MAGNESIUM ALLOY THAT EXHIBITS SUPERELASTIC EFFECT AND/OR SHAPE-MEMORY EFFECT

Abstract

 An object of the present invention is to provide a Mg alloy that exhibits a superelastic effect and a shape memory effect, and is excellent in cold workability. The Mg alloy is prepared so as to have a composition in which Sc is contained in a content of more than 13 at% and 30 at% or less, and the balance is Mg and inevitable impurities. In addition, the alloy may contain, in addition to the above-described composition, at least one or more additive elements selected from Li, Al, Zn, Y, Ag, In, Sn and Bi, in a total content of 0.001 at% or more and 9 at% or less in relation to the amount of the whole alloy defined to be 100 at%.

Description

Technical Field

[0001] The present invention relates to a magnesium alloy (hereinafter, referred to as a Mg alloy) that exhibits superelastic effect and/or shape memory effect. In particular, the present invention relates to a Mg alloy containing a certain amount of scandium (Sc). The present application is an application related to Japanese Patent Application No. 2015-201830, filed at Japanese Patent Office on October 13, 2015, and claims the priority based on the foregoing Japanese Patent Application. In addition, the whole of the contents of the following papers of the present inventors are cited: Ando, D., et al., Materials Letters, Vol. 161, p. 5-8; Ogawa, Y., et al., Science, 2016, Vol. 353(6297), pp. 368-370; Ogawa, Y., et al., Scripta Materialia, doi.org/10.1016/j.scriptamat.2016.09.024.

Background Art

[0002] Mg alloys are the lowest in density and the lightest in weight among the metals used for structural materials. Accordingly, when Mg alloys are used as the structural materials for automobiles, aircraft and the like, the Mg alloys contribute to weight saving and energy saving effect can be expected. Mg alloys are also excellent in recyclability, and have an advantage that Mg alloys can be more easily recycled as compared with plastics. Moreover, Mg alloys are high in specific strength, the resources for Mg alloys are abundant, and thus, a few tens of years have passed since Mg alloys began to be referred to as next-generation structural materials and attract attention. However, widely used Mg alloys have never been developed. As one of the reasons why Mg alloys have not yet been sufficiently practically used although there have been developed Mg alloys light in weight, high in specific rigidity and excellent in shock absorption, insufficient mechanical properties such as poor cold workability, or low strength may be mentioned.

[0003] Alloys have been developed in which Al is added to Mg in order to increase the strength, but suffer from the drawback that cold workability is poor. For example, examples of typical Mg alloys containing Al as added therein include AZ31 (Al: 3% by mass, Zn: 1% by mass, the balance: Mg), AZ61 (Al: 6% by mass, Zn: 1% by mass, the balance: Mg), AZ91 (Al 9% by mass, Zn 1% by mass, the balance: Mg), AM (Al 6% by mass, Mn: less than 1% by mass, the balance: Mg). Among these, only AZ31 allows rolled materials highly versatile as structural materials to be easily obtained; however, even rolled materials of AZ31 allow press working only at approximately 250°C, and find difficulty in press working at room temperature. The drawback of being poor in cold workability inhibits the practical use in various applications.

[0004] As a cause for poor cold workability and poor strength of common magnesium alloys, the HCP (hexagonal close-packed) structure of the main phase is quoted; it has been pointed out that premature fracture occurs because a local large deformation is caused in the interior of the double twins formed during deformation. As the solutions for such problems, there have been attempted controls of crystals such as crystal grain refinement or crystal grain randomization (Non Patent Literature 1, 2). However, even when the crystal microstructure control such as the crystal grain refinement is applied, the crystal structure remains as HCP, and the improvement of the ductility is limited because of the presence of the anisotropy due to the structure.

[0005] As a technique for improving the cold workability of Mg alloys, a Mg-Li alloy may be mentioned (Patent Literature 1, Patent Literature 2, Non Patent Literature 3). When Li is added to Mg in a content of 24.5 at%, the crystal structure is changed from the HCP structure to the BCC (body-centered cubic) structure, and consequently the cold workability is improved. However, with the increase of the lithium content, the corrosion resistance is degraded. In addition, Mg-Li alloys are low in hardness and strength, and poor in thermal stability. Therefore, Mg-Li alloys cannot be used as materials requiring strength, such as materials for automobiles or aircraft. In addition, Mg-Li alloys are poor in corrosion resistance, and accordingly require surface treatment, and hence the applications of Mg-Li alloys are extremely limited.

[0006] In addition, as a second cause for no wide use of Mg alloys, there may be mentioned a fact that Mg alloys have no such functionality as the functionality of Ti alloys, and thus the application range of Mg alloys is not widened. Ti alloys have high specific strength, and are excellent in ductility, and in particular, Ti alloys having BCC structure are known to exhibit superelastic effect (Patent Literature 3). It is also known that fundamentally, materials exhibiting superelastic effect due to the martensite transformation caused by loading stress exhibit shape memory effect depending on the transformation temperature in the state free from loading of stress. By utilizing these properties, Ti alloys are increasingly applied to accessories such as frames of spectacles, and to medical fields involving stents, catheters and guide wires.

[0007] The superelastic effect means a property getting back to the original shape immediately after the removal of stress even when a large deformation strain is applied. The shape memory effect means a property of an object getting back to the original memorized shape when the temperature is equal to or higher than a certain temperature even when the object is deformed by an external force. As a shape memory alloy having superelastic effect, there have been developed alloys having various metals as bases such as Ni-Ti, Cu-Al-Ni, Cu-Zn, Cu-Zn-Al, Cu-Al-Mn, Ti-Nb-Al, and Ni-Al.

[0008] It has recently been disclosed that a Mg alloy mainly composed of Mg, containing as an alloy element at least one element selected from Sc, Y, La, Ce, Pr and the like, and having a unidirectional crystal structure has a pseudoelasticity (Patent Literature 4). As a mechanism allowing a Mg alloy to have a pseudoelasticity, there has been disclosed a mechanism in which the addition of Sc, Y, La, Ce, Pr or the like suppresses the bottom plane sliding of the hexagonal crystal of Mg, and promotes the generation of twin crystals. Patent Literature 4 discloses, as an Example, a Mg alloy including 1.0 to 1.7 at% of Y as added therein; the pseudoelasticity in the case of including other elements is not disclosed, but it is recognized that the content of the element component to be added to the matrix phase is assumed to fall within a range of 1.0 to 6.0 at%. However, in the pseudoelasticity originating from the reversible change of the twin crystals, a plenty of residual strain is found, and a nearly perfect shape recovery as high as 90% or more cannot be expected. In addition, in order to achieve a good shape recovery, it is necessary to prepare a single crystal, and thus, the practical use of such a Mg alloy is limited.

[0009] The present inventors have made a study while focusing attention on the crystal structure of Mg alloys. The present inventors have considered that Mg alloys are poor in cold workability because of taking HCP structure high in anisotropy, and accordingly have searched Mg alloys having BCC structure. From the analysis of the phase diagrams, in addition to the Mg-Li alloy, the Mg-Sc alloy including Sc as added therein has been anticipated to have a BCC structure at a high Mg concentration. The present inventors have already produced Mg alloys including Sc as added therein, and have analyzed and reported the possibility of the two-phase microstructure control, the relation with mechanical properties, and moreover, the crystal orientation (Non Patent Literature 4 to Non Patent Literature 8). In particular, it has been shown that restriction to the two phases, namely, the BCC phase and the HCP phase, allows the achievement of high strength to be performed (Non Patent Literature 4). In addition, the present inventors have found that an aging treatment at a temperature of 175°C to 400°C produces fine HCP structure deposits in the BCC phase, and consequently the Mg alloy is hardened (Non Patent Literature 5, 6).

Citation List

Patent Literature

[0010] 

Patent Literature 1: Japanese Patent Laid-Open No. 2011-58089

Patent Literature 2: Japanese Patent Laid-Open No. 2001-40445

Patent Literature 3: Japanese Patent Laid-Open No. 2004-124156

Patent Literature 4: Japanese Patent Laid-Open No. 2015-63746

Non Patent Literature

[0011] 

Non Patent Literature 1: Miura, H. et al., 2010, Trans. Nonferrous Met. Soc. China, Vol. 20, p.1294-1298.

Non Patent Literature 2: Kim, W.J. et al., Acta Materialia, 2003, Vol.51, pp.3293-3307.

Non Patent Literature 3: Sanschagrin, A. et al., 1996, Mater. Sci. Eng.A, A220, pp.69-77.

Non Patent Literature 4: Ando, D. et al., Abstracts of 126th Spring Conference of Japan Institute of Light Metals (2014), pp.147-148.

Non Patent Literature 5: Ogawa, Y. et al., Abstracts of 128th Spring Conference of Japan Institute of Light Metals (2015), pp.47-48.

Non Patent Literature 6: Ando, D. et al., Materials Letters, 2015, Vol.161, pp.5-8, (available online 17 Jun 2015)

Non Patent Literature 7: Ogawa, Y., et al., Mater. Sci. Eng.A, 2016, A670, p.335-341.

Non Patent Literature 8: Ogawa, Y., et al., Scripta Materialia, doi.org/10.1016/j.scriptamat.2016.09.024

Non Patent Literature 9: Ogawa Y., et al., Science, 2016, Vol.353(6297), pp.368-370.

Non Patent Literature 10: Ogawa, Y. et al., Abstracts of Meeting of Japan Institute of Metals and Materials CD (159th Autumn Annual Meeting of Institute of Metals and Materials), 2016, ISSN 1342-5730, 339

Non Patent Literature 11: Handbook of Advanced Magnesium Technology, edited by the Japan Magnesium Association, published by Kallos Publishing Co. Ltd., 2000, Chapters 4 and 5, pp.71 to 129.

Summary of Invention

Technical Problem

[0012] Analyses have been performed on the Mg-Sc alloys as described above; however, there are still many unclear points with respect to the method for controlling the microstructures of the Mg-Sc alloys, and details of the mechanical properties of the Mg-Sc alloys. In addition, Mg alloys having superelasticity and shape memory property and being excellent in cold workability have never been developed yet. An object of the present invention is to provide a Mg alloy having superelastic effect and/or shape memory effect, and being excellent in cold workability.

Solution to Problem

[0013] The present inventors made a diligent study, and consequently have discovered that a Mg-Sc alloy having a BCC structure having a specific composition range exhibits superelastic effect concomitantly with stress-induced transformation. Moreover, the present inventors have discovered that the foregoing Mg-Sc alloy has shape memory effect (Non Patent Literature 9 and Non Patent Literature 10). The present invention relates to the following alloy in which a certain amount of Sc is added to Mg, and a method for producing the same.

[0014] 

(1) A Mg alloy having a BCC phase, and having superelastic effect and/or shape memory effect, wherein the alloy comprises Mg as a main component, the alloy contains Sc in a range of more than 13 at% and 30 at% or less, and the balance is Mg and inevitable impurities.

(2) The Mg alloy having superelastic effect and/or shape memory effect according to (1), containing in addition to the above-described composition, as an additive element(s), at least one or more selected from the group consisting of Li, Al, Zn, Y, Ag, In, Sn and Bi, in a total content of 0.001 or more and 9 at% or less in relation to the amount of the whole alloy defined to be 100 at%.

(3) The Mg alloy having superelastic effect and/or shape memory effect according to (1) or (2), containing in addition to the above-described composition, as an additive element(s), at least one or more selected from the group consisting of Ca, Mn, Zr and Ce, in a total content of 0.01 or more and 2.0 at% or less, and the total content of the additive element(s) is 9 at% or less, in relation to the amount of the whole alloy defined to be 100 at%.

(4) A method for producing a Mg alloy having superelastic effect and/or shape memory effect, wherein
a solution treatment is performed at a temperature of 500°C or higher such that the alloy comprises Mg as a main component and the alloy contains Sc in a range of more than 13 at% and 30 at% or less, and the balance is Mg and inevitable impurities, and the solution is subjected to a cooling treatment at a cooling rate faster than 1000°C/min.

(5) The method for producing a Mg alloy according to (4), wherein the solution treatment is performed such that in addition to the above-described composition, as an additive element(s), at least one or more selected from the group consisting of Li, Al, Zn, Y, Ag, In, Sn and Bi are contained in a total content of 0.001 or more and 9 at% or less in relation to the amount of the whole alloy defined to be 100 at%.

(6) The method for producing a Mg alloy according to (4) or (5), wherein the solution treatment is performed such that in addition to the above-described composition, as an additive element(s), at least one or more selected from the group consisting of Ca, Mn, Zr, and Ce are contained in a total content of 0.01 or more and 2.0 at% or less, and the total content of the additive element(s) is 9 at% or less, in relation to the amount of the whole alloy defined to be 100 at%.

(7) The method for producing a Mg alloy according to any one of (4) to (6), wherein an aging treatment is performed in a temperature range from 100°C to 400°C.

(8) A Mg alloy having superelastic effect and/or shape memory effect produced by the production method according to any one of (4) to (7).

Advantageous Effects of Invention

[0015] The Mg alloy of the present invention is excellent in cold workability, and also exhibits a superelastic effect and a shape memory effect. Accordingly, applications in various fields can be expected for the Mg alloy of the present invention. In particular, Mg dissolves in living organisms, accordingly when Mg is used for medical materials such as stents to be left in living organisms, such materials are not required to be exenterated from patients, and therefore burdens to patients can be reduced in an extremely useful manner.

[0016] In addition to the characteristics of Mg alloys that Mg alloys are light in weight and high in specific strength, Mg alloys are excellent in cold workability, and therefore, Mg alloys can be expected to be applied to various structural materials in the aerospace field, the automobile field and the like.

Brief Description of Drawings

[0017] 

[Figure 1] Figure 1 is a graph showing a stress-strain curve of the Mg alloy in Example 1.

[Figure 2A] Figure 2A is a stress-strain cycle test graph of the Mg alloy of Example 1.

[Figure 2B] Figure 2B is a graph showing the relation between ε_t and ε_SE obtained from the stress-strain curve of Figure 2A.

[Figure 3] Figure 3 is a chart showing the X-ray diffraction results after heat treatment of Examples 1, 4, and 6, and Comparative Example 3.

[Figure 4] Figure 4 is a chart showing the results of an X-ray analysis of the Mg alloy of Example 1 performed while a stress was being loaded on the Mg alloy.

[Figure 5] Figure 5 is charts showing the X-ray diffraction patterns of Mg alloys. Figure 5A shows the results of a Mg alloy containing Sc in a content of 20.5 at%, and Figure 5B shows the results of a Mg alloy containing Sc in a content of 19.2 at%.

[Figure 6] Figure 6 is a sequence of the photographs showing how the temperature change recovered the shape of a plate-shaped Mg alloy sample.

[Figure 7] Figure 7 is a graph showing the relation between the yield stress σ_y and the ratio of the relative crystal grain size to the sample plate thickness, and the relation between the superelastic recovery strain magnitude ε_SEⁱ⁼³ and the ratio of the relative crystal grain size to the sample plate thickness.

Description of Embodiments

[0018]  Hereinafter, the present invention is described by way of Examples, but the present invention is not limited by following Examples at all. Specifically, the present invention naturally includes, for example, other examples and embodiments within the scope of the technical concept of the present invention.

[0019] First, the alloy composition of the present invention is described. The Mg alloy of the present invention includes Sc within the range of more than 13 at% and 30 at% or less. When the addition content of Sc is 13 at% or less, the BCC phase is not obtained, and the superelastic effect and the shape memory effect cannot be obtained. When the addition content of Sc is 30 at% or more, the alloy is poor in ductility and undergoes grain boundary fracture.

[0020] The Mg alloy of the present invention may include, if necessary, at least one or more additive elements selected from the group consisting of Li, Al, Zn, Y, Ag, In, Sn and Bi, in a total content of 0.001 to 9 at% in relation to the amount of the whole alloy defined to be 100 at%. The inclusion of these elements allows the further improvement of the superelastic effect and the regulation of the mechanical strength to be expected. When the content of the additive element(s) exceeds 9 at%, the alloy is embrittled, and is hence liable to be poor in workability. When the content of the additive element(s) is less than 0.001 at%, no effect can be expected. Herein, Li is an element stabilizing the BCC structure, and is regarded as effective in the improvement of the workability. Al, Zn, Y, Ag, In and Sn have an effect of improving the strength through solid-solution hardening or precipitation hardening, and suppress the dislocation migration so as to be regarded as effective in improving the superelastic effect.

[0021] Moreover, at least one or more elements selected from the group consisting of Ca, Mn, Zr, and Ce, refining the crystal microstructure without impairing the superelastic effect may also be added. These elements are known to be able to achieve high strength and high ductility by refining the crystal grains, and accordingly a high strength and a high ductility of the Mg alloy can be expected to be achieved (Non Patent Literature 11). These additive elements can be included in a content of 0.01 to 2 at% in relation to the amount of the whole alloy defined to be 100 at%. When the content of the additive element(s) exceeds 2 at%, embrittlement is liable to occur. When the content of the additive element(s) is less than 0.01 at%, the effects of the high strength and the high ductility cannot be expected.

[0022]  Successively, the method for producing an alloy of the present invention is described. When the Mg alloy of the present invention is produced, a predetermined amount of each of the foregoing elements is added, and the resulting mixture is melted in an inert gas atmosphere. For the melting, high frequency heating melting is preferable. The molten alloy is turned into a molten ingot, and the ingot is subjected to hot rolling and cold rolling to be processed into a predetermined shape.

[0023] Next, a solution treatment is performed in which the Mg alloy processed into a predetermined shape is heated to a solution treatment temperature range, to transform the crystal microstructure into the BCC phase, and then rapidly cooled. The solution treatment is performed at a temperature of 500°C or higher. The solution treatment temperature is varied depending on the composition of the sample; in general, with the increase of the Sc content, the temperature can be decreased. In an alloy having a relatively larger Sc content, a perfect solution treatment is possible at a temperature of approximately 500°C; however, in an alloy having a lower Sc content, the solution treatment is required to be performed at a higher temperature. Because when a solution treatment is performed at 550°C or higher, a perfect solution treatment is performed, the treatment temperature is preferably 550°C or higher and 800°C or lower. At a temperature of 550°C or lower, alloys lower in the Sc content sometimes undergo formation of a large amount of the HCP phase to fail in obtaining the superelastic effect. On the other hand, at a temperature of 800°C or higher, the material starts to melt. The retention time at the treatment temperature may be 1 minute or more; however, when the retention time exceeds 24 hours, the effect of oxidation cannot be ignored. Accordingly, the retention time at the treatment temperature preferably falls within a range from 1 minute to 24 hours. By rapidly cooling after heating to the solution treatment temperature range, it is possible to produce the Mg-Sc alloy having the BCC phase. From the viewpoint of the superelasticity, the cooling rate is preferably 1000°C/min or more.

[0024] By further performing an aging treatment, it is possible to increase the hardness of the material. The achievement of a high hardness allows the superelasticity, in particular the repeating properties, to be improved. The aging treatment temperature is preferably 100°C or higher and 400°C or lower.

Examples

[0025] Next, the present invention is described in more detail by way of Examples and Comparative Examples. According to the compositions shown in Table 1, Mg alloys were produced by mixing Sc alone in Mg (Examples 1 to 6), and by further mixing Li, Al, Zn, Y, Ag, In, Sn, and Bi (Examples 7 to 16).

[0026] Specifically, individual materials were weighed out so as to give the alloy compositions of Examples 1 to 16 in Table 1 presented below, and were melted in an argon atmosphere, by using a high frequency melting furnace. Alumina crucibles were used as crucibles, and after melting, the molten materials were retained in the crucibles to produce molten ingots. Next, each of the ingots was hot rolled at a temperature of 600°C to a thickness of approximately 2 mm, and then cold rolled to a thickness of approximately 0.7 mm at a temperature of 600°C, while annealing was being repeated. The obtained sample was subjected to a solution treatment at a temperature of 500°C to 700°C for 30 minutes, and then rapidly cooled at a rate of 1000°C/min or more to prepare a Mg alloy sample. The solution treatment temperature is verified by investigating the temperature allowing the BCC phase to be obtained as a single phase by using an optical microscope.

[0027] The alloys of Comparative Examples 1 to 4 were prepared as follows. The materials were weighed out according to the compositions shown in Table 1, and were melted by using a high frequency melting furnace in the same manner as in Examples. Next, in each of Comparative Examples 1 and 2, the ingot was hot rolled to a thickness of approximately 2 mm at a temperature of 600°C, and then cold rolled to a thickness of approximately 0.7 mm at a temperature of 600°C, while annealing was being repeated. On the other hand, in each of Comparative Examples 3 and 4, the ingot was hot rolled to a thickness of approximately 2 mm at a temperature of 300°C, and cold rolled to a thickness of approximately 0.7 mm at a temperature of 300°C while annealing was being repeated. The obtained samples were heat treated at a temperature of 300°C for 30 minutes, and then rapidly cooled at a rate of 1000°C/min or more to prepare Mg alloy samples. The hot rolling temperatures and the subsequent heat treatment temperatures of the samples are different from each other because the melting temperatures are different depending on the compositions of the samples.

[0028] Next, specimens were prepared with the alloys, and measurements were performed to determine whether or not the superelasticity was exhibited. Each of the specimens was subjected to mechanical surface polishing so as to have a final thickness of 0.5 mm. The size of each of the specimens was set to be 3.5 mm in width, 0.5 mm in thickness, and 10 mm in gauge length; a test was performed at a test temperature of -150°C, and at a tensile rate of 0.5 mm/min. After loading a 4% pre-strain, the stress was unloaded, and thus the superelastic shape recovery rate of the given strain was determined.

[0029] Here, the superelastic shape recovery rate was defined as the shape recovery magnitude due to the superelasticity after the unloading of the load of the 4% tensile strain, and was evaluated on the basis of the following formula:

[0030] As an example, the stress-strain curve obtained in the sample of Example 1 is shown in Figure 1. When a stress is applied, first an elastic strain proportional to the stress is generated. When the yield point (around 1% strain in Figure 1) is reached, subsequently strain is generated without largely increasing the stress. As can be seen from Figure 1, by unloading the stress after loading of the 4% pre-strain, the sample of Example 1 manifested an excellent superelastic effect such that the given strain was restored to a nearly original state.

[0031] It is to be noted that as shown in Figure 1, ε_t is "the pre-strain obtained by subtracting the recovery due to elastic distortion from the tensile load strain (4%)," and ε_SE is "the superelastic recovery strain." By using alloys having various compositions, the superelastic shape recovery rates were determined. The results thus obtained are shown in Table 1.

[Table 1]

	Mg	Sc (at.%)	Additive elements	Superelastic shape recovery rate (%)
Example 1	Balance	20.5	-	90
Example 2	"	19.5	-	77
Example 3	"	14.5	-	75
Example 4	"	22	-	90
Example 5	"	26.5	-	93
Example 6	"	29.5	-	90
Example 7	"	21	Li:5at.%	92
Example 8	"	20	Li:8.6at.%	90
Example9	"	20.5	Li:5at.%, Y:2at.%	85
Example 10	"	18	Li:5at.%, Al:2at.%	88
Example 11	"	20	Li:5at.%, Zn:2at.%	85
Example 12	"	18	Li:5at.%, Sn:2at.%	80
Example 13	"	20	Li:5at.%, Bi:2at.%	80
Example 14	"	20	Li:3at.%, Ag:3at.%, In:2at.%	75
Example 15	"	21	Ag:2at.%, Y:2at.%	80
Example 16	"	22	Li:3at.%, Al: 2at.%, Y:2at.%	80
Comparative Example1	"	10	Al:4at.%	0
Comparative Example 2	"	13	-	0
Comparative Example 3	"	-	Al:2.7at.%, Zn:0.4at.% (AZ31)	0
Comparative Example 4	"	-	Zn:2.1at.%,Zr:0.15at.% (ZK60)	0

[0032] As shown in Table 1, in the case (Comparative Example 2) where 13 at% of Sc alone was added to Mg, absolutely no superelasticity was exhibited. On the other hand, in the case (Example 3) where 14.5 at% of Sc was added, a superelastic shape recovery rate of 75% was exhibited. In the case where the content of Sc was less than 13 at%, even when the composition included 14 at% of Sc and another element in combination (Sc 10 at%-Al 4 at%, Comparative Example 1), absolutely no superelasticity was exhibited. Consequently, it has been concluded that the addition of Sc in a content of more than 13 at% is required for the purpose of having a superelastic effect.

[0033] In addition, in the case where Sc is alone added to Mg, the addition of Sc in a content of 20.5 at% or more allows the superelastic shape recovery rate of 90% or more to be obtained (Example 1). Accordingly, it is preferable to prepare an alloy composition including Sc as added therein in a content of 20.5 at% or more. When Example 5 in which Sc was added in a content of 26.5 at% is compared with Example 6 in which Sc was added in a content of 29.5 at%, it is found that Example 5 smaller in the Sc content was higher in the superelastic shape recovery rate. In the case where Sc is added alone, it is understood that a high superelastic shape recovery rate can be obtained with a peak around the addition amount of Sc of 26.5 at%.

[0034] In the cases where, Li, Al, Zn, Y, Ag, In, Sn and Bi were further added as the additive elements in addition to Sc, high superelastic shape recovery rates were exhibited in the same manner (Examples 7 to 16). The superelastic shape recovery rate is varied depending on the elements added in addition to Sc and the addition amounts of the elements; however, the improvement of the superelasticity can be obtained as compared with the case where Sc is added alone. For example, the addition amount of Sc in the Mg alloy of Example 10 is as small as 18 at%, but the superelastic recovery rate is 88%. In contrast, the superelastic recovery rate of the alloy of Example 2 in which Sc was added alone in a content of 19.5 at% is 77%, so as for the superelastic recovery rate of the Mg alloy of Example 10 to be higher than this value.

[0035] In addition, although not shown herein, as described above, Li contributes to the workability improvement, and Al, Zn, Y, Ag, In and Sn contribute to the strength improvement through solid-solution hardening or precipitation hardening, and accordingly, the addition of these additive elements allows the improvement of the mechanical properties other than the improvement of the superelastic effect to be expected. Accordingly, the addition of a plurality of additive elements allows the improvement of different mechanical properties other than the superelastic effect to be expected.

[0036]  Moreover, at least one or more additive elements selected from the group consisting of Ca, Mn, Zr, and Ce may be added. The addition of Ca, Mn, Zr and Ce refines the crystal microstructure, and accordingly allows the strength improvement and the workability improvement to be expected.

[0037] The Mg alloy sample of Example 1 was subjected to a tensile cycle test, and the obtained maximum superelastic strain magnitude was evaluated. The tensile cycle test gives the results of the superelastic recovery strain magnitude (ε_SE) measured while the tensile load strain magnitude (ε_t) is being gradually increased. Figure 2A shows the stress-strain cycle test chart. In Figure 2A, σ_y denotes the yield stress, ε_tⁱ denotes the tensile load strain magnitude in the cycle i, ε_eⁱ denotes the pure elastic recovery strain magnitude in the cycle i, ε_SEⁱ denotes the superelastic recovery strain magnitude in the cycle i, and ε_rⁱ denotes the residual strain magnitude in the cycle i. In the first cycle, the alloy sample is loaded with a tensile force up to a strain magnitude of 1%, and then unloaded. In the second cycle, the alloy sample is loaded with a tensile force up to a strain magnitude of 2%, and then unloaded. While this operation was repeated to the eighth cycle, the stress was measured. Figure 2B shows the relation between the tensile load strain magnitude and the superelastic recovery strain magnitude, obtained from the measurement results of the tensile cycle test, and the maximum pure elastic recovery strain magnitude of the Mg alloy of Example 1 was 4.4%. In addition, although no results are shown herein, the Mg alloys of other Examples also exhibited equivalent maximum pure elastic recover strain magnitudes.

[0038] In addition, as shown in Table 1, no superelasticity was exhibited by the existing Mg alloys (AZ31: Comparative Example 3, ZK60: Comparative Example 4) in which Sc was not added at all. These existing Mg alloys have been shown to take the HCP structure, and thus, it is suggested that the participation of the BCC structure is important for exhibiting the superelasticity in the case of the Mg alloys.

[0039] The present inventors have already revealed that some Mg-Sc alloys are provided with the BCC structure, and in addition, in order to elucidate the relation between the Mg alloys exhibiting the superelasticity and the BCC structure, an crystal structure analysis was performed on the basis of X-ray diffraction.

[0040] The specimens of the alloys of Examples 1, 4, 6, and Comparative Example 3 were prepared by performing solution treatment by heat treatment and performing rapid cooling in the same manner as described above. Each of the specimens was set to have a size of 10 mm × 20 mm × 0.7 mm, and the surface of the specimen was mirror-finished by physical polishing. The prepared specimens were subjected to X-ray diffraction. An X-ray diffractometer, Ultima, manufactured by Rigaku Corporation was used, the θ/2θ method was adopted and Cu K-α was used as an X-ray source. The results thus obtained are shown in Figure 3. Herein, the ordinate is given in logarithmic scale.

[0041] In Examples 1, 4 and 6, the peaks (marked with ○ in the chart) indicating the BCC phases are large in strength, showing that substantially the BCC phases are present as single phases. It is to be noted that in Example 1, the peaks (marked with ● in the chart) indicating the HCP phase were observed to some extent, but these were produced during rapid cooling after heat treatment, and the fraction of the HCP phase was 10% or less. On the other hand, in Comparative Example 3, strong peaks of the HCP phase were observed, showing that the HCP phase is present as a single phase. From these results, it has been shown that the presence of the BCC phase is important for exhibiting the superelasticity.

[0042] In addition, when the sample of Example 1 was subjected to an X-ray diffraction at -150°C while being loaded with a stress, it has been found that a phase having an orthorhombic crystal structure is generated from the BCC structure. Figure 4 shows the results of an X-ray diffraction of the sample of Example 1 performed at -150°C while a stress was being loaded on the sample of Example 1.

[0043] In the sample of Example 1, in the state of being free from stress loading at -150°C, the BCC phase is observed as the main phase in the same manner as in the results (measured at room temperature in a state of being free from stress loading) of Example 1 of Figure 3, and the HCP phase produced during cooling is observed to some extent. On the other hand, as shown in Figure 4, in the state of being loaded with a stress at -150°C, additionally phases being probably orthorhombic crystal structures are observed (marked with arrows in the chart). The orthorhombic crystal products disappear after unloading the stress. This means that the superelastic effect is obtained in the Mg-Sc alloy having the BCC phase in connection with the stress-induced transformation, in the same manner as in common shape memory alloys. In this way, in the Mg-Sc alloy, an excellent superelastic shape recovery rate is obtained in connection with the reversible transformation in association with the stress loading-unloading in the BCC phase.

[0044]  Next, an analysis was performed on the correlation between the cooling rate after the solution treatment and the exhibition of the superelastic property. The Mg alloy (Mg alloy containing 20.5 at% of Sc) having the same composition as the composition of Example 1 was subjected to the solution treatment, and then Mg alloys were produced by varying the cooling rate so as to be 1000°C/sec, 1000°C/min, 100°C/min, and 20°C/min. The produced Mg alloys were subjected to a tensile test to measure the superelastic shape recovery rate. The produced Mg alloys were also subjected to X-ray diffraction to analyze the phase structure. The results thus obtained are shown in Table 2.

[Table 2]

Cooling rate	Superelastic shape recovery rate	Phase structure
1000°C/sec	90%	BCC (+HCP)
1000°C/min	70%	BCC (+HCP)
100°C/min	0%	HCP
20°C/min	0%	HCP

[0045] When the cooling was performed at 1000°C/sec and 1000°C/min, the superelastic recovery rates of 70% or more were obtained, but when the cooling was performed at 100°C/min and 20°C/min, no superelastic property was obtained. When a Mg alloy containing 20.5 at% of Sc was used, as the X-ray diffraction results shows, even a cooling at 1000°C/sec or 1000°C/min allowed a small amount of HCP phase to be contained. Fundamentally, the slower the cooling after the heat treatment, the more grows the HCP phase. With growth of the HCP phase, the superelastic recovery rate is also decreasingly exhibited. In the compositions of the Mg-Sc alloys, the superelastic shape recovery rates due to the cooling temperature are different from each other; however, the cooling performed at a rate faster than 1000°C/min allows any alloys shown in Examples to exhibit superelasticity.

[0046] From the above-described results, in order to for the Mg alloy to have the superelastic property, it has been shown to be very important that Sc is contained in a range from more than 13 at% to 30 at% or less, and the cooling rate after the solution treatment allows the BCC phase to be taken as the crystal structure.

[0047] Next, an analysis was performed as to whether or not these Mg alloys undergo martensitic transformation under stress-free conditions. The samples of the Mg alloy (containing Sc in a content of 20.5 at%) of Example 1 and an alloy containing Sc in a content of 19.2 at% were subjected to an X-ray diffraction at 20°C and -190°C (Figure 5).

[0048] Figure 5A shows the X-ray diffraction patters at 20°C and -190°C, of a Mg alloy having the BCC phase, and containing Sc in a content of 20.5 at%. The results obtained as follows are shown: first, an X-ray diffraction was performed at 20°C, and then the sample was cooled to -190°C and subjected to an X-ray diffraction. The Mg alloy sample containing Sc in a content of 20.5 at% did not show any change between 20°C to -190°C, and it is shown that martensitic transformation did not occur at this temperature.

[0049] The Mg alloy sample containing Sc in a content of 19.2 at% underwent a temperature change from 20°C to - 190°C, and back to 20°C, and was subjected to X-ray diffraction at the respective temperatures (Figure 5B). In this composition, the cooling to -190°C caused a martensitic transformation from a body-centered cubic structure to an orthorhombic structure (orthorhombic martensite phase, denoted as ortho-M in the chart). The martensite phase is reversibly changed into the BCC phase by again increasing the temperature to 20°C. Because the Mg alloy having this composition undergoes temperature-dependent martensitic transformation between 20°C and-190°C, the shape memory property was suggested to be exhibited.

[0050] Accordingly, an analysis was performed as to whether or not the shape memory property of a Mg alloy containing Sc was exhibited. A plate-shaped sample of a Mg alloy containing Sc in a content of 18.3 at% was deformed so as to have a surface distortion of approximately 5% at the liquid nitrogen temperature, and then the shape was observed when the temperature was slowly increased while the sample temperature was being monitored (Figure 6). It has been verified that the sample having this composition undergoes the start of the shape recovery from around -30°C. This result shows that the smaller the Sc content, the higher the martensitic transformation temperature.

[0051] Next, an analysis of the shape memory property of the Mg alloy containing Sc in a content of 16.2 at%, Zn in a content of 1.0 at%, and Zr in a content of 0.1 at%. A sample having the aforementioned composition was analyzed with respect to the martensitic transformation start temperature (Ms), and the finish temperature (Mf), and the martensitic reverse transformation start temperature (As), and the finish temperature (Af), by using a differential scanning calorimeter (DSC). Consequently, it was found that Ms=5°C, Mf=-30°C, As=20°C, and Af=50°C.

[0052] Moreover, by using a sample having this composition, the shape memory property was analyzed. A plate-shaped sample having this composition was bend-deformed so as to have a surface distortion of approximately 3% at the liquid nitrogen temperature, and then heated to 50°C or higher, and thus recovered to a nearly straight shape. The shape recovery rate was found to be 95% or more, so as to be in good agreement with the above result obtained by using DSC. This result shows that when Sc is contained in a certain content, even a sample containing elements other than Sc has the shape memory property. In addition, this alloy composition allows the shape recovery at room temperature or higher to be achieved, and allows the alloy having this alloy composition to be used at an environmental temperature in the vicinity of room temperature. By regulating the composition in the manner as in present Example, an alloy that exhibits the shape memory effect at an environmental temperature in the vicinity of room temperature is obtained, and thus, the application range of such an alloy can be widened.

[0053] Next, the Mg alloy containing Sc in a content of 20.5 at% was investigated with respect to the yield stress σ_y, the pure elastic recovery strain magnitude, and the relation of the relative crystal grain diameter to the plate thickness of the sample (crystal grain diameter d/sample plate thickness t). A stress-strain cycle test as shown in Figure 2 was performed, and the yield stress and the superelastic strain magnitude (ε_SEⁱ⁼³) obtained by applying a 3% strain and then unloading the strain were respectively plotted against the plate thickness of the sample (Figure 7).

[0054] It has been shown that the yield stress is decreased with the increase of the relative crystal grain diameter in relation to the plate thickness of the sample, and on the other hand, the superelastic property is improved. This is the same tendency as the characteristics seen in other shape memory alloys. Figure 5 shows the XRD results down to -190°C; however, in the case of the Mg alloy having the composition of a Sc content of 20.5 at%, no martensitic transformation thermally occurs in the temperature range of absolute zero degree or higher. However, even a Mg alloy having a composition free from the thermal occurrence of the martensitic transformation in the temperature range of absolute zero degree or higher shows the same characteristics as the characteristics shown by other shape memory alloys, as shown in Figure 7, and therefore, such a Mg alloy has a possibility of recovering the shape depending on the conditions.

Industrial Applicability

[0055] The Mg alloy of the present invention is excellent in cold workability, and also exhibits superelastic property and shape memory property. The Mg alloy of the present invention having superelastic property and shape memory property, can be utilized in the aerospace field, the automobile field and the like, because of the feature of being "light" in weight. In addition, because Mg has biodegradability, when the Mg alloy having superelastic effect is used in medical tools such as stents, such medical tools are expected to be dissolved after being left in living bodies for certain periods, and thus the Mg alloy provides significant benefits to patients.

Claims

1. A Mg alloy having a BCC phase, and having superelastic effect and/or shape memory effect,
wherein the alloy comprises Mg as a main component, the alloy contains Sc in a range of more than 13 at% and 30 at% or less, and
the balance is Mg and inevitable impurities.

2. The Mg alloy having superelastic effect and/or shape memory effect according to claim 1, containing in addition to the above-described composition, as an additive element(s), at least one or more selected from the group consisting of Li, Al, Zn, Y, Ag, In, Sn and Bi, in a total content of 0.001 or more and 9 at% or less in relation to the amount of the whole alloy defined to be 100 at%.

3. The Mg alloy having superelastic effect and/or shape memory effect according to claim 1 or 2, containing in addition to the above-described composition, as an additive element(s), at least one or more selected from the group consisting of Ca, Mn, Zr and Ce,
in a total content of 0.01 or more and 2.0 at% or less, and the total content of the additive element(s) is 9 at% or less, in relation to the amount of the whole alloy defined to be 100 at%.

4. A method for producing a Mg alloy having superelastic effect and/or shape memory effect, wherein
a solution treatment is performed at a temperature of 500°C or higher such that
the alloy comprises Mg as a main component and
the alloy contains Sc in a range of more than 13 at% and 30 at% or less, and the balance is Mg and inevitable impurities, and
the solution is subjected to a cooling treatment at a cooling rate faster than 1000°C/min.

5. The method for producing a Mg alloy according to claim 4, wherein the solution treatment is performed such that
in addition to the above-described composition, as an additive element(s), at least one or more selected from the group consisting of Li, Al, Zn, Y, Ag, In, Sn and Bi are contained
in a total content of 0.001 or more and 9 at% or less in relation to the amount of the whole alloy defined to be 100 at%.

6.  The method for producing a Mg alloy according to claim 4 or 5, wherein the solution treatment is performed such that
in addition to the above-described composition, as an additive element(s), at least one or more selected from the group consisting of Ca, Mn, Zr, and Ce are contained in a total content of 0.01 or more and 2.0 at% or less, and the total content of the additive element(s) is 9 at% or less, in relation to the amount of the whole alloy defined to be 100 at%.

7. The method for producing a Mg alloy according to any one of claims 4 to 6, wherein an aging treatment is performed in a temperature range from 100°C to 400°C.

8. A Mg alloy having superelastic effect and/or shape memory effect produced by the production method according to any one of claims 4 to 7.

Drawing