, Tb, and Tm, while a and b satisfying the following formulae (1) to (3), then by carrying out plastic-working of the magnesium alloy cast, and preferably is manufactured thereby while a and b satisfying the following formulae (1′) to (3′),

0.2≦a≦10  (1)

0.2≦b≦10  (2)

2/3a−2/3<b  (3)

0.2≦a≦5  (1′)

0.2≦b≦5  (2′)

2/3a−1/6<b.  (3′)

Regarding the high-strength and high-toughness magnesium alloy in the present invention, the high-strength and high-toughness magnesium alloy can also be heat-treated.
Regarding the high-strength and high-toughness magnesium alloy in the present invention, the plastic work product can also be heat-treated.
Regarding the high-strength and high-toughness magnesium alloy in the present invention, the plastic work product can have an α-Mg phase, and the α-Mg phase can have a lamellar structure.
Regarding the high-strength and high-toughness magnesium alloy in the present invention, the plastic work product can also have a compound phase.
Regarding the high-strength and high-toughness magnesium alloy in the present invention, the plastic-working preferably includes at least one of rolling, extruding, ECAE, drawing, forging, pressing, form-rolling, bending, FSW working, and repeating thereof.
Regarding the high-strength and high-toughness magnesium alloy in the present invention, the plastic-working preferably gives an amount of equivalent strain per at least one cycle thereof within the range of more than zero to not more than 5.
The high-strength and high-toughness magnesium alloy in the present invention is composed of a powder, a sheet, or a thin wire, which is prepared by forming a liquid having a composition containing a atom % in total of at least one metal of Cu, Ni, and Co, and b atom % in total of at least one element selected from the group consisting of Y, Dy, Er, Ho, Gd, Tb, and Tm, with a and b satisfying the following formulae (1) to (3), then by rapidly cooling the liquid to coagulate, and more preferably by forming a liquid having a composition in which a and b satisfy the following formulae (1′) to (3′),

0.2≦a≦10  (1)

0.2≦b≦10  (2)

2/3a−2/3<b  (3)

0.2≦a≦5  (1′)

0.2≦b≦5  (2′)

2/3a−1/6<b.  (3′)

Regarding the high-strength and high-toughness magnesium alloy in the present invention, the powder, the sheet, or the thin wire can also have a crystal structure of long-period stacking ordered structure phase.
Regarding the high-strength and high-toughness magnesium alloy in the present invention, the powder, the sheet, or the thin wire can also have an α-Mg phase, and the α-Mg phase can also have a lamellar structure.
Regarding the high-strength and high-toughness magnesium alloy in the present invention, the powder, the sheet, or the thin wire can also have a compound phase.
Regarding the high-strength and high-toughness magnesium alloy in the present invention, the powder, the sheet, or the thin wire can also be the one solidified so that shear is applied thereto.
Regarding the high-strength and high-toughness magnesium alloy in the present invention, the long-period stacking ordered structure phase can also kink.
Regarding the high-strength and high-toughness magnesium alloy in the present invention, the Mg can be added with c atom % of Zn, while the a and c can also satisfy the following formula (4), and more preferably the a and c satisfy the following formula (4′),

0.2<a+c≦15  (4)

0.2<a+c≦5.  (4′)

Regarding the high-strength and high-toughness magnesium alloy in the present invention, the a and c can also further satisfy the following formula (5),

c/a≦1/2.  (5)

Regarding the high-strength and high-toughness magnesium alloy in the present invention, the Mg can also be added with d atom % in total of at least one element selected from the group consisting of La, Ce, Pr, Nd, Sm, Eu, Yb, and Lu, while the b and d can also satisfy the following formula (6), and more preferably the b and d satisfy the following formula (6′),

0.2<b+d≦15  (6)

0.2<b+d≦5.  (6′)

Regarding the high-strength and high-toughness magnesium alloy in the present invention, b and d can also further satisfy the following formula (7),

d/b≦1/2.  (7)

Regarding the high-strength and high-toughness magnesium alloy in the present invention, the Mg can also be added with e atom % in total of at least one element selected from the group consisting of Zr, Ti, Mn, Al, Ag, Sc, Sr, Ca, Si, Hf, Nb, B, C, Sn, Au, Ba, Ge, Bi, Ga, In, Ir, Li, Pd, Sb, V, Fe, Cr, and Mo, while e can satisfy the following formula (8),

0<e≦2.5.  (8)

Regarding the high-strength and high-toughness magnesium alloy in the present invention, e, a, b, and d can also further satisfy the following formula (9),

e/(a+b+c+d)≦1/2.  (9)

The method for manufacturing high-strength and high-toughness magnesium alloy in the present invention has the steps of: preparing a magnesium alloy cast containing a atom % in total of at least one metal of Cu, Ni, and Co, and b atom % in total of at least one element selected from the group consisting of Y, Dy, Er, Ho, Gd, Tb, and Tm, while a and b satisfying the following formulae (1) to (3); and preparing a plastic work product by carrying out plastic-working of the magnesium alloy cast, and more preferably has the step of preparing a magnesium alloy cast in which a and b satisfy the following formulae (1′) to (3′),

0.2≦a≦10  (1)

0.2≦b≦10  (2)

2/3a−2/3<b  (3)

0.2≦a≦5  (1′)

0.2≦b≦5  (2′)

2/3a−1/6<b.  (3′)

The high-strength and high-toughness magnesium alloy in the present invention can also further have the step of cutting the magnesium alloy cast between the step of preparing the magnesium alloy cast and the step of preparing the plastic work product.
The method for manufacturing high-strength and high-toughness magnesium alloy in to the present invention can also further comprise the step of conducting heat treatment of the magnesium alloy cast after the step of preparing the magnesium alloy cast.
The method for manufacturing high-strength and high-toughness in the present invention can also further comprise the step of conducting heat treatment of the plastic work product after the step of preparing the plastic work product.
The method for manufacturing high-strength and high-toughness magnesium alloy in the present invention has the steps of: preparing a liquid having a composition containing a atom % in total of at least one metal of Cu, Ni, and Co, and b atom % in total of at least one element selected from the group consisting of Y, Dy, Er, Ho, Gd, Tb, and Tm, while a and b satisfying the following formulae (1) to (3); and forming a powder, a sheet, or a thin wire by rapidly cooling the liquid to coagulate, then by solidifying the powder, the sheet, or the thin wire so that shear is applied thereto, and more preferably preparing a liquid having a composition in which a an b satisfy the following formulae (1′) to (3′),

0.2≦a≦10  (1)

0.2≦b≦10  (2)

2/3a−2/3<b  (3)

0.2≦a≦5  (1′)

0.2≦b≦5  (2′)

2/3a−1/6<b.  (3′)

As described above, the present invention can provide a high-strength and high-toughness magnesium alloy which has practical level of both the strength and the toughness for expanded applications of the magnesium alloys, and to provide a method for manufacturing thereof.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1(A) is an SEM micrograph of an ingot of Mg97Co1Y2 alloy, FIG. 1(B) is an SEM micrograph of an ingot of Mg97Ni1Y2 alloy, and FIG. 1(C) is an SEM micrograph of an ingot of Mg97Cu1Y2 alloy.
FIG. 2 shows a TEM micrograph of a long-period stacking ordered structure phase of an ingot of Mg97Cu1Y2 alloy, and a diagram of electron beam diffraction on [1120].
FIG. 3 shows a result of tensile test for the extruded materials of Mg97X1Y2 (X═Fe, Co, Ni, or Cu) alloy at room temperature, which are the materials of Example 1 and Comparative Example 1.
FIG. 4 shows a result of tensile test for the extruded materials of Mg97X1Y2 (X═Fe, Co, Ni, or Cu) alloy at 473K, which are the materials of Example 1 and Comparative Examples.
FIG. 5 illustrates the system preparing rapidly coagulated powder by the gas-atomizing method, and manufacturing an extrusion billet.
FIG. 6 illustrates the process of heating and pressing, thus solidifying and forming the billet.
FIG. 7 is an SEM micrograph of an ingot of Mg85Cu6Y9 alloy in Example 2.
FIG. 8 is an SEM micrograph of an ingot of Mg85Ni6Y9 alloy in Example 2.
FIG. 9 is an SEM micrograph of an ingot of Mg85CO6Y9 alloy in Example 2.
FIG. 10 shows a TEM micrograph of a long-period stacking ordered structure phase of an ingot of Mg85Cu6Y9 alloy in Example 2.
FIG. 11 shows a diffraction pattern of a long-period stacking ordered structure phase of 18R type formed in an ingot of Mg85Cu6Y9 alloy in Example 2.
FIG. 12 shows a diffraction pattern of a long-period stacking ordered structure phase of 10H type formed in an ingot of Mg85Cu6Y9 alloy in Example 2.
FIG. 13 shows a TEM micrograph and an electron beam diffraction pattern of a heat-treated Mg91Cu3Y6 alloy in Example 3.



DESCRIPTION OF THE REFERENCE SYMBOLS













100
high pressure gas atomizer


110
melting chamber


112
stopper


114
induction coil


116
crucible


130
atomizing chamber


131
heater


132
nozzle


140
cyclone classifier


150
filter


162, 166
oxygen analyzer


164
vacuum gauge


200
vacuum glove box


210
argon gas refiner


220
hopper


230
sieve


240
vacuum hot press


242
vacuum chamber


244
punch


246
die


248
heater


252
cap


254
can


256
welding machine


258
rotary disk


260
billet


262
valve


270
oxidation box


280
entrance box


292
vacuum gauge


294
hygrometer


296
oxygen analyzer


340
spot-welding machine


400
extrusion press


410
heater


420
container


430
die


450
main stem


460
die backer


470
back stem









BEST MODE FOR CARRYING OUT THE INVENTION
The embodiments of the present invention are described below.
The inventors of the present invention have substituted Zn in Mg—Zn-RE (rare earth element) alloys with other metals, and investigated strength and toughness thereof, and found that there are attained magnesium alloys having high level of both the strength and the toughness even when Zn is substituted with other metals, and also found that there are attained higher strength and toughness than ever with the magnesium alloys of a series of Mg-(substituted metal)-RE (rare earth element), in which the substituted metal is at least one metal of Cu, Ni, and Co, and the rare earth element is at least one element selected from the group consisting of Y, Dy, Er, Ho, Gd, Tb, and Tm, and further the content of the substituted metal is as low as 5 atom % or less, and the content of the rare earth element is as low as 5 atom % or less.
The inventors of the present invention have further found that plastic-working of a metal having a long-period stacking ordered structure phase can curve or bend at least a part of the long-period stacking ordered structure phase, thereby obtaining a metal having high strength, high ductility, and high toughness.
The inventors of the present invention have found that a cast alloy forming a long-period stacking ordered structure phase provides a magnesium alloy having high strength, high ductility, and high toughness, after plastic-working or by conducting heat treatment after plastic-working. Also the inventors of the present invention have found an alloy composition which forms a long-period stacking ordered structure and provides high strength, high ductility, and high toughness after plastic-working, or after both plastic-working and subsequent heat treatment.
Furthermore, the inventors of the present invention have found that even an alloy which does not form a long-period stacking structure phase in a state immediately after casting, forms a long-period stacking structure phase by conducting heat treatment to the alloy. The inventors of the present invention have found an alloy composition which provides high strength, high ductility, and high toughness by carrying out plastic-working or by conducting heat treatment after plastic-working thereof.
Furthermore, it was found that, by preparing chip-shaped casts by cutting a cast alloy in which a long-period stacking ordered structure is formed, and by carrying out plastic-working thereof or by conducting heat treatment thereof after plastic-working, there is attained a magnesium alloy having higher strength, higher ductility, and higher toughness, compared with the case where a process of cutting into chip-shape in not conducted. Further, the inventors of the present invention have found an alloy composition which provides high strength, high ductility, and high toughness by forming a long-period stacking ordered structure, by cutting the alloy in chip shape, and then carrying out plastic-working or by conducting heat treatment after plastic-working thereof.
Embodiment 1
The magnesium alloy according to the Embodiment 1 of the present invention is an alloy of ternary or higher order, containing at least one metal of Cu, Ni, and Co, and containing rare earth elements that are one or more elements selected from the group consisting of Y, Dy, Er, Ho, Gd, Tb, and Tm.
The composition range of the magnesium alloy according to the Embodiment 1 is the one in which a and b satisfy the following formulae (1) to (3), and more preferably a and b satisfy the following formulae (1′) to (3′), (the total content of the above-described one metal is defined as a atom %, while the total content of the above-described one or more rare earth elements is defined as b atom %),

0.2≦a≦10  (1)

0.2≦b≦10  (2)

2/3a−2/3<b  (3)

0.2≦a≦5  (1′)

0.2≦b≦5  (2′)

2/3a−1/6<b.  (3′)

The reason of above is that, if the total content of above-described one metal exceeds 10 atom %, specifically the toughness (or the ductility) tends to decrease, and that, if the total content of the rare earth elements exceeds 10 atom %, specifically the toughness (or the ductility) tends to decrease.
If the total content of above-described one metal is less than 0.2 atom %, or if the total content of the rare earth elements is less than 0.2 atom %, at least any of the strength and the toughness becomes insufficient. Therefore, the lower limit of the total content of the above-described one metal is specified to 0.2 atom %, and the lower limit of the total content of the rare earth elements is specified to 0.2 atom %.
In the magnesium alloy according to the Embodiment 1, the component other than the above-described one metal and the rare earth element, having the above-mentioned range of content, is magnesium. However, the magnesium alloy may contain amounts of impurities not affecting the alloy characteristics.
Embodiment 2
The magnesium alloy according to the Embodiment 2 of the present invention is the one in which the composition of the Embodiment 1 contains Zn.
That is, the magnesium alloy according to the Embodiment 2 is one of quaternary or higher order, containing at least one metal of Cu, Ni, and Co, and Zn, and rare earth elements that are one or more elements selected from the group consisting of Y, Dy, Er, Ho, Gd, Tb, and Tm.
The composition range of the magnesium alloy according to the Embodiment 2 is the one in which a, b, and c satisfy the following formulae (1) to (3), and preferably a, b, and c satisfy the following formulae (1′) to (3′), (the total content of above-described one metal is defined as a atom %, the total content of the above-described one or more rare earth elements is defined as b atom %, and the content of Zn is defined as c atom %),

0.2≦a+c≦15  (1)

0.2≦b≦10  (2)

2/3a−2/3<b  (3)

0.2<a+c≦5  (1′)

0.2≦b≦5  (2′)

2/3a−1/6<b.  (3′)

More preferably, the composition range thereof is the one in which a, b, and c satisfy the following formulae (1) to (4), and most preferably a, b, and c satisfy the following formulae (1′) to (4′),

0.2≦a+b≦15  (1)

0.2≦b≦10  (2)

2/3a−2/3<b  (3)

c/a≦1/2  (4)

0.2<a+c≦5  (1′)

0.2≦b≦5  (2′)

2/3a−1/6<b  (3′)

c/a≦1/2.  (4′)

The reason of above is that, if the total content of the above-described one metal and Zn exceeds 15 atom %, specifically the toughness (or the ductility) tends to decrease, and if the total content of the rare earth elements exceeds 10 atoms, specifically the toughness (or the ductility) tends to decrease.
Further, if the total content of the above-described one metal and Zn is less than 0.2 atom %, or if the total content of the rare earth elements is less than 0.2 atom %, at least any of the strength and the toughness becomes insufficient. Therefore, the lower limit of the total content of above-described one metal and Zn is specified to 0.2 atom %, and the lower limit of the total content of the rare earth elements is specified to 0.2 atoms.
In the magnesium alloy according to the Embodiment 2, the component other than the above-described one metal and the rare earth elements, having the above-mentioned range of content, is magnesium. However, the magnesium alloy may contain amounts of impurities not affecting the alloy characteristics.
Embodiment 3
The magnesium alloy according to the Embodiment 3 of the present invention is the one in which the composition of the Embodiment 1 contains one or more elements selected from the group consisting of La, Ce, Pr, Nd, Sm, Eu, Yb, and Lu.
That is, the magnesium alloy according to the Embodiment 3 is one of quaternary or higher order, containing at least one metal of Cu, Ni, and Co, and containing first rare earth elements and second rare earth elements, in which first rare earth elements are one or more elements selected from the group consisting of Y, Dy, Er, Ho, Gd, Tb, and Tm, and in which second rare earth elements are one or more elements selected from the group consisting of La, Ce, Pr, Nd, Sm, Eu, Yb, and Lu.
The composition range of the magnesium alloy according to the Embodiment 3 is the one in which a, b, and d satisfy the following formulae (1) to (3), and more preferably a, b, and d satisfy the following formulae (1′) to (3′), (the total content of the above-described one metal is defined as a atom %, the total content of the above-described one or more of the first rare earth element is defined as b atom %, and the total content of the above-described one or more of the second rare earth elements is defined as d atom %),

0.2≦a≦10  (1)

0.2<b+d≦15  (2)

2/3a−2/3<b  (3)

0.2≦a≦5  (1′)

0.2<b+d≦5  (2′)

0.2<b+d≦5.  (3′)

The reason of above is that, if the total content of the first rare earth elements and the second rare earth elements exceeds 15 atom %, specifically the toughness (or the ductility) tends to decrease. The reason for adding the second rare earth elements is that the second rare earth elements have an effect of refining crystal grains and have an effect of precipitating intermetallic compounds.
If the total content of the first rare earth elements and the second rare earth elements is less than 0.2 atom %, at least any of the strength and the toughness becomes insufficient. Therefore, the lower limit of the total content of the first rare earth elements and the second rare earth elements is specified to 0.2 atom %.
The reason for specifying the content of the above-described one metal as above is similar to that of the Embodiment 1.
Embodiment 4
The magnesium alloy according to the Embodiment 4 of the present invention is the one in which the composition of the Embodiment 2 contains one or more element selected from the group consisting of La, Ce, Pr, Nd, Sm, Eu, Yb, and Lu.
That is, the magnesium alloy according to the Embodiment 4 is an alloy of pentagonary or higher order, containing at least one metal of Cu, Ni, and Co, and Zn, a first rare earth element, and a second element, in which first rare earth elements are one or more element selected from the group consisting of Y, Dy, Er, Ho, Gd, Tb, and Tm, and in which second rare earth elements are one or more element selected from the group consisting of La, Ce, Pr, Nd, Sm, Eu, Yb, and Lu.
The composition range of the magnesium alloy according to the Embodiment 4 is the one in which a, b, c, and d satisfy the following formulae (1) to (3), and preferably a, b, c, and d satisfy the following formulae (1′) to (3′), (the total content of the above-described one metal is defined as a atom %, the total content of the above-described one or more of the first rare earth elements is defined as b atom %, the content of Zn is defined as c atom %, and the total content of the above-described one or more of the second rare earth elements is defined as d atom %),

0.2<a+c≦15  (1)

0.2<b+d≦15  (2)

2/3a−2/3<b  (3)

0.2<a+c≦5  (1′)

0.2<b+d≦5  (2′)

2/3a−2/3<b.  (3′)

More preferably a, b, c, and d satisfy the following formulae (1) to (4), and most preferably a, b, c, and d satisfy the following formulae (1′) to (4′),

0.2<a+c≦15  (1)

0.2<b+d≦15  (2)

2/3a−2/3<b  (3)

c/a≦1/2  (4)

0.2<a+c≦5  (1′)

0.2<b+d≦5  (2′)

2/3a−2/3<b.  (3′)

c/a≦1/2.  (4′)

The reason of the above is that, if the total content of the first rare earth elements and the second rare earth elements exceeds 15 atom %, specifically the toughness (or the ductility) tends to decrease. The reason for adding the second rare earth elements is that the second rare earth elements have an effect of refining crystal grains and have an effect of precipitating intermetallic compounds.
If the total content of the first rare earth elements and the second rare earth elements is less than 0.2 atom %, at least any of the strength and the toughness becomes insufficient. Therefore, the lower limit of the total content of the first rare earth elements and the second rare earth elements is specified to 0.2 atom %.
The reason for specifying the total content of the above-described one metal and zinc as the above range is similar to that of the Embodiment 2.
Embodiment 5
The magnesium alloy according to the Embodiment 5 of the present invention includes the one in which the composition of any of the Embodiments 1 to 4 contains Me. The Me is at least one element selected from the group consisting of Zr, Ti, Mn, Al, Ag, Sc, Sr, Ca, Si, Hf, Nb, B, C, Sn, Au, Ba, Ge, Bi, Ga, In, Ir, Li, Pd, Sb, V, Fe, Cr, and Mo. If the total content of Me is defined as e atom %, the e satisfies the following formula (5), preferably e and the a, b, and d further satisfy the following formula (6),

0<e≦2.5  (5)

e/(a+b+c+d)≦1/2.  (6)

The addition of Me can improve other properties while maintaining high strength and high toughness. For example, this provides an effect of corrosion resistance and crystal grain refinement.
The magnesium alloy according to each of above Embodiments 1 to 5 can also be applied to pluralities of chip-shaped casts each having several millimeters square or less prepared by cutting the cast.
Embodiment 6
The method for manufacturing the magnesium alloy according to the Embodiment 6 of the present invention is described below.
A magnesium alloy having the composition of any of the Embodiments 1 to 5 is melted to cast, thus preparing a magnesium alloy cast. The cooling speed of casting is within the range of 0.05K/sec to 1000 (103) K/sec, preferably 0.5K/sec to 1000 (103) K/sec. As the magnesium alloy cast, the one cut to a specific shape from an ingot is used.
Further, the magnesium alloy cast may be heat-treated. The condition of the heat treatment is preferably at temperatures ranging from 200° C. to 550° C. with treatment time ranging from 1 minute to 3600 minutes (or 60 hours).
The magnesium alloy cast has a crystal structure of long-period stacking ordered structure phase. The magnesium alloy cast has an α-Mg phase, which has a lamellar structure. In addition, the long-period stacking ordered structure phase kinks. The word “kink” referred to herein signifies that an intensely worked long-period stacking ordered structure phase has no specific directional relation, induces bending within the phase, and refines the long-period structure phase.
In some instances, the magnesium alloy contains other compound phases, in addition to the long-period stacking ordered structure phase and the α-Mg phase.
Next, plastic-working is carried out to the magnesium alloy cast. The method of plastic-working includes extrusion, ECAE (equal-channel-angular-extrusion) working method, rolling, drawing and forging, repeated working of above methods, and FSW working. The plastic-working preferably gives an amount of equivalent strain per at least one cycle within the range of more than zero to not more than 5. When the stress component in a multiaxial stress state is converted into a corresponding uniaxial stress, the converted stress is called the “equivalent stress”. The term “amount of equivalent strain” signifies the amount of strain under the equivalent stress.
When the plastic-working is carried out by extrusion, it is preferable to select the extrusion temperature ranging from 200° C. to 500° C., and to select the reduction in area by extrusion of 5% or more.
The ECAE working method is one in which the sample longitudinal direction is rotated by 90° at every pass in order to introduce strain uniformly into the sample. Specifically, the method is to forcefully insert the magnesium alloy cast as the forming material into the molding hole of the molding die formed in a cross-sectional L shape, and a stress is applied to the magnesium alloy cast at a portion of 90° bend of the L-shaped molding hole, thus obtaining a molded article having excellent strength and toughness. The number of passes of ECAE is preferably within the range of 1 to 8, and more preferably 3 to 5. The temperature of ECAE working is preferably within the range of 200° C. to 500° C.
When the plastic-working is carried out by rolling, it is preferable to select the rolling temperature within the range of 200° C. to 500° C., and to select the reduction in thickness of 5% or more.
When the plastic-working is carried out by drawing, it is preferable to select the drawing temperature within the range of 200° C. to 500° C., and to select the reduction in area in the drawing of 5% or more.
When the plastic-working is carried out by forging, it is preferable to select the forging temperature within the range of 200° C. to 500° C., and to select the working rate of the forging of 5% or more.
The plastic work product prepared by carrying out plastic-working of the magnesium alloy cast, as described above, has a crystal structure of long-period stacking ordered structure phase at normal temperature. The plastic work product has an α-Mg phase, which has a lamellar structure. In addition, the long-period stacking ordered structure phase kinks. At least a part of the long-period stacking ordered structure phase is curved or bent. In some instances, the plastic work product contains other compound phases, in addition to the long-period stacking ordered structure phase and the α-Mg phase. For example, the plastic work product may contain at least one precipitate selected from the precipitate groups of: a compound of Mg with rare earth element; a compound of Mg with the above-described one metal; a compound of the above-described one metal with rare earth element; and a compound of Mg, the above-described one metal, and rare earth element. The plastic work product contains hcp-Mg. The plastic work product after treated by the plastic-working increases both the Vickers hardness and the yield strength compared with those of the cast before being subjected to plastic-working.
The plastic work product prepared by carrying out plastic-working of the magnesium alloy cast may be subjected to heat treatment. A preferable condition of the heat treatment is a temperature ranging from 200° C. to 500° C., and a heat-treatment time ranging from 1 minute to 3600 minutes (or 60 hours). The heat-treated plastic work product increases both the Vickers hardness and the yield strength compared with those of the plastic work product before being subjected to heat treatment. Similar to the case before conducting heat treatment, the heat-treated plastic work product has a crystal structure of long-period stacking ordered structure phase at normal temperature, and has an α-Mg phase, which has a lamellar structure. In addition, the long-period stacking ordered structure phase kinks. At least a part of the long-period stacking ordered structure phase is curved or bent. The plastic work product may contain at least one precipitate selected from the precipitate groups of: a compound of Mg with rare earth element; a compound of Mg with the above-described one metal; a compound of the above-described one metal with rare earth element; and a compound of Mg, the above-described one metal, and rare earth element. The plastic work product contains hcp-Mg.
According to the Embodiments 1 to 6, for expanded applications of magnesium alloys, such as applications as alloys for high-tech fields requiring high performance of both the strength and the toughness, there can be provided a high-strength and high-toughness magnesium alloy giving practical application level of both the strength and the toughness, and can be provided a method for manufacturing thereof.
When the magnesium alloy which is prepared by adding Zr by more than 0 atom % and not more than 2.5 atom % to the composition of any of the Embodiments 1 to 4 is melted and cast, the obtained magnesium alloy cast suppresses the precipitation of chemical compound, enhances the formation of long-period stacking ordered structure phase, and refines the crystal structure. Consequently, the magnesium alloy cast allows easy plastic-working such as extrusion, and the plastic work product after being treated by plastic-working has a large amount of long-period stacking ordered structure phase and of refined crystal structure compared with the amount thereof in the plastic work product of a magnesium alloy without the addition of Zr. With that large amount of long-period stacking ordered structure phase, both the strength and the toughness can be increased.
The long-period stacking ordered structure phase has a concentration modulation. The term “concentration modulation” means periodical variations in the solute element concentration at every atom layer.
Embodiment 7
The method for manufacturing the magnesium alloy according to the Embodiment 7 of the present invention is described below.
Similar to the method of the Embodiment 6, the magnesium alloy having the composition of any of the Embodiments 1 to 5 is melted to cast, thus preparing a magnesium alloy cast. Then, the magnesium alloy cast may be subjected to homogenized heat treatment.
Afterwards, pluralities of chip-shaped casts each having several millimeters square or less are prepared by cutting the magnesium alloy cast.
The chip-shaped casts may then be preformed by means of compression or plastic-working, and be heat-treated. The condition of the heat treatment is preferably at a temperature ranging from 200° C. to 550° C. for a treatment time ranging from 1 minute to 3600 minutes (or 60 hours).
The chip-shaped casts are commonly used as a raw material of thixotropic molding, for example.
A mixture of chip-shaped casts and ceramic particles may be preformed by means of compression or plastic-working, followed by heat treatment. The chip-shaped casts may be subjected to additional intense-strain working before applying performing.
Then, the chip-shaped casts are subjected to plastic-working. Varieties of plastic-working methods are applicable as in the case of the Embodiment 6.
Similar to the Embodiment 6, the plastic work product treated by plastic-working has a crystal structure of long-period stacking ordered structure at normal temperature. At least a part of the long-period stacking ordered structure phase is curved or bent. The plastic work product after treated by the plastic-working increases in both the Vickers hardness and the yield strength compared with those of the cast before the treatment of plastic-working.
The plastic work product after carrying out the plastic-working of the chip-shaped casts may be subjected to heat treatment. The condition of the heat treatment is preferably at a temperature ranging from 200° C. to 550° C. for a treatment time ranging from 1 minute to 3600 minutes (or 60 hours). The plastic work product after treated by the plastic-working increases in both the Vickers hardness and the yield strength compared with those of the plastic work product before the treatment of plastic-working. The plastic work product after the heat treatment has a crystal structure of long-period stacking ordered structure at normal temperature, similar to the case of the plastic work product before the heat treatment. At least a part of the long-period stacking ordered structure phase is curved or bent.
According to the Embodiment 7, since the cutting of casts to prepare the chip-shaped casts refines the structure, it is possible to manufacture a plastic work product or the like having higher strength, higher ductility, and higher toughness than those of the Embodiment 6. In addition, the magnesium alloy according to the Embodiment 7 can attain the characteristics of high strength and high toughness even when Zinc and rare earth element are at lower concentration than those of magnesium alloy in the Embodiment 6.
According to the Embodiment 7, for expanded applications of magnesium alloys, such as applications as alloys for high-tech fields requiring high performance of both the strength and the toughness, for example, there can be provided a high-strength and high-toughness magnesium alloy giving practical level of both the strength and the toughness, and can be provided a method for manufacturing thereof.
The long-period stacking ordered structure phase has a concentration modulation. The term “concentration modulation” means periodical variations in the solute element concentration in every atom layer.
Embodiment 8
The method for manufacturing the magnesium alloy according to the Embodiment 8 of the present invention is described below.
Preparation of Rapidly Coagulated Powder and the solidification forming thereof use a closed P/M processing system. The system applied to preparing thereof is illustrated in FIG. 5 and FIG. 6. FIG. 5 illustrates the process of preparing rapidly coagulated powder using the gas atomize method, and of forming a billet from thus prepared rapidly coagulated powder by extrusion forming. FIG. 6 illustrates the process up to the extrusion forming of the prepared billet. The preparation of rapidly coagulated powder and the solidification forming thereof are described below in detail referring to FIG. 5 and FIG. 6.
Referring to FIG. 5, the powder of magnesium alloy having a target component ratio is prepared using a high pressure gas atomizer 100. That is, the alloy having the target components ratio is melted in a crucible 116 in a melting chamber 110 using an induction coil 114. The material of the alloy is the magnesium alloy having the composition of any of Embodiments 1 to 5.
The melted alloy is ejected by lifting a stopper 112, to which a high pressure inert gas (such as helium gas and argon gas) is blown to spray thereof through a nozzle 132, thus preparing the alloy powder. The cooling speed in the preparation step is within the range of 1000 (103) K/sec to 10000000 (107) K/sec, preferably 10000 (104) K/sec to 10000000 (107) K/sec. The nozzle and other parts are heated by a heater 131. In addition, an atomizing chamber 130 is monitored by an oxygen analyzer 162 and a vacuum gauge 164.
The prepared magnesium alloy powder has a crystal structure of long-period stacking ordered structure phase. The powder has an α-Mg phase, which the α-Mg phase has a lamellar structure. Further, the long-period stacking ordered structure phase kinks. In some instances, the powder contains other compound phases, in addition to the long-period stacking ordered structure phase and the α-Mg phase.
The prepared alloy powder is collected in a hopper 220 in a vacuum glove box 200 via a cyclone classifier 140. Succeeding treatments are given in the vacuum glove box 200. Then, the powder passes through a series of sieves 230, which stepwise refine the mesh opening, in the vacuum glove box 200 to obtain powder having a target fineness. According to the present invention, 32 μm or smaller size of powder was obtained. Instead of the powder, sheet or thin wire can also be prepared.
For forming a billet from the alloy powder, firstly the pre-compression is given to the powder using a vacuum hot press 240. The vacuum hot press applied was the one which can press 30 tons.
The alloy powder is packed in a copper can 254 using the hot press 240, and a cap 252 is applied onto the can. The can 254 with the cap 252 are welded together by a welding machine 256 while rotating them on a rotary disk 258, thus forming a billet 260. For leakage check of the billet 260, the billet 260 is connected to a vacuum pump via a valve 262, thus checking the leakage of the billet 260. If no leakage occurred, the valve 262 is closed, and the alloy billet 260 equipped with the valve 262, together with the vessel, is taken out from an entrance box 280 of the vacuum glove box 200.
As illustrated in FIG. 6, the billet 260 taken out is put in a heating furnace, which is connected to a vacuum pump for degassing while preheating the billet 260, (refer to FIG. 6(a)). Then, the cap of the billet 260 is squeezed, and the cap is spot-welded by a spot-welding machine 340, thus shutting off the connection between the billet 260 and external environment, (refer to FIG. 6(b)). After that, the alloy billet 260 together with the vessel is placed in an extrusion press 400 to form into the final shape, (refer to FIG. 6(c)). The extrusion press has a performance of 100 ton of the main press (at main stem 450 side) and 20 ton of the back press (at back stem 470 side). By heating the container 420 using a heater 410, the extrusion temperature can be adjusted.
As described above, the rapidly coagulated powder according to the Embodiment 8 was prepared by the high pressure He gas atomizing method. Thus prepared powder having particle size of 32 μm or less was packed in a copper can, which was vacuum-sealed to form the billet. The solidification forming was conducted by extrusion forming under the condition of extrusion temperature within the range of 623K to 723K, and extrusion ratio of 10:1. The extrusion forming applied pressure and shear to the powder, thus attaining densification and bonding between powder particles. The forming by rolling method or forging method also generates shear.
The magnesium alloy obtained by the above-described solidification forming has a crystal structure of long-period stacking ordered structure phase. The powder has an α-Mg phase, which has a lamellar structure. In addition, the long-period stacking ordered structure phase kinks. In some instances, the powder contains other compound phases, in addition to the long-period stacking ordered structure phase and the α-Mg phase
According to the Embodiment 8, there is provided a magnesium alloy having high strength and high toughness. The magnesium alloy has a fine crystal structure having average crystal grain size of 1 μm or less.
EXAMPLES
Examples of the present invention are described as follows.
Example 1
There were ingoted Mg97Co1Y2 alloy, Mg97Ni1Y2 alloy, and Mg97Cu1Y2 alloy for the Example 1, and Mg97Fe1Y2 alloy for the Comparative Example 1 through high frequency induction melting in an Ar gas atmosphere. From each of the ingots, an extrusion billet, cut to a shape of 29 mm in diameter and 65 mm in length, was prepared.
Then, the extrusion billet was extruded under the condition of extrusion ratio of 10, extrusion temperature of 623K, and extrusion speed of 2.5 mm/sec, after preheating it at 623K for 20 minutes.
(Observation of Structure of Ingot)
The structure observation for the ingot was conducted by SEM and TEM. FIGS. 1(A) to 1(C) and FIG. 2 show the micrographs of these crystal structures. FIG. 1(A) shows an SEM micrograph of the ingot of Mg97CO1Y2 alloy, FIG. 1(B) shows an SEM micrograph of the ingot of Mg97Ni1Y2 alloy, and FIG. 1(C) shows an SEM micrograph of the ingot of Mg97Cu1Y2 alloy. FIG. 2 shows a TEM micrograph of the long-period stacking ordered structure phase of the ingot of Mg97Cu1Y2 alloy, and the electron beam diffraction image on [1120].
The ingot of Mg97Fe1Y2 alloy as the Comparative Example 1 did not show long-period stacking ordered structure phase. To the contrary, as shown in FIG. 1(A), the ingot of Mg97CO1Y2 alloy as the Example 1 showed a lamellar structure indicating the formation of long-period stacking ordered structure phase other than the compound phase. Further, as shown in FIGS. 1(B) and 1(C), each ingot of Mg97Ni1Y2 alloy and Mg97Cu1Y2 alloy showed a significant lamellar structure indicating the formation of long-period stacking ordered structure phase, and specifically the Mg97Cu1Y2 alloy showed a long-period stacking ordered structure phase at the highest volume fraction.
According to the electron beam diffraction image given in FIG. 2, it was confirmed that the long-period stacking ordered structure phase observed in the Mg97Cu1Y2 alloy is the same 18R type as that of the Mg—Zn—Y series alloys.
(Vickers Hardness Test)
The Vickers hardness of the extruded material of the Mg97Cu1Y2 alloy was 87HV0.5. The Vickers hardness of the extruded material of the Mg97Ni1Y2 alloy was 90.1HV0.5. The Vickers hardness of the extruded material of the Mg97CO1Y2 alloy was 81HV0.5. The Vickers hardness of the extruded material of the Mg97Fe1Y2 alloy was 77.6HV0.5.
FIG. 3 shows the result of tensile test for the extruded materials of Mg97X1Y2 (X═Fe, Co, Ni, or Cu) alloys at room temperature, which materials are for the Example 1 and the Comparative Example. Table 1 shows the result of tensile test for the extruded materials of the Example 1 at room temperature, (YS: yield strength, UTS: tensile strength, and elongation (%)), and hardness Hv.







TABLE 1







Result of Mg—X—Y tensile test at room temperature












Alloy
Extrusion






component
temperature
Hv
YS
UTS
Elongation
















Mg97Fe1Y2
623 K
77.6
255
308
10.5
Comparative








Example


Mg97Co1Y2
623 K
81.3
315
326
2.3
Example


Mg97Ni1Y2
623 K
90.1
293
373
13.6
Example


Mg97Cu1Y2
623 K
87.7
276
363
12.5
Example


Mg97Cu1Y2
598 K

297
377
8.1
Example









As shown in FIG. 3 and Table 1, the Mg97Fe1Y2 alloy not forming long-period stacking ordered structure phase had only a relatively low strength. On the other hand, the Mg97CO1Y2 alloy, the Mg97Ni1Y2 alloy, and the Mg97Cu1Y2 alloy, forming a long-period stacking ordered structure phase, had high strength, giving the yield strength (YS) of 315 MPa, 293 MPa, and 276 MPa, respectively. The Mg97Ni1Y2 alloy and the Mg97Cu1Y2 alloy having large amount of formed long-period stacking ordered structure phase, exhibited good ductility of 12% or more. However, the mg97Co1Y2 alloy exhibited only relatively low ductility caused by the presence of chemical compounds.
FIG. 4 shows the result of tensile test for the extruded materials of Mg97X1Y2 (X═Fe, Co, Ni, or Cu) alloys at 473K, which are for the Example 1 and the Comparative Example. Table 2 shows the result of tensile test at 473K for the extruded materials of the Example 1, (YS: yield strength, UTS: tensile strength, and elongation (%)).







TABLE 2







Result of Mg—X—Y high temperature tensile test


Test temperature: 473 K












Extrusion





Alloy component
temperature
YS
UTS
Elongation















Mg97Fe1Y2
623 K
217
266
19.4
Comparative







Example


Mg97Co1Y2
623 K
269
299
11.8
Example


Mg97Ni1Y2
623 K
262
312
20.7
Example


Mg97Cu1Y2
623 K
245
334
18
Example


Mg97Cu1Y2
598 K
273
344
16.3
Example









As shown in Table 2, though the Mg97CO1Y2 alloy had large high-temperature strength, giving yield strength of 269 MPa, the high-temperature strength was somewhat low compared with the room-temperature strength. On the other hand, the Mg97Ni1Y2 alloy and the Mg97Cu1Y2 alloy gave relatively small difference between the room-temperature strength and the high-temperature strength, and thus these alloys maintained high strength even in high-temperature zone. Consequently, it was confirmed that the long-period stacking ordered structure phase significantly contributes to the improvement in the mechanical properties, or significantly contributes to the high strength and high ductility, in high-temperature zone.
Example 2
There were ingoted Mg85Cu6Y9 alloy, Mg85Ni6Y9 alloy, and Mg85CO6Y9 alloy, respectively, for the Example 2 through high frequency induction melting in an Ar gas atmosphere.
Then, the ingot was treated by hot-rolling. The hot-rolling was carried out at the condition of rolling rate of 50 to 70% and rolling temperature of 250° C. to 400° C., after preheating at 200° C. for 30 minutes.
(Observation of Structure of Ingot)
The observation of structure of ingot was given by SEM and TEM. FIGS. 7 to 12 show the photographs of crystal structures of the respective ingots. FIG. 7 is an SEM micrograph of the ingot of Mg85Cu6Y9 alloy. FIG. 8 is an SEM micrograph of the ingot of Mg85Ni6Y9 alloy. FIG. 9 is an SEM micrograph of the ingot of Mg85CO6Y9 alloy. FIG. 10 is a TEM micrograph of a long-period stacking ordered structure phase of the ingot of Mg85Cu6Y9 alloy. FIG. 11 shows the diffraction pattern of the long-period stacking ordered structure phase of 18R type formed in the ingot of Mg85Cu6Y9 alloy. FIG. 12 shows the diffraction pattern of the long-period stacking ordered structure phase of 10H type formed in the ingot of Mg85Cu6Y9 alloy.
As shown in FIGS. 7 to 9, each ingot of Mg85Cu6Y9 alloy, Mg85Ni6Y9 alloy, and Mg85CO6Y9 alloy in the Example 2 showed a plate-shaped structure having a size of about 10 to 30 μm. The sheet-shaped structure was 10H type or 18R type long-period stacking ordered structure phase. The scale bar given in FIGS. 7 to 9 indicates 100 μm.
On the TEM micrographs and the electron beam diffraction image given in FIG. 10 and FIG. 11, there was identified the 18R type long-period stacking ordered structure phase in the Mg85Cu6Y9 alloy. On the electron beam diffraction image given in FIG. 12, there was identified the 10H type long-period stacking ordered structure phase in the Mg85Cu6Y9 alloy.
In addition, 18R type and 10H type long-period stacking ordered structure phases were identified in the respective ingots of Mg85Ni6Y9 alloy and Mg85CO6Y9 alloy.
(Vickers Hardness Test)
Vickers hardness test was performed for both the ingots and the hot-rolled materials.
The Vickers hardnesses of the ingot and the hot-rolled material of Mg85Cu6Y9 alloy were 108HV0.5 and 150HV0.5, respectively. The Vickers hardnesses of the ingot and the hot-rolled material of Mg85Ni6Y9 alloy were 110HV0.5 and 147HV0.5, respectively. The Vickers hardnesses of the ingot and the hot-rolled material of Mg85CO6Y9 alloy were 105HV0.5 and 138HV0.5, respectively.
As described above, since the ingots and the hot-rolled materials of the Example 2 have high hardnesses, the magnesium alloys in the Example 2 also presumably have high strength.
Example 3
<Sample Preparation>
(Preparation of Ingot)
An Mg alloy was melted in an iron crucible using an electric furnace while introducing CO2 gas into the crucible. The melted Mg alloy was poured in an iron mold to prepare the ingot sample. In detail, the respective materials were weighed. After weighing, the Mg was first poured in the iron crucible to melt. After melting the Mg, elements were added, and the mixture was heated up to 1123K, and held the temperature for 10 minutes. Afterwards, the mixture was agitated by an iron rod to tap into the mold.
(Preparation of Rapidly Cooled Material)
An Mg alloy was melted in an iron crucible using an electric furnace while introducing CO2 gas into the crucible. The melted Mg alloy was poured in a copper mold to prepare the rapidly cooling sample. In detail, the respective ingots were placed in the respective crucibles. The Mg97X1Y2 (X═Cu or Ni) alloy was heated up to 1123K, the Mg94X2Y4 (X═Cu or Ni) alloy was heated up to 1098K, and the Mg100-A-BXAYB (X═Cu or Ni, A=3 to 3.5, and B=6 to 7) alloy was heated up to 1073K, and was kept at the temperature for 10 minutes. Afterwards, the alloy was tapped into a water-cooling type copper mold to rapidly cool the alloy.
(Preparation of Rolled Material)
The rapidly cooled Mg91X3Y6 (X═Cu or Ni) alloy was treated by hot-rolling at 623K to 70% of reduction in area to prepare the rolled sample. The rolling was conducted by rotating the mill-roll at a speed of 8.6 rpm while heating the mill-roll by a gas burner, and the rapidly cooled Mg91X3Y6 (X═Cu or Ni) alloy kept at 623K in an electric furnace was rolled.
(Preparation of Tensile Test Piece)
Sheet-shaped test piece of 14B grade specified by JIS was prepared using a discharge wire working machine (FA20, manufactured by Mitsubishi Electric Corporation). The dimensions of the prepared tensile test piece were 9.45 mm of distance between gauge marks, 12.8 mm of length of parallel section, and 15.0 mm of shoulder radius. After working, the test piece was polished by a water-proof abrasive paper and by a buff-polisher.
(Preparation of Heat-Treated Material)
The prepared tensile test piece of rolled Mg91X3Y6 (X═Cu or Ni) alloy was treated by strain-removing annealing. The rolled material was held at 673K in air for 6 hours in an electric furnace, and then was immediately immersed in, water to rapidly cool.
(Mechanical Characteristics of Rapidly Cooled Mg100-A-BCuAYB (A=1 to 3.5, B=2 to 7) Alloy)
The rapidly cooled Mg100-A-BCuAYB (A=1 to 3.5, B=2 to 7) alloy was subjected to tensile test at room temperature. The rapidly cooled Mg97Cu1Y2 alloy showed the proof stress (hereinafter referred to as σ0.2) of 121 MPa, the tensile strength (hereinafter referred to as σB) of 215 MPa, and the elongation (hereinafter referred to as δ) of 14% at room temperature. The rapidly cooled Mg94Cu2Y4 alloy showed σ0.2 of 191 MPa, σB of 257 MPa, and δ of 8%, which showed increased strength compared with that of the Mg97Cu1Y2 alloy, though the elongation becomes smaller. Furthermore, the rapidly cooled Mg91Cu3Y6 alloy showed σ0.2 of 257 MPa, σB of 312 MPa, and δ of 6%, and the rapidly cooled Mg90.5Cu3.25Y6.25 alloy showed σ0.2 of 277 MPa, σB of 328 MPa, and δ of 5%, both of which showed a tendency to increase in the strength, though the elongation becomes smaller with an increase in the amount of added element. However, the rapidly cooled Mg89.5Cu3.5Y7 alloy showed δ of 1%, and it was fractured in brittle mode in the elastic region so that the strength also decreased to σB of 221 MPa. The above result showed that the increase in the amount of added elements of Cu and Y increases long-period phase, and increases the strength. However, the above result also showed that the increase in the amount of added element to the level of the Mg89.5Cu3.5Y7 alloy generates brittle fracture. Consequently, it was found that the ductility can be increased by dispersing an adequate amount of Mg phase in the long-period phase to establish a multiple phase.
(Rolling Work and Mechanical Characteristics of Mg91Cu3Y6 Alloy)
Since the tensile test of rapidly cooled materials showed that the Mg91Cu3Y6 alloy has high strength and adequate ductility, giving yield strength of 257 MPa and elongation of 6%, the inventors of the present invention have conducted tensile test to the rapidly cooled Mg91Cu3Y6 alloy, and to the rolled product thereof, and further to the heat-treated material thereof after rolling, in the temperature range of room temperature to 623K, and have investigated the mechanical characteristics after the rolling.
(Mechanical Characteristics of Rapidly Cooled Mg91Cu3Y6 Alloy)
The rapidly cooled Mg91Cu3Y6 alloy showed the proof stress (hereinafter referred to as σ0.2) of 257 MPa, the tensile strength (hereinafter referred to as σB) of 312 MPa, and the elongation (hereinafter referred to as δ) of 6% at room temperature. At 525K, the rapidly cooled Mg91Cu3Y6 alloy showed σ0.2 of 203 MPa, σB of 250 MPa, and δ of 7%. At 573K, the rapidly cooled Mg91Cu3Y6 alloy showed σ0.2 of 152 MPa, σB of 192 MPa, and δ of 11%. At 598K, the rapidly cooled Mg91Cu3Y6 alloy showed σ0.2 of 109 MPa, σB of 125 MPa, and δ of 34%. At 623K, the rapidly cooled Mg91Cu3Y6 alloy showed σ0.2 of 61 MPa, σB of 74 MPa, and δ of 100%. The tendency showed that the strength decreases and the elongation increases with the increase in the temperature. In addition, even at a high temperature of 523K, the high yield strength of 150 MPa was maintained so that the rapidly cooled Mg91Cu3Y6 alloy was found to be as an alloy having high strength even in high temperature range.
(Hardness of Mg91Cu3Y6 Alloy)
The hardness of rolled Mg91Cu3Y6 alloy was 119Hv0.5, showing the increase in the hardness compared with 100Hv0.5 of the rapidly cooled Mg91Cu3Y6 alloy. Also for the heat-treated Mg91Cu3Y6 alloy, the hardness test was conducted. Since the heat-treated Mg91Cu3Y6 alloy showed the hardness of 108Hv0.5 and the decrease in the hardness by heat treatment, the strain of Mg and of long-period was presumably relaxed.
(Mechanical Characteristics of Heat-Treated Mg91Cu3Y6 Alloy)
It is known that a material in as-rolled state accumulates strain therein, and that fracture occurs almost within the elastic region. Based on the phenomenon, stress-removing annealing was given to the rolled Mg91Cu3Y6 alloy at 673K for 6 hours. Tensile test was given to the heat-treated Mg91Cu3Y6 alloy to investigate the mechanical characteristics. The heat-treated Mg91Cu3Y6 alloy showed the proof stress (hereinafter referred to as σ0.2) of 412 MPa, the tensile strength (hereinafter referred to as σB) of 477 MPa, and the elongation (hereinafter referred to as δ) of 6% at room temperature. At 523K, the heat-treated Mg91Cu3Y6 alloy showed σ0.2 of 254 MPa, σB of 284 MPa, and δ of 24%. At 573K, the heat-treated Mg91Cu3Y6 alloy showed σ0.2 of 199 MPa, σB of 223 MPa, and δ of 46%. At 598K, the heat-treated Mg91Cu3Y6 alloy showed σ0.2 of 105 MPa, σB of 134 MPa, and δ of 69%. At 623K, the heat-treated Mg91Cu3Y6 alloy showed σ0.2 of 66 MPa, σB of 81 MPa, and did not fracture even at δ of 63%. Similar to the case of rapidly cooled material, the above phenomenon showed a tendency of decrease in the strength and increase in the elongation with increase in the temperature. For the heat-treated material, the yield strength σ0.2 gave as high as 400 MPa or more at room temperature. In addition, in a high temperature range, the heat-treated material gave high strength and increased elongation compared with those of the rapidly cooled material. A presumable reason of the phenomenon is that the material-defects such as cast-defects (voids) in the sample, which supposedly existed in the rapidly cooled material, are collapsed by the rolling work. Particularly in view of strength, it is presumed that the bottom plane (0018) of the long-period phase formed a texture in parallel with the rolled sheet plane. In hexagonal system, if the direction of external force during deformation of the material is in parallel with or vertical to the bottom plane, the shearing force applied to the bottom plane becomes zero, which prevents the generation of sliding deformation, and increases the yield strength, though no plastic deformation occurs. Therefore, the Mg91Cu3Y6 alloy further significantly increases the strength by applying hot-rolling, thus obtaining an Mg alloy having also adequate ductility.
(Rolling Work and Mechanical Characteristics of Mg90.5Cu3.25Y6.25 Alloy)
Tensile test was conducted for a rolled Mg91Cu3Y6 alloy. It was found that the Mg91Cu3Y6 alloy has excellent characteristics, giving high yield strength of 400 MPa or more, and elongation of 6%, at room temperature. To create an alloy having further high strength, it is expected to apply rolling to the Mg90.5Cu3.25Y6.25 alloy which has higher strength than Mg91Cu3Y6.25 and has ductility to some degree, giving 4.6% elongation of 4.6%. Thus, the inventors of the present invention have prepared a rapidly cooled Mg90.5Cu3.25Y6.25 alloy, to which the rolling was applied to form a sample. The sample was subjected to tensile test to investigate the mechanical characteristics.
(Mechanical Characteristics of Heat-Treated Mg90.5Cu3.25Y6.25 alloy)
Concerning thus prepared heat-treated Mg90.5Cu3.25Y6.25 alloy, tensile test was conducted in a temperature range of room temperature to 623K to determine the mechanical characteristics. Table 3 shows the result. At room temperature, the Mg90.5Cu3.25Y6.25 alloy showed the proof stress (hereinafter referred to as σ0.2) of 448 MPa, the tensile strength (hereinafter referred to as σB) of 512 MPa, and the elongation (hereinafter referred to as δ) of 6%. At 523K, the Mg90.5Cu3.25Y6.25 alloy showed σ0.2 of 342 MPa, σB of 375 MPa, and δ of 25%. At 573K, the Mg90.5Cu3.25Y6.25 alloy showed ° 0.2 of 228 MPa, σB of 245 MPa, and δ of 44%. At 598K, the Mg90.5Cu3.25Y6.25 alloy showed σ0.2 of 177 MPa, σB of 189 MPa, and δ of 47%. At 623K, the Mg90.5Cu3.25Y6.25 alloy showed σ0.2 of 54 MPa, σB of 61 MPa, and δ of 143%. These values show higher strength and equivalent to or somewhat lower ductility than those of the heat-treated Mg91Cu3Y6 alloy. This is attributed to the increase in the area percent of the long-period phase and the increase in the work rate through rolling.
In addition, it was observed that there is a decreasing tendency in the strength and an increasing tendency in the elongation with the increase in the temperature, similar to the case of heat-treated Mg91Cu3Y6 alloy. Since in the heat-treated material, σ0.2 indicates 448 MPa and σB is higher than 500 MPa at room temperature, it can be said that the heat-treated Mg90.5Cu3.25Y6.25 alloy is a material which has an adequate ductility while keeping very high strength exceeding that of the heat-treated Mg91Cu3Y6 alloy.
FIG. 13 shows a TEM micrograph and an electron beam diffraction pattern of the heat-treated Mg91Cu3Y6 alloy. As seen in FIG. 13, the structure is in a two-phase state of Mg grains and long-period phase. It was found that a structural bend (curve) occurred at long intervals, which also presumably contributes to the increase in strength. Although the structure in FIG. 13 is for the heat-treated Mg91Cu3Y6 alloy, it is considered that the same is true of the heat-treated Mg90.5Cu3.25Y6.25 alloy.







TABLE 3







Mechanical characteristics of heat-treated rolled Mg90.5X3.25Y6.25 (X = Cu, Zn, or Ni)
















Tensile

Specific
Specific



Tensile
Proof stress
strength
Elongation
strength
gravity


Sample
temperature
σ0.2/MPa
σB/MPa
δ/%
σ0.2/ρ/(2)
ρ/Mg · m−3
















Heat-treated rolled
Room
448
512
6
214
2.098


Mg90.5Cu3.25Y6.25 alloy
temperature



523 K
342
375
25
163



573 K
228
245
44
109



598 K
177
189
47
84



673 K
54
61
143
26


Heat-treated rolled
Room
353
400
5
169
2.093


Mg90.5Zn3.25Y6.25 alloy
temperature



523 K
279
317
14
133



573 K
150
170
23
72



598 K
131
145
32
63



673 K
80
88
57
38


Heat-treated rolled
Room
460
526
8
220
2.090


Mg90.5Ni3.25Y6.25 alloy
temperature



523 K
301
245
12
144



573 K
224
236
25
107



598 K
159
176
34
76



673 K
114
126
43
55









Table 3 shows the mechanical characteristics of the alloys prepared in the Example 3. At room temperature, the heat-treated Mg90.5Cu3.25Y6.25 alloy and the heat-treated Mg90.5Ni3.25Y6.25 alloy showed higher specific strength than that of A7075-T6 (A7075: Al-1.2% Cu-6% Zn-2% Mg-0.25% Cr-0.25% Mn, T6: state treated through artificial aging effect after solution treatment), giving very high specific strength, though the specific strength was slightly lower than that of Ti-6Al-4V. Further, the specific strength of the heat-treated Mg90.5Zn3.25Y6.25 alloy exceeded that of the commercialized magnesium alloys. Regarding the specific strength at 523K, all the alloys of heat-treated Mg90.5Cu3.25Y6.25, heat-treated Mg90.5Zn3.25Y6.25, and heat-treated Mg90.5Ni3.25Y6.25 exceeded the strength of the heat-resistant magnesium alloy WE54A-T6 (WE54A: Mg-5% Y-4% RE, T6: state treated through artificial aging effect after solution treatment), and the heat-resistant aluminum alloy A2219-T81 (A2219: Al-6% Cu-0.3% Mn-0.5% Zr, T81: state treated through artificial aging effect after solution treatment followed by 1% cold-rolling). Further at 598K, the proof stress of them was 100 MPa or more, keeping the high strength. At 623K, the heat-treated Mg90.5Ni3.25Y6.25 alloy kept high strength, giving 100 MPa or higher proof stress, and the heat-treated Mg90.5Cu3.25Y6.25 alloy showed as high as 143% of ductility.
From the above result, it can be said that the Mg-TM-Y alloy (TM: transition metal) prepared in the Example 3 is an Mg alloy having high specific strength within a wide range of room temperature to high temperatures.
A presumable reason of the above high strength of the alloy “sheet” in the Example 3 is that the hot-rolling brings Mg and (001) and (0018) planes of the long-period phase orient (forming texture) in parallel with the sheet plane so that the deformation in the tensile direction becomes difficult. The result of tensile test for a non-oriented as-rapidly cooled material also showed high strength, giving tensile strength of 300 MP because the long-period itself has high strength. The rapid cooling effect using a copper mold also contributes to the increase in strength to some extent. In addition to the above, hot-rolling presumably forms a texture to further increase the strength. The reason for high strength even in high temperature range is that the long-period phase itself endures high temperatures and that the texture remains even after heat treatment at 400° C. for 6 hours so that the high strength is attained similar to the case of room temperature. The heat treatment after rolling is critical, and without the heat treatment, the elongation at room temperature cannot improve. The elongation at room temperature is a phenomenon in which the heat treatment brings Mg to recovery and recrystallization to induce elongation. Although Mg is recovered, the long-period phase itself remains in a texture form even after the heat treatment at 400° C. as described above, the remained texture significantly contributes to the increase in strength.
Example 4
First, there were prepared ingots having the respective compositions given in Tables 4 to 6 through high frequency induction melting in an Ar gas atmosphere. The ingot was cut to prepare an extrusion billet in a shape of 29 mm in diameter and 65 mm in length.
Then, extrusion was given to the extrusion billet conducting preheating at 623K for 20 minutes, followed by extrusion at the respective extrusion ratios, extrusion temperatures, and extrusion speeds indicated in Tables 4 to 6. Thus extruded material was subjected to tensile test at the respective temperatures indicated in Tables 4 to 6. The result is indicated in Tables 4 to 6.
As seen in Tables 4 to 6, the magnesium alloy that forms the long-period stacking ordered structure phase has high yield strength.
The present invention is not limited to the above-described embodiments and examples, and various modifications can be possible within a range not departing from the spirit and scope of the present invention.







TABLE 4





Tensile characteristics of extrusion ingot of Mg—Ni—Y alloy

















Tensile temperature











Extrusion condition
Room temperature
200°0 C.

















Extrusion

Extrusion
Yield
Tensile

Yield
Tensile



Composition
temperature
Extrusion
speed
strength
strength
Elongation
strength
strength
Elongation


(at. %)
(° C.)
ratio
(mm/s)
(MPa)
(MPa)
(%)
(MPa)
(MPa)
(%)





Mg93.5Ni3Y3.5
400
10
2.5
485
551
8.9
320
387
21.9


Mg89Ni4Y7
500
10
2.5
480
507
0.2
392
450
1.7


Mg93Ni3Y4
400
10
2.5
440
521
2.6
320
389
17


Mg94.5Ni3Y2.5
350
10
2.5
461
513
2.7
320
359
20.4


Mg94Ni2.5Y3.5
350
10
2.5
445
520
8.4
324
387
16.7


Mg94Ni3Y3
350
10
2.5
475
542
6.7
322
375
21.1


Mg91.5Ni4Y4.5
500
10
2.5
407
496
2.4
281
355
19.4


Mg92.5Ni3.5Y4
450
10
2.5
465
516
2.7
305
371
22.3


Mg92.5Ni4Y3.5
450
10
2.5
456
531
4.5
300
365
20.3


Mg92Ni3.5Y4.5
450
10
2.5
464
536
2.6
310
385
21.9


Mg92Ni4Y4
450
10
2.5
455
532
3.2
303
374
21.8


Mg93.5Ni2.5Y4
450
10
2.5
405
475
7.2
293
374
20.2


Mg93.5Ni3.5Y3
400
10
2.5
480
534
5.6
310
385
24


Mg93Ni2.5Y4.5
450
10
2.5
355
487
5.9
315
290
19.4


Mg93Ni3.5Y3.5
450
10
2.5
456
516
5.8
304
365
26


Mg95Ni2Y3
350
10
2.5
311
448
10.8
320
370
10


Mg92.5Ni3Y4.5
400
10
2.5
405
518
4
330
409
17


Mg90Ni4Y6
500
10
2.5
470
470
0.2
375
442
7.7


Mg96Ni2Y2
350
10
2.5
445
473
5.8
289
325
13.2


Mg97Ni1Y2
350
10
2.5
293
373
13.6
262
312
20.7












Tensile tempetature











250° C.
300° C.
Result of 350° C. tensile test



















Yield
Tensile

Yield
Tensile

Yield
Tensile




Composition
strength
strength
Elongation
strength
strength
Elongation
strength
strength
Elongation



(at. %)
(MPa)
(MPa)
(%)
(MPa)
(MPa)
(%)
(MPa)
(MPa)
(%)







Mg93.5Ni3Y3.5
250
311
31.2
162
199
70
81
97
31.9












(stopped












before












completion)



Mg89Ni4Y7
320
402
6.7
230
274
282
119
143
48.2



Mg93Ni3Y4



Mg94.5Ni3Y2.5



Mg94Ni2.5Y3.5



Mg94Ni3Y3



Mg91.5Ni4Y4.5



Mg92.5Ni3.5Y4



Mg92.5Ni4Y3.5



Mg92Ni3.5Y4.5



Mg92Ni4Y4



Mg93.5Ni2.5Y4



Mg93.5Ni3.5Y3



Mg93Ni2.5Y4.5



Mg93Ni3.5Y3.5



Mg95Ni2Y3



Mg92.5Ni3Y4.5



Mg90Ni4Y6



Mg96Ni2Y2



Mg97Ni1Y2

















TABLE 5







Tensile characteristics of extrusion ingot of Mg—Cu—Y alloy









Tensile temperature











Extrusion condition
Room temperature
200° C.

















Extrusion

Extrusion
Yield
Tensile

Yield
Tensile



Composition
temperature
Extrusion
speed
strength
strength
Elongation
strength
strength
Elongation


(at. %)
(° C.)
ratio
(mm/s)
(MPa)
(MPa)
(%)
(MPa)
(MPa)
(%)



















Mg92.5Cu3Y4.5
500
10
2.5
310
441
7.8
303
405
17.6


Mg94Cu2.5Y3.5
500
10
2.5
305
410
7.6
281
365
17.6


Mg90Cu4Y6
500
10
2.5
375
526
3.9
371
456
13.9


Mg95Cu2Y3
500
10
2.5
303
398
10.7
272
353
19.4


Mg97Cu1Y2
350
10
2.5
276
363
12.5
245
334
18


Mg96Cu2Y2
350
10
2.5
330
414
6.3
305
359
13.6
















TABLE 6







Tensile characteristics of extrusion ingot of Mg—Co—Y alloy









Tensile temperature











Extrusion condition
Room temperature
200° C.

















Extrusion

Extrusion
Yield
Tensile

Yield
Tensile



Composition
temperature
Extrusion
speed
strength
strength
Elongation
strength
strength
Elongation


(at. %)
(° C.)
ratio
(mm/s)
(MPa)
(MPa)
(%)
(MPa)
(MPa)
(%)





Mg97Co1Y2
350
10
2.5
315
326
2.3
269
299
11.8


Mg96Co2Y2
350
10
2.5
265
311
9.2
239
283
12.9

## Claims

1. A high-strength and high-toughness magnesium alloy comprising:
a atom % in total of at least one metal of Cu, Ni, and Co; and
b atom % in total of at least one element selected from the group consisting of Y, Dy, Er, Ho, Gd, Tb, and Tm,
wherein a and b satisfy the following formulae (1) to (3),


0.2≦a≦10  (1)



0.2≦b≦10  (2)



2/3a−2/3<b,  (3)


said high-strength and high-toughness magnesium alloy has a long-period stacking ordered structure phase, and
said high-strength and high-toughness magnesium alloy does not contain Zn.

2. The high-strength and high-toughness magnesium alloy according to claim 1, wherein said high-strength and high-toughness magnesium alloy has an α-Mg phase, and said α-Mg phase has a lamellar structure.

3. The high-strength and high-toughness magnesium alloy according to claim 1, wherein said high-strength and high-toughness magnesium alloy has a chemical compound phase.

4. The high-strength and high-toughness magnesium alloy according to claim 1, wherein said high-strength and high-toughness magnesium alloy is a magnesium alloy cast, and said magnesium alloy cast is heat-treated.

5. The high-strength and high-toughness magnesium alloy according to claim 4, wherein said high-strength and high-toughness magnesium alloy is a plastic work product obtained by carrying out plastic-working of said magnesium alloy cast.

6. A high-strength and high-toughness magnesium alloy comprising a plastic work product having a long-period stacking ordered structure phase, said plastic work product being manufactured by preparing a magnesium alloy cast comprising a atom % in total of at least one metal of Cu, Ni, and Co, and b atom % in total of at least one element selected from the group consisting of Y, Dy, Er, Ho, Gd, Tb, and Tm, while a and b satisfy the following formulae (1) to (3), then by cutting said magnesium alloy cast into a chip-shaped cast, and then by solidifying said cast by plastic-working,


0.2≦a≦10  (1)



0.2≦b≦10  (2)



2/3a−2/3<b,  (3)

said high-strength and high-toughness magnesium alloy does not contain Zn.

7. A high-strength and high-toughness magnesium alloy comprising a plastic work product having a long-period stacking ordered structure phase, said plastic work product being manufactured by preparing a magnesium alloy cast comprising a atom % in total of at least one metal of Cu, Ni, and Co, and b atom % in total of at least one element selected from the group consisting of Y, Dy, Er, Ho, Gd, Tb, and Tm, while a and b satisfy the following formulae (1) to (3), then by carrying out plastic-working of said magnesium alloy cast,


0.2≦a≦10  (1)



0.2≦b≦10  (2)



2/3a−2/3<b,  (3)

said high-strength and high-toughness magnesium alloy does not contain Zn.

8. The high-strength and high-toughness magnesium alloy according to claim 6, wherein said magnesium alloy cast is heat-treated.

9. The high-strength and high-toughness magnesium alloy according to claim 6, wherein said plastic work product is heat-treated.

10. The high-strength and high-toughness magnesium alloy according to claim 5, wherein said plastic work product has an α-Mg phase, and said α-Mg phase has a lamellar structure.

11. The high-strength and high-toughness magnesium alloy according to claim 5, wherein said plastic work product has a chemical compound phase.

12. The high-strength and high-toughness magnesium alloy according to claim 5, wherein said plastic-working includes at least one of rolling, extruding, ECAE, drawing, forging, pressing, form-rolling, bending, FSW working, and repeating thereof.

13. The high-strength and high-toughness magnesium alloy according to claim 5, wherein said plastic-working gives an amount of equivalent strain per at least one cycle thereof within the range of more than zero to not more than 5.

14. The high-strength and high-toughness magnesium alloy according to claim 1, said high-strength and high-toughness magnesium alloy comprising a powder, a sheet, or a thin wire, which is prepared by forming a liquid having a composition containing a atom % in total of at least one metal of Cu, Ni, and Co, and b atom % in total of at least one element selected from the group consisting of Y, Dy, Er, Ho, Gd, Tb, and Tm,
wherein said liquid does not contain Zn, and
wherein a and b satisfy the following formulae (1) to (3),


0.2≦a≦10  (1)



0.2≦b≦10  (2)



2/3a−2/3<b, and  (3)



then rapidly cooling said liquid to coagulate, said powder, sheet, or thin wire to give a crystal structure of a long-period stacking ordered structure phase.

15. The high-strength and high-toughness magnesium alloy according to claim 14, wherein said powder, sheet, or thin wire has an α-Mg phase, and said α-Mg phase has a lamellar structure.

16. The high-strength and high-toughness magnesium alloy according to claim 14, wherein said powder, sheet, or thin wire has a chemical compound phase.

17. The high-strength and high-toughness magnesium alloy according to claim 1, wherein said long-period stacking ordered structure phase kinks.

18. The high-strength and high-toughness magnesium alloy according to claim 1, wherein said magnesium alloy comprises d atom % in total of at least one element selected from the group consisting of La, Ce, Pr, Nd, Sm, Eu, Yb, and Lu, while said b and d satisfy the following formula (6),


0.2<b+d≦15.  (6)

19. The high-strength and high-toughness magnesium alloy according to claim 18, wherein said b and d further satisfy the following formula (7),


d/b≦1/2.  (7)

20. The high-strength and high-toughness magnesium alloy according to claim 1, wherein said magnesium alloy comprises e atom % in total of at least one element selected from the group consisting of Zr, Ti, Mn, Al, Ag, Sc, Sr, Ca, Si, Hf, Nb, B, C, Sn, Au, Ba, Ge, Bi, Ga, In, Ir, Li, Pd, Sb, V, Fe, Cr, and Mo, while e satisfies the following formula (8),


0<e≦2.5.  (8)

21. A method for manufacturing high-strength and high-toughness magnesium alloy according to claim 1, comprising the steps of: preparing a magnesium alloy cast containing a atom % in total of at least one metal of Cu, Ni, and Co, and b atom % in total of at least one element selected from the group consisting of Y, Dy, Er, Ho, Gd, Tb, and Tm, while a and b satisfy the following formulae (1) to (3); and preparing a plastic work product by carrying out plastic-working of said magnesium alloy cast,


0.2≦a≦10  (1)



0.2≦b≦10  (2)



2/3a−2/3<b.  (3)

22. The high-strength and high-toughness magnesium alloy according to claim 21, further comprising the step of cutting said magnesium alloy cast between the step of preparing said magnesium alloy cast and the step of preparing said plastic work product.

23. The method for manufacturing high-strength and high-toughness magnesium alloy according to claim 21, wherein said magnesium alloy cast has a long-period stacking ordered structure phase.

24. The method for manufacturing high-strength and high-toughness magnesium alloy according to claim 21, wherein said plastic work product has a long-period stacking ordered structure phase.

25. The method for manufacturing high-strength and high-toughness magnesium alloy according to 23, wherein said plastic work product has an α-Mg phase, and said α-Mg phase has a lamella structure.

26. The method for manufacturing high-strength and high-toughness magnesium alloy according to claim 23, wherein said plastic work product has a chemical compound phase.

27. The method for manufacturing high-strength and high-toughness magnesium alloy according to claim 23, wherein said long-period stacking ordered structure phase kinks.

28. The method for manufacturing high-strength and high-toughness magnesium alloy according to claim 21, further comprising the step of conducting heat treatment of said magnesium alloy cast after the step of preparing said magnesium alloy cast.

29. The method for manufacturing high-strength and high-toughness magnesium alloy according to claim 21, further comprising the step of conducting heat treatment of said plastic work product after the step of preparing said plastic work product.

30. The method for manufacturing high-strength and high-toughness magnesium alloy according to claim 21, wherein said plastic-working includes at least one of rolling, extruding, ECAE, drawing, forging, pressing, form-rolling, bending, FSW working, and repeating thereof.

31. The method for manufacturing high-strength and high-toughness magnesium alloy according to claim 21, wherein said plastic working gives an amount of equivalent strain per at least one cycle thereof within the range of more than zero and not more than 5.

32. The method for manufacturing high-strength and high-toughness magnesium alloy according to claim 21, wherein the magnesium alloy comprises d atom % in total of at least one element selected from the group consisting of La, Ce, Pr, Nd, Sm, Eu, Yb, and Lu, while said b and d satisfy the following formula (6),


0.2<b+d≦15.  (6)

33. The method for manufacturing high-strength and high-toughness magnesium alloy according to claim 32, wherein said b and d further satisfy the following formula (7),


d/b≦1/2.  (7)

34. The method for manufacturing high-strength and high-toughness magnesium alloy according to claim 21, wherein said magnesium alloy comprises e atom % in total of at least one element selected from the group consisting of Zr, Ti, Mn, Al, Ag, Sc, Sr, Ca, Si, Hf, Nb, B, C, Sn, Au, Ba, Ge, Bi, Ga, In, Ir, Li, Pd, Sb, V, Fe, Cr, and Mo, while e satisfies the following formula (8),


0<e≦2.5.  (8)

35. A method for manufacturing high-strength and high-toughness magnesium alloy according to claim 1, comprising the steps of: preparing a liquid having a composition containing a atom % in total of at least one metal of Cu, Ni, and Co, and b atom % in total of at least one element selected from the group consisting of Y, Dy, Er, Ho, Gd, Tb, and Tm, while a and b satisfy the following formulae (1) to (3); forming a powder, a sheet, or a thin wire, having a crystal structure of a long-period stacking ordered structure phase, by rapidly cooling said liquid to coagulate, and solidifying said powder, sheet, or thin wire so that shear is applied thereto,


0.2≦a≦10  (1)



0.2≦b≦10  (2)



2/3a−2/3<b.  (3)