Magnesium-based wrought alloy material and manufacturing method therefor

Abstract

Adding multiple solute elements could create fracture origin through formation of intermetallic compound due to bonding of added elements. While maintaining microstructure for activating non-basal dislocation movement, additive elements not to create fracture origin, but to promote grain boundary sliding are preferably found from among inexpensive and versatile elements. Provided is Mg-based wrought alloy material including two or more among group consisting of Mn, Zr, Bi, and Sn; and Mg and unavoidable constituents, having excellent room-temperature ductility and characterized by having finer crystal grain size in Mg parent phase during room-temperature deformation and in that mean grain size in matrix thereof is 20 μm or smaller; rate of (σ.sub.max−σ.sub.bk)/σ.sub.max (maximum load stress (σ.sub.max), breaking stress (σ.sub.bk)) in stress-strain curve obtained by tension-compression test of the wrought material is 0.2 or higher; and resistance against breakage shows 200 kJ or higher.

Claims

1. A Mg-based alloy wrought material comprising: Mg-A mol % Bi—B mol % Sn wherein a remainder comprises Mg and unavoidable impurities, wherein a value of A is at least 0.3 mol % and not exceeding 0.5 mol %, wherein, with respect to a relationship of A and B, A≥B and an upper limit of B is not exceeding 0.8 times as large as an upper limit of A and a lower limit of B is at least 0.1 mol %, and wherein an average crystal grain size of the Mg-based alloy wrought material is not exceeding 10 micrometers.

2. The Mg-based alloy wrought material according to claim 1, wherein intermetallic compound particles constituted of Mg and Bi; Mg and Sn; or Mg, Bi, and Sn and having an average diameter of not exceeding 0.5 micrometers exist in a Mg mother phase and/or crystal grain boundaries of a metallographic structure of the Mg-based alloy wrought material.

3. The Mg-based alloy wrought material according to claim 1, wherein a value of a formula of (σ.sub.max−σ.sub.bk)/σ.sub.max is at least 0.2 when a maximum applied stress is defined as σ.sub.max and a stress at break is defined as σ.sub.bk in a stress-strain diagram obtained by a room temperature tensile test with an initial strain rate not exceeding 1×10.sup.−3 s.sup.−1.

4. The Mg-based alloy wrought material according to claim 1, wherein the Mg-based alloy wrought material does not break even if a nominal strain of at least 0.2 is applied in a room temperature tensile test or compression test with an initial strain rate not exceeding 1×10.sup.−3 s.sup.−1.

5. The Mg-based alloy wrought material according to claim 1, wherein an area enclosed by a nominal stress-and-nominal strain curve in a stress-strain diagram obtained by a room temperature compression test with an initial strain rate of at least 1×10.sup.−3 s.sup.−1 exhibits at least 200 kJ with respect to the Mg-based alloy wrought material.

6. A method of manufacturing a Mg-based alloy wrought material as described in claim 1, the method comprising: performing a solution treatment of a Mg-based alloy cast material having been melted and cast at a temperature of at least 400 degrees Celsius and not exceeding 650 degrees Celsius for at least 0.5 hours and not exceeding 48 hours and, performing a hot plastic working for the Mg-based alloy cast material having been treated by the solution treatment at a temperature of at least 50 degrees Celsius and not exceeding 550 degrees Celsius with at least 80% of cross-section reduction rate as a process of applying plastic strain.

7. The method of manufacturing the Mg-based alloy wrought material according to claim 6, wherein the process of applying plastic strain comprises any one of extrusion, forging, rolling, and drawing.

8. The Mg-based alloy wrought material according to claim 2, wherein a value of a formula of (σ.sub.max−σ.sub.bk)/σ.sub.max is at least 0.2 when a maximum applied stress is defined as σ.sub.max and a stress at break is defined as σ.sub.bk in a stress-strain diagram obtained by a room temperature tensile test with an initial strain rate not exceeding 1×10.sup.−3 s.sup.−1.

9. The Mg-based alloy wrought material according to claim 2, wherein the Mg-based alloy wrought material does not break even if a nominal strain of at least 0.2 is applied in a room temperature tensile test or compression test with an initial strain rate not exceeding 1×10.sup.−3 s.sup.−1.

10. The Mg-based alloy wrought material according to claim 3, wherein the Mg-based alloy wrought material does not break even if a nominal strain of at least 0.2 is applied in a room temperature tensile test or compression test with an initial strain rate not exceeding 1×10.sup.−3 s.sup.−1.

11. The Mg-based alloy wrought material according to claim 8, wherein the Mg-based alloy wrought material does not break even if a nominal strain of at least 0.2 is applied in a room temperature tensile test or compression test with an initial strain rate not exceeding 1×10.sup.−3 s.sup.−1.

12. The Mg-based alloy wrought material according to claim 2, wherein an area enclosed by a nominal stress-and-nominal strain curve in a stress-strain diagram obtained by a room temperature compression test with an initial strain rate of at least 1×10.sup.−3 s.sup.−1 exhibits at least 200 kJ with respect to the Mg-based alloy wrought material.

13. The Mg-based alloy wrought material according to claim 3, wherein an area enclosed by a nominal stress-and-nominal strain curve in a stress-strain diagram obtained by a room temperature compression test with an initial strain rate of at least 1×10.sup.−3 s.sup.−1 exhibits at least 200 kJ with respect to the Mg-based alloy wrought material.

14. The Mg-based alloy wrought material according to claim 4, wherein an area enclosed by a nominal stress-and-nominal strain curve in a stress-strain diagram obtained by a room temperature compression test with an initial strain rate of at least 1×10.sup.−3 s.sup.−1 exhibits at least 200 kJ with respect to the Mg-based alloy wrought material.

15. The Mg-based alloy wrought material according to claim 8, wherein an area enclosed by a nominal stress-and-nominal strain curve in a stress-strain diagram obtained by a room temperature compression test with an initial strain rate of at least 1×10.sup.−3 s.sup.−1 exhibits at least 200 kJ with respect to the Mg-based alloy wrought material.

16. The Mg-based alloy wrought material according to claim 9, wherein an area enclosed by a nominal stress-and-nominal strain curve in a stress-strain diagram obtained by a room temperature compression test with an initial strain rate of at least 1×10.sup.−3 s.sup.−1 exhibits at least 200 kJ with respect to the Mg-based alloy wrought material.

17. The Mg-based alloy wrought material according to claim 10, wherein an area enclosed by a nominal stress-and-nominal strain curve in a stress-strain diagram obtained by a room temperature compression test with an initial strain rate of at least 1×10.sup.−3 s.sup.−1 exhibits at least 200 kJ with respect to the Mg-based alloy wrought material.

18. The Mg-based alloy wrought material according to claim 11, wherein an area enclosed by a nominal stress-and-nominal strain curve in a stress-strain diagram obtained by a room temperature compression test with an initial strain rate of at least 1×10.sup.−3 s.sup.−1 exhibits at least 200 kJ with respect to the Mg-based alloy wrought material.

19. A method of manufacturing a Mg-based alloy wrought material as described in claim 2, the method comprising: performing a solution treatment of a Mg-based alloy cast material having been melted and cast at a temperature of at least 400 degrees Celsius and not exceeding 650 degrees Celsius for at least 0.5 hours and not exceeding 48 hours and, performing a hot plastic working for the Mg-based alloy cast material having been treated by the solution treatment at a temperature of at least 50 degrees Celsius and not exceeding 550 degrees Celsius with at least 80% of cross-section reduction rate as a process of applying plastic strain.

20. The method of manufacturing the Mg-based alloy wrought material according to claim 19, wherein the process of applying plastic strain comprises any one of extrusion, forging, rolling, and drawing.

Description

BRIEF EXPLANATIONS OF DRAWINGS

(1) FIG. 1 shows a nominal stress-nominal strain curve obtained by a room temperature tensile test of a Mg-3Al-1Zn alloy extruded material.

(2) FIG. 2 shows a nominal stress-nominal strain curve obtained by a room temperature compression test of the Mg-3Al-1Zn alloy extruded material.

(3) FIG. 3 shows a nominal stress-nominal strain curve obtained by a room temperature tensile test of a Mg-based alloy extruded material of an embodiment.

(4) FIG. 4 shows a nominal stress-nominal strain curve obtained by a room temperature compression test of a Mg—Mn—Zr alloy extruded material of an embodiment.

(5) FIG. 5 shows a microstructure diagram obtained by the electron backscatter diffraction method of the Mg—Mn—Zr alloy extruded material of an embodiment.

(6) FIG. 6 shows a microstructure diagram obtained by the transmission electron microscope observation of the Mg-based alloy wrought material of an embodiment.

(7) FIG. 7 shows a microstructure diagram obtained by the optical microscope observation of the Mg-3Al-1Zn alloy extruded material.

EMBODIMENT CARRYING OUT INVENTION

(8) In embodiments of the present invention, a Mg-based alloy raw material comprises: Mg-A mol % X-B mol % Z wherein X is any one kind of element of Mn, Bi, and Sn and wherein Z is any one or more kinds of elements selected from a group consisting of Mn, Bi, Sn, and Zr. That is, if X is Mn, Z should be at least one kind of element from Bi, Sn, and Zr. If X is Sn, Z should be at least one kind of element from Bi, Mn, and Zr. And if X is Bi, Z should be at least one kind of element from Mn, Sn, and Zr. With respect to the relationship between A and B, A≥B and A is preferably not exceeding 1 mol %, more preferably not exceeding 0.5 mol %, yet more preferably at least 0.3 mol %. The lower limit of A is at least 0.03 mol %. The upper limit of B is preferably not exceeding 1.0 times as large as the upper limit of A, more preferably not exceeding 0.9 times, and yet more preferably not exceeding 0.8 times. The lower limit of B is at least 0.03 mol %.

(9) Here, 0.03 mol % is a value to define a boundary between unavoidable impurities and added elements. If a recycled Mg-based alloy is used as a raw material of Mg-based alloy raw material, various kinds of alloy elements may be originally included such that the content amount usually contained therein should be excluded in the case where the Mg-based alloy raw material is used. Examples of elements contained in the unavoidable impurities may include Fe (iron), Si (silicon), Cu (copper), and Ni (nickel).

(10) Here, in embodiments of the present invention, the Mg-based alloy raw material may be represented by Mg-aMn-bBi-cSn-dZr (a, b, c, and d represent amounts of mol %, respectively) and could be treated as a material that satisfies any one of the following conditions. Here, a, b, c, and d are at least 0, respectively. (1) Condition 1 (a corresponds to A. b+c+d corresponds to B.)
1≥a≥b+c+d≥0.03;
(2) Condition 2 (b corresponds to A. a+c+d corresponds to B.)
1≥b≥a+c+d≥0.03; and
(3) Condition 3 (c corresponds to A. a+b+d corresponds to B.)
1≥c≥a+b+d≥0.03.

(11) The average crystal grain size of the Mg parent phase, that is, crystal grains after hot-working is preferably not exceeding 20 micrometer. More preferably it is not exceeding 10 micrometer and further preferably it is not exceeding 5 micrometer. The measurement of the crystal grain size is preferably conducted by an intersection method (G 0551: 2013) based on the JIS standard through the optical microscope observation of the intersection (A conceptual diagram in which crystal grains and grain boundaries appear in the microscopic field of view is shown in FIG. 7.). In the case where the crystal grain size is so fine or crystal grain boundaries are not so clear, it is not easy to employ the intersection method such that the measurement may be conducted by the bright-field image and the dark-field image obtained by the transmission electron microscope observation or the electron backscatter diffraction image. Here, in the case where the crystal grain size is larger than 20 micrometer, the grain boundary compatibility stress arising near the crystal grain boundaries does not affect all region of grain interior. That is to say, it is difficult for the non-basal dislocation slip to make an occurrence in all region of grain interior such that it cannot be expected that the ductility would be improved. If the average crystal grain size is not exceeding 20 micrometer, of course, the intermetallic compounds having the size of 0.5 micrometer or less could be dispersed inside the Mg crystal grains and the crystal grain boundaries. And if the average crystal grain size is maintained not exceeding 20 micrometer, it is OK to conduct a heat treatment such as a strain annihilation via annealing after the hot working. Here, it is OK either the added elements may be segregated or may not be segregated at the crystal grain boundaries.

(12) Next, a method of manufacturing in order to obtain a fine structure will be explained. The solution treatment is performed with respect to the melt Mg-based alloy cast material at a temperature of at least 400 degree Celsius and not exceeding 650 degree Celsius. Here, in the case where the temperature of the solution treatment is less than 400 degree Celsius, it is not preferable from the industrial point of view since it is necessary to hold the temperature for a long period of time in order to have the added solute elements homogeneously solid solved. On the other hand, if the temperature exceeds 650 degree Celsius, it may not be safe for operation since the localized melting begins because it is at a solid phase temperature or higher. And the period of time for the solution treatment is at least 0.5 hours and not exceeding 48 hours. If it is less than 0.5 hours, it is insufficient for the solute elements to be dispersed in all region inside the parent phase such that segregation during the casting remains and a good raw material cannot be manufactured. If it is longer than 48 hours, the operation time becomes longer so as not to be preferable from the industrial point of view. With respect to the casting method, any method such as gravity casting, sand casting, die casting, continuous casting, etc. that can manufacture the Mg-based alloy cast material of the present invention of course may be employed.

(13) After the solution treatment, a hot strain application process is conducted. The temperature during the hot working is preferably at least 50 degree Celsius and not exceeding 550 degree Celsius; more preferably at least 75 degree Celsius and not exceeding 525 degree Celsius; and further preferably at least 100 degree Celsius and not exceeding 500 degree Celsius. If the working temperature is less than 50 degree Celsius, so many deformation twins that may be an origin of break or crack are caused such that a good wrought material could not be manufactured. If the working temperature is higher than 550 degree Celsius, the recrystallization may proceed during the working process such that refinement of the crystal grains would be prevented and further cause the lifetime of the mold for the working to be shortened.

(14) The application of strain during the hot working is characterized by the total cross-section reduction rate of at least 70%, preferably at least 80%, and more preferably at least 90%. If the total cross-section reduction rate is less than 70%, the strain application is not enough such that the crystal grain size cannot be refined. It is also considered that the structure with a mixture of fine grains and coarse grains may be formed. In such a case, the room temperature ductility is lowered because the coarse grain may become a fracture origin. With respect to the hot working process, typically extrusion, forging, rolling, drawing and so on may be representative, but any processing method that is a plastic working method that can apply strain could be employed. However, it cannot be said that it is preferable only to perform the solution treatment for the cast material without conducting the hot working since the crystal grain size in the Mg parent phase tends to be coarse.

(15) Now, the indices to evaluate the ductility and formability of the Mg-based alloy wrought material at the room temperature, that is, the degree of stress reduction and the resistance (hereinafter defined as F) against the fracture are explained. Both indices could be calculated from the nominal stress-and-nominal strain curve obtained by the room temperature tensile test and compression test, respectively. Here, since the speeding-up in the rate is important, it is assumed that the nominal stress-and-nominal strain curve is obtained with the initial strain rate of 1×10.sup.−3 s.sup.−1 or higher in both tensile and compression tests.

(16) In FIGS. 1 and 2, the nominal stress-and-nominal strain curves obtained by the room temperature tensile test and compression test using a commercially available magnesium alloy (Mg-3 mass % Al-1 mass % Zn: commonly known as AZ31) are shown. In the stress-strain curve during the tensile test as shown in FIG. 1, a slight work-hardening occurs after yielding, and then, the specimen breaks when the nominal strain reaches about 0.2. On the other hand, in the stress-strain curve during the compression test as shown in FIG. 2, a large work-hardening occurs after yielding, and then, the specimen breaks around 0.2 of the nominal strain. In both tensile and compression tests, it should be understood that the specimens break at an early stage of deformation with respect to the conventional Mg-based alloy.

(17) The degree of stress reduction may be obtained by the formula (1) and preferably is at least 0.2 and more preferably is at least 0.25.

(18) $\begin{array}{cc}\left[\mathrm{Formula}1\right]& \\ \mathrm{Degree}\mathrm{of}\mathrm{stress}\mathrm{reduction}=\frac{{\sigma }_{\max }-{\sigma }_{\mathrm{bk}}}{{\sigma }_{\max }}& \left(\mathrm{Formula}1\right)\end{array}$
Here, σ.sub.max is the maximum applied stress and σ.sub.bk is the stress at break and their examples are shown in FIG. 1.

(19) Next, the resistance against the fracture: F corresponds to the area enclosed by the nominal stress-and-nominal strain curve obtained by the room temperature compression test as shown in FIG. 2 and the larger the area is, the larger the resistance against the fracture (=energy absorption capacity) is (cf. shaded area in the figure). This resistance: F is also obtained, as the area enclosed by the nominal stress-and-nominal strain curve, from the nominal stress-and-nominal strain curve obtained by the room temperature tensile test in the same way. The F tends to increase as the testing rate is speeded up since it is affected by the strain rate. Therefore, when the value of F may be obtained under the condition that the initial strain rate is 1×10.sup.−3 s.sup.−1, it is preferably 200 kJ or more, and more preferably 250 kJ or more, yet more preferably 300 kJ or more. Here, a similar nominal stress-and-nominal strain curve (FIG. 1) to that of the compression test can be obtained by the tensile test, but the resistance against the fracture may be evaluated more strictly by the compression test than by the tensile test since the specimen breaks with a slight nominal strain in the case of the Mg-based alloy.

Embodiments

(20) A Mg—Mn mother alloy was manufactured with an iron crucible from a commercially available pure Mn (99.9 mass %) and a commercially available pure Mg (99.98 mass %). In a similar manner, a Mg—Zr mother alloy was manufactured using a commercially available pure Zr and a commercially available pure Mg. Using the respective mother alloys, a Mg—Mn—Zr alloy cast material was manufactured by adjusting the composition to the target constituent contents of 0.1 mol % Mn-0.1 mol % Zr and melting it in an iron crucible. Here, the cast material was made by melting the composition in an Ar atmosphere at a melting temperature of 700 degree Celsius for a melt holding time of 5 minutes and pouring the melt into an iron mold having a diameter of 50 mm and a height of 200 mm. Then, the cast material was heat-treated for the solution treatment at 500 degree Celsius for 8 hours.

(21) The cast material after the solution treatment was machined into a cylindrical extrusion billet having a diameter of 40 mm and a length of 60 mm by the machine working. After the thus-machined billet was held in a container kept at 165 degree Celsius for 30 minutes, an extruded material in a shape having a diameter of 8 mm and a length of 500 mm or longer (hereinafter referred to as “extruded material”) was manufactured by the extrusion with the extrusion ratio of 25:1 (=reduction rate: 94%) through the hot strain application process.

(22) In the case where Mn and Zr were used as the additive, the above-mentioned respective mother alloys were used and, in the case where Bi and Sn were added, commercially available pure Bi and pure Sn were used, and the composition was adjusted to the target composition and was melted in an iron crucible such that respective kinds of cast materials were manufactured by casting respective melts. Then, respective kinds of extruded materials were made through the solution treatment with the same condition (temperature and time) as mentioned above, the machine working process to form the cylindrical extrusion billet in the same dimension, and the extrusion working process with the same extrusion rate and the same holding time as mentioned above. Here, the extrusion temperatures are summarized in Table 1.

(23) Fine structure appearances of the respective extruded materials were photographed with the optical microscope and average crystal grain sizes were obtained by the intersection method such that they are summarized in Table 1. In any of the extruded materials, the average crystal grain sizes were 5 micrometer or less. Here, the microstructural image obtained by the electron backscatter diffraction method is shown in FIG. 5. In the figure, a portion composed of the same contrast indicates one crystal grain, that is, the Mg parent phase and it can be confirmed that a size thereof is 5 micrometer or less. And the microstructural image obtained by the transmission electron microscope observation is shown in FIG. 6. An aggregate composed of black contrast indicates that of intermetallic compound. It can be confirmed that there are aggregates of intermetallic compound having diameters of 100 to 200 nm.

(24) With respect to specimens cut out of the Mg-based alloy extruded material, a room temperature tensile test was conducted with the initial strain rate of 1×10.sup.−3 s.sup.−1. Round bar specimens a parallel portion length of 10 mm and having a parallel portion diameter of 2.5 mm were used with all tensile tests. The test pieces were cut out from the extruded material in the parallel direction to the extrusion direction. A nominal stress-nominal strain curve obtained by the room temperature tensile test with respect to Embodiment 2 is shown in FIG. 3. With respect to the Mg-0.3 Bi-0.1 Zr alloy extruded material, it can be confirmed that the tensile breaking strain was beyond 1.0 and an excellent ductility was exhibited. Here, when the nominal stress was suddenly dropped (20% during each measurement), it was defined as “breaking” such that the nominal strain at the time of breaking is referred to as the tensile breaking strain: eT, which is summarized in Table 1. It should be understood that every tensile breaking strain of the extruded materials exceeds 0.03 so as to exhibit an excellent tensile ductility.

(25) TABLE-US-00001 TABLE 1 Degree of Intermetallic Heat stress compound grain No. T, °C. treatment d, um F, kJ eC eT reduction diameter/μm 1 Extruded material Mg-0.1Mn-0.1Zr 165 No ≤5 ≥350 ≥0.5 0.42 0.24 — 2 Extruded material Mg-0.1Mn-0.1Zr 165 Yes ≤8 336 ≥0.5 0.40 0.22 — 3 Extruded material Mg-0.3Mn-0.1Zr 130 No ≤5 504 ≥0.5 1.04 0.56 0.4 4 Extruded material Mg-0.3Mn-0.1Zr 130 Yes ≤8 475 ≥0.5 0.50 0.50 0.4 5 Groove-rolled material Mg-0.3Bi-0.1Zr 400 No ≤5 484 ≥0.5 0.75 0.64 0.4 6 Extruded material Mg-0.6Mn-0.1Zr 150 No ≤5 500 ≥0.5 0.70 0.50 0.5 7 Extruded material Mg-0.3Bi-0.1Zr 130 No ≤5 ≥350 ≥0.5 1.05 0.75 — 8 Extruded material Mg-0.3Bi-0.1Zr 130 Yes ≤8 345 ≥0.5 0.55 0.34 — 9 Groove-rolled material Mg-0.3Si-0.1Zr 400 No ≤5 450 ≥0.5 0.71 0.89 — 10 Extruded material Mg-0.6Bi-0.1Zr 135 No ≤5 500 ≥0.5 0.70 0.50 0.5 11 Extruded material Mg-0.45Bi-0.15Mn 180 No ≤5 ≥350 ≥0.5 0.32 0.25 0.3 12 Extruded material Mg-0.6Bi-0.3Mn 150 No ≤5 604 ≥0.5 0.55 0.38 0.5 13 Extruded material Mg-0.6Bi-0.3Mn 150 Yes ≤8 550 ≥0.5 0.37 0.33 0.5 14 Extruded material Mg-0.9Bi-0.3Mn 150 No ≤5 579 ≥0.5 0.57 0.47 0.5 15 Extruded material Mg-0.9Bi-0.3Mn 150 Yes ≤8 525 ≥0.5 0.33 0.24 0.5 16 Extruded material Mg-0.3Bi-0.1Mn 155 No ≤5 549 ≥0.5 0.78 0.51 — 17 Extruded material Mg-0.3Bi-0.1Mn 155 Yes ≤8 523 ≥0.5 0.22 0.22 — 18 Extruded material Mg-0.9Mn-0.1Bi 210 No ≤5 565 0.46 0.38 0.23 0.5 19 Extruded material Mg-0.9Mn-0.1Bi 210 Yes ≤8 515 0.40 0.23 0.21 0.5 20 Extruded material Mg-0.3Mn-0.1Bi 150 No ≤5 588 ≥0.5 0.77 0.50 — 21 Extruded material Mg-0.6Mn-0.1Bi 210 No ≤5 320 0.25 0.30 0.25 0.3 22 Groove-rolled material Mg-0.3Mn-0.1Bi 400 No ≤5 562 ≥0.5 0.65 0.62 — 23 Extruded material Mg-0.9Mn-0.1Sn 220 No ≤5 298 0.22 0.27 0.24 0.5 24 Groove-rolled material Mg-0.3Mn-0.1Sn 300 No ≤5 380 0.25 0.48 0.39 — 25 Extruded material Mg-0.6Mn-0.1Sn 150 No ≤5 839 ≥0.5 0.41 0.35 0.3 26 Extruded material Mg-0.6Mn-0.1Sn 150 Yes ≤8 674 0.45 0.33 0.22 0.3 27 Extruded material Mg-0.3Mn-0.1Sn 170 No ≤5 437 0.32 0.51 0.31 — 28 Extruded material Mg-0.3Si-0.1Sn 170 No ≤5 316 0.26 0.22 0.23 — 29 Comparative material AZ31 210 No ≤3 255 0.16 0.22 0.05 — 30 Comparative material AZ31 — No 20 196 0.17 0.23 0.10 — T: Extruding temperature d: Average crystal grain size F: Absorbed energy for break eC: Compressive breaking strain eT: Tensile breaking strain Heat treatment: 200 degree Celsius to one hour

(26) In the nominal stress-and-nominal strain curve of the Mg-based alloy extruded material in the tensile test as shown in FIG. 3, it should be understood that a large stress reduction is shown after the maximum applied stress is reached. For example, in the case of the Mg-0.3 Bi-0.1 Zr alloy extruded material, the value of (σ.sub.max−σ.sub.bk)/σ.sub.max shows 0.75 such that it is suggested that the plastic deformation limit is large and the formability is excellent. From Table 1, it should be understood that every value of (α.sub.max−σ.sub.bk)/σ.sub.max of the extruded materials is larger than that of the commercially available magnesium alloy: AZ31 such that an excellent formability is shown.

(27) With respect to test pieces cut out from the Mg-based alloy extruded material, room temperature compression tests were conducted with the initial strain rates of 1×10.sup.−2 and 1×10.sup.−3 s.sup.−1. As the specimen, a cylindrical test piece having a height of 8 mm and a diameter of 4 mm was used. The test piece was taken in the parallel direction to the extrusion direction. In FIG. 4, a nominal stress-nominal strain curve with Embodiment 2 obtained by the room temperature compression test is shown. It should be understood that, even after the nominal strain in the compression test reaches 0.5, stress reduction as shown in FIG. 2 does not appear, but the deformation continues. And the shaded area in the figure corresponds to the resistance against the fracture, which is determined to be 403 kJ. It should be understood that the area enclosed by the stress-and-strain is increased when the initial strain rate of the compression test is higher by one order of magnitude. In Table 1, values of F are summarized with the initial strain rate: 1×10.sup.−3 s.sup.−1. It can be confirmed that every extruded material exhibits an excellent resistance against the fracture. And when the nominal stress in the compression test was suddenly dropped (20% during each measurement), it was defined as “breaking” such that the nominal strain at the time of breaking is referred to as the compressive breaking strain: eC, which are summarized in Table 1. Here, it is suggested that, even if the compression nominal strain of 0.50 is applied, no breaking occurs such that it has an excellent compression deformability.

(28) Here, concrete procedures of the groove rolling process are described as follows. Each kind of cast material after the solution treatment was machined into a cylindrical extrusion billet having a diameter of 40 mm and a length of 80 mm through the mechanical working. The thus-machined billet was held in an electric furnace kept at 400 degree Celsius for 30 minutes or longer. hen, rolling was repeatedly performed in the condition that the rolling temperature was set to the room temperature and that the cross-section reduction rate for one rolling was set to 18% such that the total cross-section reduction rate might be 92%. (Hereinafter, it is referred to as “groove-rolled material”.)

(29) Each room temperature property of the groove-rolled materials is summarized in Table 1. It can be confirmed that excellent values are shown as compared to those of the commercially available magnesium alloy: AZ31 even if the groove-rolling method was employed as the expansion forming process method. Here, the tensile and compression test pieces were taken in the parallel direction to the rolling direction and the test condition was the same as that of the above-mentioned extruded material.

(30) Further, the effect of the crystal grain size on the resistance against the fracture and the degree of stress reduction was investigated. Each kind of the Mg-based alloy extruded materials was held in a muffle furnace kept at 200 degree Celsius for one hour. Then, the room temperature tensile and compression tests were performed with test pieces of the same size and shape in the same procedures as mentioned above. The obtained results are summarized in Table 1. It can be confirmed that excellent values are shown as compared to those of the commercially available magnesium alloy: AZ31 even if the average crystal grain sizes were coarsened.

Comparative Embodiment

(31) The room tensile and compression tests were performed with the extruded material of the commercially available magnesium alloy (Mg-3 mass % Al-1 mass % Zn: commonly known as AZ31). The same test piece size and shape and the same test condition were employed as those of the above-mentioned embodiments. The breaking elongations, degrees of stress reduction, values of F, and so on obtained by the tensile and compression tests are summarized in Table 1. And a microstructural image obtained with the optical microscope is shown in FIG. 7. The crystal grain boundaries are indicated by line in a black color and the area enclosed by a black line corresponds to one crystal grain.

(32) Here, in embodiments of the present invention, the refinement of the internal structure was attempted by the one-time plastic-strain application method, but the plastic-strain application can be performed for a plurality of times in the case where the cross-section reduction rate is smaller than a predetermined value.

INDUSTRIAL APPLICABILITY

(33) The Mg-based alloy of the present invention exhibits an excellent room temperature ductility so as to have a good secondary workability and be easily formed into a complicated shape such as a plate shape. In particular, it has an excellent property for the stretch forming, the deep drawing, and so on. And, since the grain boundary sliding is caused, it has an excellent internal friction property so as to be applied possibly to the part in which vibration and noise are to be a technical problem. Further, since a small amount of versatile element is added such that the rare earth element is not used, it is possible to reduce the price of the raw material as compared to the conventional rare earth added Mg alloy.

EXPLANATION OF REFERENCE NUMERALS

(34) σ.sub.max maximum applied stress; σ.sub.bk stress at beak: F resistance against fracture (=energy absorption capacity)