Patent Publication Number: US-2015064053-A1

Title: Pseudoelastic magnesium alloy, pseudoelastic magnesium alloy component, and production method thereof

Description:
INCORPORATION BY REFERENCE 
     The disclosures of Japanese Patent Application Nos. 2013-181682 and 2014-043677 filed on Sep. 2, 2013 and Mar. 6, 2014 including the specification, drawings and abstract are incorporated herein by reference in their entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a magnesium alloy, and more particularly to a pseudoelastic magnesium alloy. 
     2. Description of Related Art 
     Magnesium has the lowest density among metals commonly used for structural materials and the like. Moreover, because of its abundant natural resources and excellent recyclability, magnesium has received considerable attention as a next-generation structural material. In particular, magnesium alloys prepared by adding various additive elements to magnesium are lightweight, have high levels of specific strength and rigidity, and excellent shock absorption. Therefore, magnesium alloys have been studied for use as various structural materials such as automotive parts, housings for mobile electronic devices, and the like (e.g., Japanese Patent Application Publication No. 2005-213535 (JP 2005-213535 A), Japanese Patent Application Publication No. 2006-257478 (JP 2006-257478 A)). 
     Meanwhile, Ti—Ni series alloys have been reported as shape-memory alloys that, after being deformed at low temperature, return to a previously memorized shape when heated to a predetermined temperature, and as pseudoelastic alloys that undergo apparent plastic deformation under stress, but return to their original shape when the stress is unloaded (e.g., Japanese Patent Application Publication No. 2001-262298 (JP 2001-262298 A), Japanese Patent Application Publication No. 10-237572 (JP 10-237572)). 
     It has also been reported that when an applied stress is unloaded from a magnesium alloy, deformation is removed slightly due to the formation and extinction of twins. However, the extent of such removing is small and less than 0.5% (Reversible plastic strain during cyclic loading-unloading of Mg and Mg—Zn alloys, Materials Science and Engineering A, Vol. 456, 2007, pp. 138-146). 
     SUMMARY OF THE INVENTION 
     As noted above, there have been various reports of pseudoelastic alloys. However, a magnesium alloy that exhibits pseudoelasticity has not been reported. A lightweight magnesium alloy with pseudoelasticity will have a wide range of applications; thus, a need exists for the development thereof. Therefore, the invention provides a magnesium alloy with pseudoelasticity, and a manufactured product thereof. 
     The inventors conducted diligent and incisive research to obtain a pseudoelastic magnesium alloy that utilizes the formation and extinction of twins, and if deformed by applying stress exceeding the elastic deformation range, returns to its original shape when the stress is unloaded. As a result, the inventors discovered that by preparing a magnesium alloy containing specific elements and having an aligned crystal orientation, the strain caused by an applied stress is damped by twins that form regularly, and when the stress is unloaded, the strain is removed due to the extinction of the twins, and the alloy returns to its original shape. 
     Based on this knowledge, the invention is a magnesium alloy containing magnesium as a main component, and at least one element selected from Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu, and having a unidirectional crystal structure. A pseudoelastic magnesium alloy wherein strain removing can be obtained thereby because the regular formation of twins damps the strain from an applied stress, and the twins become extinct when the stress is unloaded. 
     In accordance with this invention, a pseudoelastic magnesium alloy that is impossible with related art can be obtained. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Features, advantages, and technical and industrial significance of exemplary embodiments of the invention will be described below with reference to the accompanying drawings, in which like numerals denote like elements, and wherein: 
         FIG. 1  is a stress-strain graph of a magnesium alloy of related art; 
         FIG. 2  is a photograph showing twins formed in two directions in an Mg-Y single crystal; 
         FIG. 3  shows twins that grow with the application of stress and become extinct with the unloading of stress; 
         FIG. 4  shows the stress-strain graph of the magnesium alloy of the invention; 
         FIG. 5A  shows the stress-strain graph of the magnesium alloy (solution) of the embodiment of the invention; 
         FIG. 5B  is a stress-strain graph of the magnesium alloy (aged 5 h, under-aged) of the embodiment of the invention; 
         FIG. 5C  is a stress-strain graph of the magnesium alloy (aged 96 h, fully aged) of the embodiment of the invention; 
         FIG. 5D  is a stress-strain graph of the magnesium alloy (aged 240 h, over-aged) of the embodiment of the invention; 
         FIG. 6A  shows a photograph of the structure of the magnesium alloy (solution, not aged) of the embodiment of the invention; 
         FIG. 6B  shows a photograph of the structure of the magnesium alloy (aged 5 h) of the embodiment of the invention; 
         FIG. 6C  shows a photograph of the structure of the magnesium alloy (aged 96 h) of the embodiment of the invention; 
         FIG. 6D  shows a photograph of the structure of the magnesium alloy (aged 240 h) of the embodiment of the invention; and 
         FIG. 7A  is a stress-strain graph of the magnesium alloy of the comparative example. 
         FIG. 7B  is a stress-strain graph of the magnesium alloy of the comparative example. 
         FIG. 7C  is a stress-strain graph of the magnesium alloy of the comparative example. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     The invention is further described in detail below. As used below, the symbol “%” refers to “atom %.” Pseudoelasticity is a phenomenon wherein a strain caused by the application of stress is removed with the unloading of the stress. The cause is believed to be as follows: when stress that exceeds the elastic region is applied, strain is stored due to the formation and growth of twins without producing irreversible plastic strain associated with dislocations such as basal slip of the crystals, and when the stress is removed, the formed and grown twins become extinct. 
       FIG. 1  is a stress-strain graph of a magnesium alloy of related art. When stress is applied to a magnesium alloy of related art, an elastic strain proportional to the stress is generated  11 , and when the yield point is reached, irreversible plastic strain accompanying dislocations such as basal slip accumulates thereafter, even without the addition of a large amount of stress. Simultaneously, local strain accumulates  12  due to the random formation of twins. When the stress is unloaded, the stored elastic strain is removed  13 . The stored strain resulting from twinning also is removed slightly  14  because some of the formed twins become extinct. 
     However, the amount of removing from strain due to extinction of the twins is extremely small, and no large removing is observed. The reason is as follows: the formation of twins is random when the stress is applied, so the formed twins mutually interfere, strain arising from local dislocations and the like occurs and becomes permanent, and even if the stress is unloaded, most of the twins do not become extinct. Moreover, once irreversible plastic strain accompanying dislocations such as basal slip accumulates, removing does not occur even if the stress is unloaded. Therefore, achieving pseudoelasticity requires both of the following two conditions. 
     (1) If a stress greater than the critical resolved shear stress (CRSS) for either slipping or twinning is applied to an alloy, then either slipping (dislocation) or twinning occurs. If the stress is no greater than the CRSS, this becomes elastic deformation. If basal slip occurs easily in the hexagonal crystals of magnesium when stress is applied, plastic strain accompanying the dislocation due to slippage will accumulate, and the strain will not be removed, even if the stress is unloaded. For strain to be removed when the stress is unloaded, the strain resulting from the formation and growth of twins need to be stored when stress exceeding the elastic region is applied. In the pseudoelastic magnesium alloy of this invention, elements that reduce this basal slip (increase the CRSS of basal slip) and also promote the formation of twins (decrease the CRSS of twins or do not increase it excessively) are selected as additive elements. 
     It has been reported that Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu are effective for reducing the basal slip of magnesium hexagonal crystals (e.g., Deformation Behavior of Mg Alloy Single Crystals at Various Temperatures, Materials Science Forum, Vol. 350-351, 2000, pp. 183-188). The mechanism has not been clarified, but the addition of these elements can promote the formation of twins. In the case of Y, for example, the addition of 1% makes the CRSS of basal slip about 10 MPa, which is approximately 10 times greater than in pure magnesium. In contrast, the CRSS of twins is about 17 MPa, which does not exceed an increase of approximately five times over pure magnesium. In addition, in an alloy with a high Y content, the CRSS of basal slip increases even more, but the CRSS of the twins barely increases. Because the effectiveness of reducing basal slip and of promoting the formation of twins differs for each element, the most suitable elements can be selected as needed. Elements can be selected separately for reducing basal slip and for promoting the formation of twins, so two or more types of elements can be added, or even three or more types of elements can be added. 
     To provide the effects noted above, these elements need to be present in the form of a solid solution in the matrix rather than as a precipitate (e.g., Mg 24 Y 5 ). If a fixed quantity of an element is present as a solid solution in the matrix, it may also be present as a precipitate simultaneously. If too much of these elements are added, however, a proeutectic solid solution will form as coarse dendrites during solidification, and thereafter a fine eutectic lamellar structure will form between the dendrites. If too many of these structures form, it will interfere with the twins that form when stress is applied, and because strain due to dislocation motion and the like will occur, the twins will not become extinct even if the stress is unloaded. 
     For example, when the added element is Y, the upper limit of the amount that can form a solid solution in the matrix is 3.4%. From the aspects of reduction of basal slip, promotion of twinning, and the formation of dendrites, the amount of Y to be added to the matrix when manufacturing the alloy need to be 1.0% to 6.0%. If the added amount is less than 1.0%, the effects of reducing basal slip and promoting twin formation cannot be adequately obtained by the addition of Y. If the added amount exceeds 6.0%, as noted above, the twins will not become extinct even if the stress is unloaded. If the added amount is 6.0% or less, the effects resulting from dendrite formation will be minimal, and even if part of the Y is present as a precipitate, the Y can effectively form a solid solution in the matrix. In consideration of the amount that can form a solid solution in the matrix, the added amount of Y is preferably 1.0% to 3.4% 
     (2) Even when selective activated twins are formed, if a plurality of twins form at random, the twins mutually interfere, and local strain (such as dislocation) occurs and becomes permanent. Therefore, the majority of twins do not become extinct even if the stress is unloaded. If the formation of twins when stress is applied is regular and they do not mutually interfere, local dislocation and the like does not occur, and greater removing from the strain occurs due to the extinction of twins when the stress is unloaded. Therefore, to make twin formation regular in the magnesium alloy of the invention the crystal orientation is aligned in one direction. In concrete terms, the crystal orientation can be aligned by making the crystal structure monocrystalline or by producing a crystal texture by rolling a polycrystalline alloy. 
     In the case of magnesium with a hexagonal crystal structure, a twin can assume six different directions with respect to the crystal, but the direction of the twin that forms is limited by the direction of the applied stress. In other words, if a compressive stress is applied from the &lt;11-20&gt; direction, twins will form in four directions, and with compression from the &lt;10-10&gt; direction, twins will form in two directions. In addition, twins will form in only one direction with compression from the &lt;10-1x&gt; direction, which is only slightly inclined in the direction of the c-axis from the &lt;10-10&gt; direction. The angle of inclination toward the direction of the c-axis from the &lt;10-10&gt; direction is 1° to 10° at that time. In other words, to cause twins to form selectively so they do not mutually interfere and will later become extinct, first the crystal orientation need to be aligned, and then the stress need to be applied from a specific direction. 
       FIG. 2  shows the structure of twins that formed in an Mg-Y single crystal when stress is applied from the &lt;10-10&gt; direction. It can be seen that a plurality of twins form as a result of the application of stress, and the direction of the formed twins is aligned in two directions.  FIG. 3  shows twins that grow with the application of stress and become extinct with the unloading of stress. If they do not interfere with other twins, the grain boundary, and the like, twins that grow as a result of the application of stress will become extinct as a result of the unloading of the stress. 
       FIG. 4  shows the stress-strain graph of the magnesium alloy of the invention. The region wherein stress is applied and elastic strain accumulates  41  is the same as in the magnesium alloys of the related art. The succeeding region  42  traces a line similar to the magnesium alloys of the related art, but plastic strain caused by dislocation and the like is reduced, and the strain is stored due to selective, regular twinning. When the stress is unloaded, the elastic strain is removed in the same manner as in magnesium alloys in the related art  43 , and the strain stored by twinning additionally is removed due to the extinction of the twins  44 . When the stress is applied, however, dislocation and the like will occur in some of the formed twins due to interference with other twins, the grain boundary, and the like, and sometimes plastic strain will remain  45 . 
     A conventional process can be used for producing the magnesium alloy of the invention. The magnesium alloy of the invention has a unidirectional crystal structure (a structure formed from crystals wherein the crystallographic orientation is aligned). A magnesium alloy wherein a unidirectional crystal structure is realized as a single crystal alloy can be produced using the Bridgman method or another conventional production method. Moreover, a magnesium alloy wherein a unidirectional crystal structure having a rolled texture with a preferred orientation can be attained as a crystal texture by producing a magnesium alloy plate material using a conventional method and then rolling the material to control the crystal orientation. 
     An aging treatment can also be applied to the magnesium alloy of the invention. If the added element is Y, for example, compounds such as Mg 24 Y 5  will precipitate when the aging treatment is performed. The stress to initiate removing from strain resulting from twin extinction and the amount of the removing can be adjusted by the amount of the precipitate. The stress to initiate removing and the amount of removing can be adjusted depending on the objective. As noted above, however, a fixed amount of the element need to still be present as a solid solution in the matrix even when a precipitate is present. 
     As noted above, the magnesium alloy of the invention is anisotropic, and the number of directions of the twins that form will differ depending on the direction of the applied stress. When twins form in a plurality of directions, they are likely to mutually interfere, and strain will remain because the twins will not become extinct even if the stress is unloaded. Therefore, when the magnesium alloy of the invention is used as a component, the direction of applied stress to the finished component becomes important. More specifically, the magnesium alloy of the invention is fabricated so that the direction of an applied compression stress to the finished component will be a direction with an inclination angle of 10° or less toward the c-axis from the &lt;10-10&gt; direction of the magnesium hexagonal crystal. By so doing, the formed equivalent twins will be limited to two or fewer types, and interference among the twins when stress is applied will be controlled, so the pseudoelastic effect of the magnesium alloy of the invention can be most advantageously realized. 
     A case wherein compression stress is applied has been described in the above explanation. Needless to say, however, the same applies to a case wherein tensile stress is applied in a direction near [0001] axis which is perpendicular to the direction wherein the abovementioned compression stress is applied. 
     Embodiment 1 
     Using the Bridgman method single crystals of two types of magnesium alloys with charge compositions of Mg-0.5%Y and Mg-1.7%Y are prepared. To reduce contamination by impurities as much as possible the crystals are grown in a high purity graphite crucible under a 50 cm 3 /min argon flow atmosphere at a growth rate of 1 mm/hour. The crystal orientation of the prepared magnesium alloys is confirmed by electron backscatter diffraction (EBSD) method, and 3 mm wide×3 mm deep×6 mm high samples are cut so that the direction of the normal prismatic plane of the hexagonal magnesium crystal would run parallel to the direction of the sample height. 
     To eliminate strain introduced to the sample during cutting, the sample cutouts are placed in a quartz tube sealed with an argon atmosphere, and strain-relief annealing is performed for five cycles by repeated two-hour cycles between 250° C. and 350° C. Then the samples are held at 500° C. for 24 hours as a solution heat treatment, and finally quenched in water. After water quenching has been performed, aging treatments are carried out on some of the samples at 200° C. to control the amount of precipitate. The duration of the aging treatments is either 5, 96, or 240 hours. 
     Stress-strain graphs are prepared by repeatedly adding compression stress at a strain rate of 2×10 −4 /sec in the direction of sample height using a universal testing machine manufactured by Instron Corporation, and then unloading the stress. The amount of strain removing is defined as the amount obtained after excluding the elastic strain component from the total removing when the stress is unloaded. 
     The results are shown in Table 1,  FIG. 5 , and  FIG. 7A-7C . In the samples subjected to the aging treatment, some of the Y precipitated as Mg 24 Y 5  and the like, so the concentration of Y in the matrix is smaller than the concentration of Y in the charge compositions. As shown in Table 1 and  FIG. 5 , a large removing of ≧1.9% is confirmed in the samples of examples Nos. 11 to 14. Moreover, in the samples subjected to the aging treatment it is confirmed that the removing initiation stress differed depending on the duration of the aging treatment.  FIGS. 6A to 6D  show photomicrographs of the examples. As shown in Table 1 and  FIG. 7A-7C , the amount of removing in samples of examples Nos. 15 to 17 is small at ≦0.25%. In this case, the amount of removing is defined as the difference in the size of the strain when the stress is 0 (intersection of  44  and the strain axis of  FIG. 4 ) and the size of the remaining strain when the elastic strain has been removed (intersection of the tangent of  43  and the strain axis of  FIG. 4 ). The removing initiation stress is defined as the stress at the intersection of the tangent of the region wherein the elastic strain is removed ( 43  of  FIG. 4 ) and the tangent of the midpoint of the region wherein removing due to pseudoelasticity occurs (in  44  of  FIG. 4 , a point that is ½ of the above amount of removing.) 
     
       
         
           
               
               
               
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                   
                   
                   
                   
                 Stress to 
                   
               
               
                   
                   
                 Aging 
                   
                 Y concen- 
                   
                 initiate 
               
               
                   
                 Charge 
                 treatment 
                   
                 tration in 
                 Removing 
                 removing 
               
               
                 No. 
                 composition 
                 time (hr) 
                 Precipitate 
                 matrix (%) 
                 (%) 
                 (MPa) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 11 
                 Mg—1.7%Y 
                 None 
                 No (solid 
                 1.7 
                 3.25 
                 40 
                 Example 
               
               
                   
                   
                   
                 solution) 
               
               
                 12 
                 Mg—1.7%Y 
                 5 
                 Yes 
                 1.0 
                 2.1 
                 100 
                 Example 
               
               
                   
                   
                   
                 (Mg 24 Y 5 , etc.) 
               
               
                 13 
                 Mg—1.7%Y 
                 96 
                 Yes 
                 1.0 
                 2.3 
                 110 
                 Example 
               
               
                   
                   
                   
                 (Mg 24 Y 5 , etc.) 
               
               
                 14 
                 Mg—1.7%Y 
                 240 
                 Yes 
                 1.0 
                 1.9 
                 80 
                 Example 
               
               
                   
                   
                   
                 (Mg 24 Y 5 , etc.) 
               
               
                 15 
                 Mg—0.5%Y 
                 None 
                 No (solid 
                 0.5 
                 0.25 
                 — 
                 Comparative 
               
               
                   
                   
                   
                 solution) 
                   
                   
                   
                 Example 
               
               
                 16 
                 Mg—0.8%Y 
                 None 
                 No (solid 
                 0.8 
                 0.2 
                 — 
                 Comparative 
               
               
                   
                   
                   
                 solution) 
                   
                   
                   
                 Example 
               
               
                 17 
                 Mg—1.2%Y 
                 None 
                 No (solid 
                 1.2 
                 0.2 
                 — 
                 Comparative 
               
               
                   
                   
                   
                 solution) 
                   
                   
                   
                 Example 
               
               
                   
               
            
           
         
       
     
     It is confirmed that in the magnesium alloy wherein the concentration of Y in the matrix is 0.5%, the removing is small, but the magnesium alloys wherein the concentration of Y in the matrix is 1.0% or 1.7% has a large removing of ≧1.9%. Moreover, it is confirmed that when precipitate due to the aging treatment is present, the amount of stress at initiation of removing changes. 
     Embodiment 2 
     The amount of removing is compared in the same manner as Embodiment 1 for single crystals of Mg-1.7%Y, polycrystals of Mg-1.7%Y, and AZ31 rolled material, a magnesium alloy commonly used as a structural material. The single crystals are prepared using the method described in Embodiment 1 (no aging treatment), and the polycrystals are prepared by a conventional casting method. For the AZ31 rolled material, samples are cut in the same manner as the single crystals so that the normal direction of the prismatic plane of the hexagonal magnesium crystals would run parallel to the direction of sample height, but because the crystal orientation in the polycrystalline material of Mg-1.7%Y is random, there is no correlation between the direction of the prismatic plane of the hexagonal magnesium crystals and the direction of the height of the cut samples. The results are shown in Table 2. 
     
       
         
           
               
               
               
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                   
                   
                   
                 Y 
                   
                   
               
               
                   
                   
                   
                 concentration 
                 Removing 
                   
               
               
                 No. 
                   
                 Crystal 
                 in matrix (%) 
                 (%) 
               
               
                   
               
             
            
               
                 21 
                 Mg-1.7% Y 
                 Single crystal 
                 1.7 
                 3.25 
                 Example 
               
               
                 22 
                 Mg-1.7% Y 
                 Polycrystal 
                 1.7 
                 0.5 
                 Comparative 
               
               
                   
                   
                 (random) 
                   
                   
                 Example 
               
               
                 23 
                 AZ31 rolled 
                 Polycrystal 
                 0 
                 0.25 
                 Comparative 
               
               
                   
                 material 
                 (texture) 
                   
                   
                 Example 
               
               
                   
               
            
           
         
       
     
     The Mg-1.7%Y single crystal of No. 21 exhibited a large removing, but the Mg-1.7%Y polycrystals of No. 22 and the AZ31 rolled material of No. 23 both has small removing values of ≦50.5%. In the case of the Mg-1.7%Y polycrystals, this occurred because twins are formed when stress is applied, but since the direction of the formed twins is random, the twins mutually interfered, strain such as local dislocation is formed, and became permanent, so most of the twins did not become extinct with unloading. In the case of the AZ31 rolled material, this occurred because the structure is a crystal texture, and although there is little mutual interference in the formed twins, Al and Zn have little effectiveness in reducing basal slip of the magnesium hexagonal crystals and promoting the growth of twins when stress is applied, so strain removing did not increase. 
     With this invention a pseudoelastic magnesium alloy that is impossible with related art can be obtained.