Magnesium-based alloy stretched material

Abstract

[Problem] To provide a Mg-based alloy extension material with which it is possible to reduce yield anisotropy while maintaining excellent tensile yield strength properties. [Solution] Provided is an Mg-based alloy extension material having excellent room-temperature strength and containing Mn and at least one element from among the four elements Al, Ca, Li, and Zn, or further containing, in addition to the above four elements, the two elements Sn and / Bi, with the remainder made up by Mg and unavoidable impurities, wherein the Mn content is 0.03–2 mol% (inclusive), and the content of the aforementioned six elements is at least 0.03 mol% but no greater than the Mn content. The extension material is characterized in that the yield stress obtained by a tensile test is 200 MPa or above and exhibits a ratio, relative to the yield stress obtained by a compressive test, of 0.6 or above.

Description

Technical Field

[0001] The present invention relates to a magnesium (Mg)-based alloy sheet having excellent room temperature strength and reduced yield anisotropy.

Background Art

[0002] Mg alloys have attracted attention as next-generation lightweight metal materials. When used as structural members, it is desired to develop materials and members with excellent strength characteristics in order to ensure safety and reliability. In physical metallurgy, refinement of grain size has been utilized since ancient times as an effective method for improving strength. In particular, Mg has a large Hall-Petch coefficient compared to other metals due to its crystal structure (= hexagonal crystal structure) (Non-Patent Document 1), and thus the effect of grain size refinement on increasing strength is significant. In addition, as another method for improving strength, it is well recognized and experimentally practiced to add one or more elements other than Mg and alloy them. In particular, the greater the atomic radius difference from the base metal, the greater the improvement effect, and in Mg alloys, adding rare earth metals is most effective. However, the use of rare earth elements is not preferable from an economic perspective because the material price soars.

[0003] Turning to general-purpose elements, Mg-Al-Zn: AZ-based alloys containing aluminum and zinc and Mg-Zn-Zr: ZK-based alloys containing zinc and zirconium are in circulation. For these Mg alloys, in order to refine the grain size, hot-rolled processing with heat treatment is applied to improve the strength. During this hot-rolled processing, the basal planes are oriented parallel to the processing direction, forming a basal texture. Therefore, although the "tensile" strength is improved due to grain size refinement, the "compressive" strength is about half of the tensile strength, and there is a problem of yield anisotropy in which the yield stress changes depending on the stress application direction. Usually, plastic deformation of metal materials is carried out by dislocations, but in the case of Mg, when a compressive stress is applied to the c-axis, deformation twins are formed with a stress smaller than that of dislocation movement. This difference in plastic deformation mechanism causes yield anisotropy and is also a factor limiting the handling of Mg sheets.

[0004] Against such a background, the inventors focused on adding only one type of solute element and investigated and studied the strengthening of Mg-based alloys. Based on the research results, a fine-grained Mg alloy with excellent strength characteristics, in which a trace amount of one element selected from rare earth elements or general-purpose elements Ca, Sr, Ba, Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Dr, Tm, Yb, Lu is contained and the crystal grains are refined, is disclosed in Patent Document 1. The strengthening of this alloy is mainly due to the segregation of the added solute element at the grain boundaries.

[0005] Patent Document 2 discloses a Mg-based alloy containing 14.5 mass% or less of Sn, having an average grain size of the Mg matrix phase of 10 μm or less, and in which the proportion of sub-grain boundaries (low-angle grain boundaries) with an average grain size of 2 μm or less among the grain boundaries surrounding the Mg matrix phase occupies the majority, and thus having excellent room-temperature strength characteristics. It has been found that the knowledge of introducing sub-grain boundaries at high density can also be applied to Mg-based alloys containing 3.5 to 11 mass% of Al, which is disclosed in Patent Document 3. The Mg-based alloys disclosed in Patent Documents 2 and 3 are characterized not only by excellent strength but also by a reduction in the yield anisotropy between the compressive yield stress and the tensile yield stress due to the presence of sub-grain boundaries.

[0006] In addition, Patent Document 4 also discloses a high-strength Mg-based alloy in which a Mg-based alloy added with two or more types of solute elements contains Ca and Zn within the solid solution amount and Ca and Zn are segregated parallel to the c-axis direction of Mg.

[0007] Regarding Mg-based alloys containing Mn, the inventors have also conducted extensive investigations and research. Based on the research results, Patent Document 5 discloses a Mg alloy containing 1 mol% or less of Mn and having excellent fracture toughness with deformation twins present in the Mg matrix phase. Patent Document 6 discloses a Mg-based alloy having excellent room-temperature ductility, with the size of the Mg matrix phase being 5 μm or less and containing 0.07 to 2 mass% of Mn. These alloys are characterized in that the elongation at break shows about 100% and the m value, which is an index of the contribution rate of grain boundary slip to deformation, shows 0.1 or more. Also, as an index of formability, the stress reduction degree is used, and its value shows 0.3 or more. Further, Patent Document 7 discloses a Mg-based alloy composed of Mg - Amol% Mn - Bmol% X, where A is 0.03 to 1 mol%, B is 1 times or less of A, containing Bi, Sn, Zr, and having excellent room-temperature ductility that does not fracture even when a nominal strain of 0.2 or more is applied. However, these Patent Documents 5 to 7 relate to improvements in fracture toughness and room-temperature ductility, and there is no description or disclosure regarding improvements in strength characteristics.

[0008] The Mn element added to Mg mostly combines with iron (Fe) and silicon (Si) during melting and is used as an impurity element removing element. It is known that ductility is imparted to Mg alloys containing Mn as the main element (the leading element) (Patent Documents 6 and 7, Non-Patent Documents 2 and 3). This is due to the activation of grain boundary slip by the addition of the Mn element. On the other hand, within the knowledge of the inventors, there are no disclosed examples or reported examples regarding the strength characteristics of Mg-Mn-based alloys. Also, since the atomic radius of the Mn element is smaller than that of Mg and the electronic state of Mn is characterized by a semi-closed shell structure, it is unclear whether there is an effect on the strength characteristics when Mn is added as the main element.

Prior Art Documents

Patent Documents

[0009]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Patent Document 6

Patent Document 7

Non-Patent Document

[0010]

Non-Patent Document 1

Non-Patent Document 2

Non-Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0011] An object of the present invention is to provide a Mg-based alloy sheet material containing Mn as a main element, having excellent room temperature strength and reduced yield anisotropy.

Means for Solving the Problems

[0012] A first aspect of the present invention is a Mg-based alloy strip containing Amol% of Mn and Bmol% of X, with the balance being Mg and inevitable impurities (hereinafter, this Mg-based alloy strip is denoted as "Mg-A(mol%)Mn-B(mol%)X"), where X is any one or more elements selected from the group consisting of Al, Ca, Li, and Zn, the value of A is 0.03 (mol%) or more and 2 (mol%) or less, the relationship between A and B is A≧B, the upper limit value of B is 1.0 times or less the upper limit value of A, and the lower limit value of B is 0.03 (mol%). A Mg-based alloy strip is provided.

[0013] A second aspect of the present invention is a Mg-based alloy strip in the first aspect of the present invention, which is a Mg-based alloy strip in which the X further contains Sn and / or Bi.

[0014] A third aspect of the present invention is a Mg-based alloy strip in the second aspect of the present invention, which is a Mg-based alloy strip containing Amol% of Mn, Bmol% of Sn, and one or more elements selected from the group consisting of Al, Ca, Li, and Zn.

[0015] A fourth aspect of the present invention is a Mg-based alloy strip in the first or second aspect of the present invention, which is a Mg-based alloy strip containing Amol% of Mn, Bmol% of Li, and one or more elements selected from the group consisting of Al, Ca, Zn, Sn, and Bi.

[0016] A fifth aspect of the present invention is a Mg-based alloy strip in the first or second aspect of the present invention, which is a Mg-based alloy strip containing Amol% of Mn, Bmol% of Zn, and one or more elements selected from the group consisting of Al, Ca, Li, Sn, and Bi.

[0017] A sixth aspect of the present invention is a Mg-based alloy strip in any one of the first to fifth aspects of the present invention, which is a Mg-based alloy strip having an average crystal grain size of the Mg matrix phase of 25 μm or less.

[0018] The seventh aspect of the present invention is an Mg-based alloy sheet in any one of the first to sixth aspects of the present invention, wherein intermetallic compound particles having a particle diameter of 0.5 μm or less are dispersed and precipitated in the Mg matrix phase and the grain boundaries in the metal structure of the Mg-based alloy sheet, and the volume fraction thereof is 10% or less.

[0019] The eighth aspect of the present invention is an Mg-based alloy sheet in any one of the first to seventh aspects of the present invention, wherein an element other than Mg is segregated at the grain boundaries of the Mg-based alloy sheet.

[0020] The ninth aspect of the present invention is an Mg-based alloy sheet in any one of the first to eighth aspects of the present invention, wherein the yield stress exceeds 200 MPa in the stress-strain curve diagram obtained by a room-temperature tensile test with an initial strain rate of 1×10 -3 s -1 .

[0021] The tenth aspect of the present invention is an Mg-based alloy sheet in any one of the first to ninth aspects of the present invention, wherein the ratio of the yield stress obtained by a room-temperature tensile and compression test with an initial strain rate of 1×10 -3 s -1 ( = compressive yield stress ÷ tensile yield stress), that is, the yield anisotropy exceeds 0.6.

Advantages of the Invention

[0022] According to the present invention described above, it is possible to provide an Mg-based alloy sheet having high room-temperature strength (yield stress) and reduced yield anisotropy.

Brief Description of the Drawings

[0023]

Figure 1

Figure 2

Figure 3

Mode for Carrying Out the Invention

[0024] The Mg-based alloy material for obtaining the effects of the present invention consists of Mg-A mol% Mn-B mol% X, where X is one or more elements selected from the group of four elements Al, Ca, Li, and Zn, or, in addition to these four elements, two elements Sn and / or Bi are selected. That is, it may be a ternary alloy such as Mg-Mn-Al, or a quaternary alloy such as Mg-Mn-Al-Bi. X may be selected from the above six elements excluding only Sn, only Bi, and the combination of only Sn and Bi. The relationship between A and B is A≧B, and the value of A is preferably 2 mol% or less, more preferably 1.5 mol% or less, and even more preferably 1.0 mol% or less. When the value of A exceeds 2 mol%, high-density α-Mn particles precipitate in the matrix phase, causing early fracture during deformation. The lower limit value of A is 0.03 mol%, preferably 0.1 mol%, and more preferably 0.3 mol%. The upper limit value of B is preferably 1.0 times or less, more preferably 0.9 times or less, and even more preferably 0.8 times or less with respect to the upper limit value of A. The lower limit value of B is 0.03 mol%. Here, 0.03 mol% is a value that defines the boundary between inevitable impurities and added elements. When using recycled Mg-based alloy as the raw material of the Mg-based alloy material, since various alloy elements may be contained in advance, it is to exclude the contents that are usually contained when used as the raw material of the Mg-based alloy material. Elements contained in inevitable impurities include, for example, Fe, Si, Cu (copper), and Ni (nickel).

[0025] The Mg-based alloy extruded material after hot extrusion preferably has an average crystal grain size of the Mg matrix phase of 25 μm or less. More preferably, it is 20 μm or less, and even more preferably 10 μm or less. For measuring the crystal grain size, it is preferable to use the sectioning method based on the G0551 JIS standard. When the crystal grain size is fine or the crystal grain boundaries are unclear, it is difficult to use the sectioning method. Therefore, measurement may be performed using a bright-field image or an electron backscatter diffraction image obtained by a transmission electron microscope. Here, when the crystal grain size is coarser than 25 μm, the volume of the crystal grain boundaries in the bulk decreases, and the number of barriers (= crystal grain boundaries) that inhibit dislocations generated during plastic deformation decreases, and the strength tends to be low. Of course, if the average crystal grain size can be maintained at 25 μm or less, heat treatment such as stress relief annealing may be performed after hot working.

[0026] In addition, either or both of α-Mn particles and intermetallic compound particles combined with Mn are dispersed in the Mg matrix phase and grain boundaries. These dispersed particles contribute to the improvement of the yield stress during compression and cause a reduction in yield anisotropy. The size of the dispersed particles is preferably 0.5 μm or less, more preferably 0.25 μm or less, and even more preferably 0.1 μm or less. Also, the ratio (= volume ratio) of the particles dispersed in the Mg matrix phase and grain boundaries is preferably 10%, more preferably 7.5%, and even more preferably 5% or less. When coarse particles exceeding 0.5 μm are dispersed, the interfaces between these coarse particles and the matrix phase are likely to peel off, becoming the starting points of microcracks and potentially causing early fracture. Even for fine particles, when they are dispersed at a high density such that the volume ratio exceeds 10%, they become barriers to dislocation movement and a factor in reducing ductility. Also, when α-Mn particles are dispersed at a high density, there is a problem that the added Mn element is consumed for the precipitation of α-Mn particles and intermetallic compound particles combined with Mn, making it difficult for Mn to segregate at the grain boundaries. Note that intermetallic compound particles include metal compound particles combined with Mn, that is, intermetallic compound particles combined with Mn and Mg, intermetallic compound particles combined with Mn and the aforementioned X (Al, Ca, Li, Zn, Sn, Bi), as well as intermetallic compound particles combined with Mg and the aforementioned X, and intermetallic compound particles combined with the aforementioned Xs. In this specification, these are collectively referred to as "intermetallic compound particles". The presence of α-Mn particles and intermetallic compound particles can be confirmed using XRD, and the particle size and volume ratio can be measured from microstructure observations using an optical microscope or a scanning electron microscope.

[0027] The strength of the Mg-based alloy sheet at room temperature is calculated from the nominal stress and nominal strain curve obtained by a room temperature tensile test. Since mechanical properties such as strength are easily affected by the test speed, the nominal stress and nominal strain curve obtained with an initial strain rate of 1x10 -3 s -1 is used. When the strength is affected by the test speed, the strength tends to increase as the test speed increases. Therefore, assuming a general-purpose test speed within the quasi-static test speed range and the minimum test speed, the above initial strain rate was set.

[0028] Figure 1 shows the nominal stress and nominal strain curve obtained from a room temperature tensile test using Mg-3 mass% Al-1 mass% Zn, a comparative example extrusion material commonly known as AZ31. In the stress-strain curve during the tensile test shown in Figure 1, after yielding, after showing slight work hardening, it fractures when the nominal strain reaches about 0.2. The yield stress is calculated based on JIS Z2241 as the value of the slope of the straight line obtained in the elastic region at a nominal strain of 0.2% offset. In the present invention, it is preferably that the calculated yield stress during tension exceeds 200 MPa, more preferably 250 MPa or more, and even more preferably 300 MPa or more. When the yield stress is 200 MPa or less, it is equivalent to a general-purpose Mg-based alloy sheet. Also, the elongation at break is calculated using the slope of the straight line obtained in the elastic region, and it is preferably more than 15%, more preferably 20% or more, and even more preferably 25% or more. When the elongation at break is 15% or less, it is equivalent to a general-purpose Mg-based alloy sheet. Note that the elongation at break may be measured by the butt-joint method using the sample after the tensile test.

[0029] The yield anisotropy is calculated using the yield stress obtained from tensile and compression tests at an initial strain rate of 1x10 -3 s -1 . That is, the value obtained by dividing the compression yield stress by the tensile yield stress is defined as the yield anisotropy, preferably more than 0.6, more preferably 0.7 or more, and even more preferably 0.75 or more. When the yield anisotropy is 0.6 or less, it is equivalent to a general-purpose Mg alloy sheet.

Examples

[0030] (Example 1) Commercial pure Mn (99.9 mass%) and commercial pure Mg (99.98 mass%) were used to produce a Mg-Mn master alloy using an iron crucible. Using commercial pure Sn, commercial pure Zn, and the Mg-Mn master alloy, the target content was adjusted to 0.3 mol% Mn - 0.1 mol% Sn - 0.07 mol% Zn, and a Mg-Mn-Sn-Zn alloy casting material was melted using an iron crucible. The melting conditions were an Ar atmosphere, a melting temperature of 700 °C, and a melting holding time of 5 minutes. Casting was performed using an iron mold with a diameter of 50 mm and a height of 200 mm. Thereafter, the casting material was solution-treated at 500 °C for 24 hours.

[0031] The casting material after solution treatment was machined by mechanical processing into a cylindrical extrusion billet with a diameter of 40 mm and a length of 40 mm. After holding the processed billet in a container set at 200 °C for 30 minutes, hot strain application processing by extrusion was performed at an extrusion ratio of 25:1 to produce a Mg-based alloy extruded material of the Mg-0.3Mn-0.1Sn-0.07Zn quaternary alloy according to Example 1 having a diameter of 8 mm and a length of 500 mm or more. (Hereinafter, the Mg-based alloy extruded material is referred to as an extruded material.)

[0032] The various extruded materials described in Examples 2 to 44 were produced as described below. When Li was used as an additive element, a Mg-Mn master alloy was produced. When Al, Bi, or Ca was used as an additive element, a master alloy was not produced, and commercial pure metals were used. Each additive element was adjusted to the target composition, and various casting materials were melted in an iron crucible. Thereafter, the solution treatment conditions (temperature and time), the dimensions of the cylindrical extrusion billet, the extrusion ratio, and the holding time during extrusion were the same as the above conditions to produce various extruded materials. The extrusion temperature is as shown in Table 1.

[0033] (Examples 2 to 4) Using Al, Ca, and Li instead of Zn and adjusting the target content to 0.07 mol% each, extrusion materials of the Mg-0.3Mn-0.1Sn-0.07(Al, Ca, Li) quaternary alloy according to Examples 2 to 4 were produced in the same manner as in Example 1.

[0034] (Examples 5 to 8) Except that the target content of Sn was changed from 0.1 mol% to 0.15 mol%, extrusion materials of Mg-0.3Mn-0.15Sn-0.07(Zn, Al, Ca, Li) quaternary alloys according to Examples 5 to 8 were produced in the same manner as in Examples 1 to 4.

[0035] (Examples 9 to 12) Except that the target content of Sn was changed from 0.1 mol% to 0.05 mol% and the target contents of Zn, Al, Ca, and Li were changed from 0.07 mol% to 0.03 mol%, extrusion materials of Mg-0.3Mn-0.05Sn-0.03(Zn, Al, Ca, Li) quaternary alloys according to Examples 9 to 12 were produced in the same manner as in Examples 1 to 4.

[0036] (Examples 13 to 16) Except that the target content of Mn was changed from 0.3 mol% to 0.52 mol% and the target contents of Zn, Al, Ca, and Li were changed from 0.07 mol% to 0.03 mol%, extrusion materials of Mg-0.52Mn-0.1Sn-0.03(Zn, Al, Ca, Li) quaternary alloys according to Examples 13 to 16 were produced in the same manner as in Examples 1 to 4.

[0037] (Examples 17 to 20) Except that the target content of Mn was changed from 0.3 mol% to 0.45 mol% and the target contents of Zn, Al, Ca, and Li were changed from 0.07 mol% to 0.05 mol%, extrusion materials of Mg-0.45Mn-0.1Sn-0.05(Zn, Al, Ca, Li) quaternary alloys according to Examples 17 to 20 were produced in the same manner as in Examples 1 to 4.

[0038] (Examples 21 to 25) Except that the target content of Mn was changed from 0.3 mol% to 0.45 mol%, Li was used instead of Sn with a target content of Li being 0.1 mol%, and the target contents of Zn, Al, Ca, Bi, and Sn were each 0.05 mol%, extrusion materials of Mg-0.45Mn-0.1Li-0.05(Zn, Al, Ca, Bi, Sn) quaternary alloys according to Examples 21 to 25 were produced in the same manner as in Example 1.

[0039] (Examples 26 to 28) Except that the target content of Mn was changed to 0.3 mol%, the target content of Li was changed to 0.15 mol%, and the target contents of Zn, Ca, and Sn were each changed to 0.07 mol%, in the same manner as in Examples 21, 23, and 25, extrusion materials of Mg-0.3Mn-0.15Li-0.07(Zn, Ca, Sn) quaternary alloys according to Examples 26 to 28 were produced.

[0040] (Examples 29 to 32) Except that the target content of Mn was changed to 0.6 mol% and the target contents of Zn, Al, Ca, and Bi were each changed to 0.05 mol%, in the same manner as in Examples 21 to 24, extrusion materials of Mg-0.6Mn-0.1Li-0.05(Zn, Al, Ca, Bi) quaternary alloys according to Examples 29 to 32 were produced.

[0041] (Examples 33, 34) Except that the target content of Zn was changed from 0.007% to 0.1 mol%, Ca and Li were used instead of Sn, and the target contents of Ca and Li were each 0.03 mol%, in the same manner as in Example 1, extrusion materials of Mg-0.3Mn-0.1Zn―0.03(Ca, Li) quaternary alloys according to Examples 33 and 34 were produced.

[0042] (Examples 35 to 38) Except that Al, Ca, Li, and Sn were used instead of Ca and Li, and the target contents of these were each 0.05 mol%, in the same manner as in Examples 33 and 34, extrusion materials of Mg-0.3Mn-0.1Zn―0.05(Al, Ca, Li, Sn) quaternary alloys according to Examples 35 to 38 were produced.

[0043] (Examples 39 to 42) Except that the target content of Mn was changed to 0.6 mol%, in the same manner as in Examples 35 to 38, extrusion materials of Mg-0.6Mn-0.1Zn―0.05(Al, Ca, Li, Sn) quaternary alloys according to Examples 39 to 42 were produced.

[0044] (Examples 43 to 46) Except that the target content of Mn was changed to 1.0 mol%, in the same manner as in Examples 35 to 38, extruded materials of Mg-1.0Mn-0.1Zn-0.05(Al, Ca, Li, Sn) quaternary alloys according to Examples 43 to 46 were produced.

[0045] (Examples 47 to 50) Except that the target content of Mn was changed to 0.15 mol%, the target content of Zn was changed to 0.07 mol%, and the target contents of Al, Ca, Li, and Sn were each changed to 0.03 mol%, in the same manner as in Examples 35 to 38, extruded materials of Mg-0.15Mn-0.07Zn-0.03(Al, Ca, Li, Sn) quaternary alloys according to Examples 47 to 50 were produced.

[0046] (Examples 51, 52) Except that Zn was used instead of Sn and Zn, and the target contents of Zn were 0.1 mol% and 0.3 mol%, in the same manner as in Example 1, extruded materials of Mg-0.3Mn-(0.1, 0.3)Zn ternary alloys according to Examples 51 and 52 were produced.

[0047] (Example 53) Except that Ca was used instead of Sn and Ca, and the target content of Ca was 0.1 mol%, in the same manner as in Example 3, an extruded material of a Mg-0.3Mn-0.1Ca ternary alloy according to Example 53 was produced.

[0048] (Examples 54, 55) Except that Li was used instead of Sn and Li, and the target contents of Li were 0.2 mol% and 0.3 mol%, in the same manner as in Example 4, extruded materials of Mg-0.3Mn-(0.2, 0.3)Li ternary alloys according to Examples 54 and 55 were produced.

[0049] (Examples 56, 57) Except that the target content of Mn was changed to 0.6 mol%, in the same manner as in Examples 51 and 52, extruded materials of Mg-0.6Mn-(0.1, 0.3)Zn ternary alloys according to Examples 56 and 57 were produced.

[0050] (Example 58) An extruded material of the Mg-0.6Mn-0.3Al ternary alloy according to Example 58 was produced in the same manner as in Example 57, except that Al was used instead of Zn.

[0051] (Example 59) An extruded material of the Mg-0.6Mn-0.2Li ternary alloy according to Example 59 was produced in the same manner as in Example 54, except that the target content of Mn was changed to 0.6 mol%.

[0052] (Examples 60 to 62) Extruded materials of the Mg-0.9Mn-0.1(Zn, Al, Li) ternary alloys according to Examples 60 to 62 were produced in the same manner as in Examples 57 to 59, except that the target content of Mn was changed to 0.9 mol% and the target contents of Zn, Al, and Li were each changed to 0.1 mol%.

[0053] The microstructures of the various extruded materials were observed by optical microscopy and electron backscatter diffraction. The average grain size of the base material was determined by the sectioning method and summarized in Table 1. In all of the extruded materials, the average grain size was 6 μm or less. Also, regarding whether intermetallic compounds with a particle size of 0.5 μm or less combined with α-Mn particles were dispersed in the Mg matrix phase, it was evaluated using X-ray diffraction (XRD), and when dispersion was confirmed, it is indicated by a circle in Table 1. It was also confirmed that elements other than Mg were segregated at the grain boundaries of some of the extruded materials. Fig. 3 shows an example of the microstructure observation of Example 51. From the EDS analysis attached to the transmission electron microscope, in the mapping image regarding Zn, the grain boundaries are clear, and it can be confirmed that the Zn element is segregated at the grain boundaries. Also, in the mapping image regarding Mn, there is a clear region with a size of 0.05 μm. Since no clear region can be confirmed from the Zn mapping image of the same location, it can be seen that α-Mn particles are dispersed in the matrix phase.

[0054] For the test pieces taken from the Mg-based alloy extruded materials, the initial strain rate was 1x10 -3 s -1Tensile and compression tests were conducted at room temperature. For the tensile test, a round bar specimen with a parallel section length of 10 mm and a parallel section diameter of 2.5 mm was used, and for the compression test, a cylindrical specimen with a diameter of 4 mm and a height of 8 mm was used. All specimens were taken from the parallel direction with respect to the extrusion direction. A typical tensile nominal stress and nominal strain curve are shown in Figure 2. It can be seen that the tensile yield stress of any of the extruded materials exceeds 200 MPa, indicating excellent strength characteristics. Also, it can be confirmed that the yield anisotropy exceeds 0.6.

[0055]

Table 1

[0056] (Comparative Example) Using a commercial magnesium alloy (Mg - 3 mass% Al - 1 mass% Zn: commonly known as AZ31) extruded material, tensile and compression tests were conducted at room temperature, and the obtained nominal stress and strain curves are shown in Figure 1. The test piece dimensions and test conditions are the same as those in the above examples. The yield stress during tensile is 200 MPa, the elongation at break is 15%, the yield stress during compression is 120 MPa, and the yield anisotropy is 0.6.

Industrial Applicability

[0057] The Mg - based alloy of the present invention exhibits excellent room - temperature strength characteristics and has a small yield anisotropy, so it has a three - dimensional isotropic deformation ability. Therefore, it can be considered for application to moving members such as automobiles. Also, since only trace amounts of general - purpose elements are added and rare - earth elements are not used, it is possible to reduce the material cost compared with conventional rare - earth - added Mg alloys.

Claims

1. A Mg-based alloy sheet containing Amol% of Mn and Bmol% of X, with the balance consisting of Mg and inevitable impurities, wherein X is one or more elements selected from the group consisting of Li, and Al, Ca, Zn, Sn, and Bi, and the value of A is 0.03 mol% or more and 2 mol% or less, and the relationship between A and B is A ≧ B, the upper limit value of B is 1.0 times or less the upper limit value of A, and the lower limit value of B is 0.03 mol%.

2. The Mg-based alloy sheet according to Claim 1, wherein the average crystal grain size of the Mg matrix phase is 25 μm or less.

3. The Mg-based alloy sheet according to Claim 1, wherein intermetallic compound particles having a particle diameter of 0.5 μm or less are dispersed and precipitated in the Mg matrix phase and crystal grain boundaries in the metal structure of the Mg-based alloy sheet, and the volume ratio thereof is 10% or less.

4. The Mg-based alloy sheet according to Claim 1, wherein elements other than Mg are segregated at the crystal grain boundaries of the Mg-based alloy sheet.

5. The Mg-based alloy sheet according to claim 1, wherein in the stress-strain curve diagram obtained by a room temperature tensile test at an initial strain rate of 1x10 －３ s －１ , the yield stress of the Mg-based alloy sheet exceeds 200 MPa.

6. The Mg-based alloy sheet according to claim 1, wherein the ratio of the yield stress obtained by tensile and compression tests at room temperature with an initial strain rate of 1x10 －３ s －１ (=compressive yield stress÷tensile yield stress), that is, the Mg-based alloy sheet having a yield anisotropy exceeding 0.6.

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