Magnesium-based alloy wrought product and method for producing same

Abstract

Provided is Mg-based alloy wrought material having improved ductility, formality, and resistance against fracture. Intermetallic compounds may be formed by mutual bonding of added elements to be a fracture origin. While maintaining microstructure for activating non-basal dislocation movement of Mg-based alloy wrought material, added elements to create no fracture origin, but to promote grain boundary sliding were found from among inexpensive and versatile elements. Provided is Mg-based alloy wrought material including at least one element from Zr, Bi, and Sn and at least one element from Al, Zn, Ca, Li, Y, and Gd wherein remainder comprises Mg and unavoidable impurities; an average grain size in a parent phase is 20 μm or smaller; a value of (σ.sub.max−σ.sub.bk)/σ.sub.max (maximum load stress (σ.sub.max), breaking stress (σ.sub.bk)) in a stress-strain curve obtained by tension-compression tests of the wrought material is 0.2 or higher; and resistance against breakage shows 100 kJ or higher.

Claims

1. A Mg-based alloy wrought material consisting of Mg-A mol % X-B mol % Z wherein a remainder consists of Mg and unavoidable impurities, wherein X is Bi, wherein Z is at least one kind of element from Al and Li, wherein a value of A is at least 0.03 mol % and not exceeding 1 mol %, wherein, with respect to a relationship of A and B, A≥B and a lower limit of B is at least 0.03 mol %, and wherein an average crystal grain size of the Mg-based alloy wrought material is not exceeding 20 micrometer.

2. The Mg-based alloy wrought material according to claim 1, wherein intermetallic compound particles having an average diameter of not exceeding 0.5 micrometer exist in Mg parent phase 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 breaking is defined as (σ.sub.bk) in a stress-strain diagram obtained by a room temperature tensile test in which an initial strain rate of the wrought material is set to not exceeding 1×10.sup.−4 s.sup.−1.

4. The Mg-based alloy wrought material according to claim 1, wherein the Mg-based alloy does not break even if a nominal strain of at least 0.2 is applied in a room temperature tensile test or compression test in which an initial strain rate is set to not exceeding 1×10.sup.−4 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 in which an initial strain rate is set to not exceeding 1×10.sup.−4 s.sup.−1 exhibits at least 100 kJ.

6. A method of manufacturing a Mg-based alloy wrought material as defined in claim 1, comprising the steps of: melting a raw material having a substantially same constituent ratios as the Mg-based alloy wrought material consisting of: A mol % X and B mol % Z, wherein a remainder thereof consists of Mg and unavoidable impurities, at a temperature of at least 650 degree Celsius, wherein X is Bi wherein Z is at least one kind of element from Al and Li, wherein a value of A is at least 0.03 mol % and not exceeding 1 mol %, wherein, with respect to a relationship of A and B, A≥B and a lower limit of B is at least 0.03 mol %; manufacturing a Mg-based cast material by pouring a thus-obtained melt into a mold; manufacturing a solution treated Mg-based alloy by performing a solution treatment of a thus-obtained Mg-based cast material at a temperature of at least 400 degree Celsius and not exceeding 650 degree Celsius for at least 0.5 hours and not exceeding 48 hours; and applying plastic strain so as to make the solution treated Mg-based alloy undergo hot plastic working at a temperature of at least 50 degree Celsius and not exceeding 550 degree Celsius with at least 70% of cross-section reduction rate.

7. The method of manufacturing the Mg-based alloy according to claim 6, wherein the step of applying plastic strain comprises any one of extruding, 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 microstructure diagram of a Mg-based alloy extruded material of an embodiment taken by the electron backscatter diffraction.

(4) FIG. 4 shows a cross-section microstructure diagram of an embodiment taken by the optical microscope.

(5) FIG. 5 shows a cross-section microstructure diagram of a comparative embodiment taken by the optical microscope.

EMBODIMENT CARRYING OUT INVENTION

(6) 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 or more kinds of elements from Mn, Bi, Sn, and Zr and wherein Z is any one or more kinds of elements selected from a group consisting of Al, Zn, Ca, Li, Y, and Gd (Here, a Mg-based alloy with addition of a Mn—Al combination, a Mg-based alloy with addition of a Mn—Zn combination, a Mg-based alloy with addition of a Mn—Ca combination, a Mg-based alloy with addition of a Mn—Li combination, and a Mg-based alloy with addition of a Mn—Y combination are excluded.). With respect to a relationship of A and B, A≥B and a value of A is preferably not exceeding 1 mol %, more preferably not exceeding 0.5 mol %, and yet more preferably not exceeding 0.3 mol %. A lower limit of A is at least 0.03 mol %. An upper limit of B is preferably 1.0 times as large as or less than an upper limit of A, more preferably 0.9 times as large as or less than the upper limit of A, and yet more preferably 0.8 times as large as or less than the upper limit of A. A lower limit of B is at least 0.03 mol %.

(7) Here, 0.03 mol % is a value to define a boundary whether or not the unavoidable impurities are. 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).

(8) 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 optical microscopic field of view is shown in FIG. 5.). 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. It is of course concerned that the crystal grain size may be coursened by the strain relief annealing, but there are no problems as far as the average crystalline grain sie of the Mg parent phase is not exceeding 20 micrometer. Here, it is OK either the added elements may be segregated or may not be segregated at the crystal grain boundaries. The temperature and the treatment time of the stress annihilation via annealing are 100 degree Celsius or higher and 400 degree Celsius or lower and 48 hours or less, respectively. Preferably, they are 125 degree Celsius or higher and 350 degree Celsius or lower and 24 hours or less, more preferably 150 degree Celsius or higher and 300 degree Celsius or lower and 12 hours or less, respectively.

(9) 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, etc. that can manufacture the Mg-based alloy cast material of the present invention of course may be employed.

(10) 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.

(11) 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 crystal grain may become a fracture origin. With respect to the hot working process, typically extruding, 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 is not 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.

(12) 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 curves obtained by the room temperature tensile test and compression test, respectively. Here, it is assumed that the nominal stress-and-nominal strain curves are obtained with the initial strain rate of 1×10.sup.−4 s.sup.−4 or lower in both tensile and compression tests.

(13) 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.

(14) 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.

(15) $\begin{array}{cc}\left[\mathrm{Formula}1\right]& \\ \mathrm{Degree}\mathrm{of}\mathrm{stress}\mathrm{reduction}=\frac{{\sigma }_{\max }-{\sigma }_{\mathrm{bk}}}{{\sigma }_{\max }}& \left(\mathrm{Formula}1\right)\end{array}$
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.

(16) 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. 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.−4 s.sup.−1, it is preferably 100 kJ or more, and more preferably 150 kJ or more, and yet more preferably 200 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. The above-mentioned enclosed area may be obtained, for example, by integrating the stress-strain curve, where the nominal stress is taken on the horizontal axis and the nominal strain is taken on the vertical axis, from 0 strain to the breaking strain.

Embodiments

(17) A Mg—Y mother alloy was manufactured by setting a commercially available pure Y (99.9 mass %) (yttrium (purity: 99.9 mass %) by Kojundo Chemical Laboratory Co., Ltd.) and a commercially available pure Mg (99.98 mass %) (magnesium (purity: 99.98 mass %) by OSAKA FUJI Corporation) into an employed iron crucible. In the case where Mn and Y were added, the mother alloy was employed, and in the case where an element or elements other than them were added, a commercially available pure element was employed and the amounts of the element or elements were adjusted so that the target content amouts summarized in Table 1 were set to be 0.15 mol % Bi-0.15 mol % Zn, and then various kinds of cast materials were melted with the 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.

(18) 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 200 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.

(19) Microstructures of the respective kinds of extruded materials were observed and was taken by the optical microscope or the electron backscatter diffraction method. A microstructural image observed with the electron backscatter diffraction method is shown in FIG. 3. A portion composed of the same contrast indicates one crystal grain and average crystal grain sizes of the respective extruded materials are summarized in Table 1. In any of the extruded materials, the average crystal grain sizea were 10 micrometer or less. And an example of an optical microscope observation after mirror polishing is shown in FIG. 4. As showm with an arrow in the figure, particles exhibiting a black color, that is, intermetallic compound particles can be confirmed. It can be confirmed that these sizes represent that the diameters are about 500 nm.

(20) 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.−4 s.sup.−1. Round bar specimens having a gauge length of 10 mm and a gauge diameter of 2.5 mm were used with the all tensile tests. When the 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, 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.

(21) TABLE-US-00001 TABLE 1 v; 1e-5/s Particle diameter of Degree intermetallic compound Heat of stress in parent phases/ No. T, ° C. d, μm treatment F, kJ eC eT reduction grain boundaries, μm 1 Extruded material Mg—0.15Bi—0.15Zn 200 8 x 149 0.31 0.48 0.28 0.5 2 Extruded material Mg—0.3Bi—0.1Li 150 ≤5 x 168 ≥0.5 1.48 0.85 0.4 3 Extruded material Mg—0.3Bi—0.1Li 150 ≤8 ∘ ≥150 0.4 ≥0.25 ≥0.25 0.4 4 Extruded material Mg—0.3Bi—0.1Ca 160 ≤5 x 200 ≥0.5 0.22 ≥0.3 0.5 5 Extruded material Mg—0.3Bi—0.1Ca 160 ≤8 ∘ ≥150 ≥0.5 ≥0.25 ≥0.25 0.5 6 Extruded material Mg—0.3Bi—0.1Sn 170 ≤5 x 317 0.25 0.28 ≥0.3 0.4 7 Extruded material Mg—0.3Bi—0.1Sn 170 ≤8 ∘ ≥150 0.3 ≥0.25 ≥0.25 0.4 8 Extruded material Mg—0.3Bi—0.1Al 180 ≤5 x 264 0.23 0.31 ≥0.25 0.5 9 Extruded material Mg—0.3Bi—0.1Al 180 ≤8 ∘ ≥150 0.3 ≥0.25 ≥0.25 0.5 10 Extruded material Mg—0.3Bi—0.1Zn 170 ≤5 x 284 0.25 0.25 0.25 0.5 11 Extruded material Mg—0.3Bi—0.1Zn 170 ≤8 ∘ ≥150 0.3 ≥0.25 ≥0.25 0.5 12 Groove-rolled Mg—0.3Bi—0.1Zn 400 ≤5 x ≥150 0.3 ≥0.25 ≥0.25 0.5 material 13 Groove-rolled Mg—0.3Bi—0.1Al 400 ≤5 x ≥150 0.3 ≥0.25 ≥0.25 0.4 material 14 Groove-rolled Mg—0.3Bi—0.1Y 400 ≤5 x ≥150 0.3 ≥0.25 ≥0.25 0.5 material Comparative AZ31 — 20 197 0.15 0.2 0.10 None material T: Extrusion temperature d: Average crystal grain size v: Strain rate eC: Compression breaking strain eT: Tensile breaking strain F: Absorption energy for fracture

(22) Further, since the value of the stress reduction: (σ.sub.max−σ.sub.bk)/σ.sub.max of 0.15 mol % Bi-0.15 mol % Zn alloy extruded material indicates 0.28, in the embodiment of the present invention, it is suggested that the plastic deformation limit of the alloy is large and the formability thereof is excellent. From Table 1, it should be understood that every value of (σ.sub.max−σ.sub.bk)/σ.sub.max of the extruded materials is larger than that of the commercially available magnesium alloy: AZ3l such that an excellent formability is shown.

(23) The resistance against the fracture (=energy absorption capacity) was evaluated by the room temperature compression test. A cylindrical test piece having a height of 8 mm and a diameter of 4 mm was cut out of each Mg-based alloy extruded material in the parallel direction to the extrusion direction. With respect to every test piece, the room temperature compression test was conducted with the initial strain rate of 1×10.sup.−5 s.sup.−1. The area enclosed by the stress-strain curve as shown in FIG. 2 was obtained and the results are listed in the column F of Table 1.

(24) Here, the process 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. Then, 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”.) The tensil test and the compression test were performed with test pieces having the same shape and the same condition as the above-mentioned extruded material, which were cut out in the parallel direction to the rolling direction.

(25) Further, the effect of the crystal grain size on the resistance against the fracture and the degree of stress reduction was investigated. In order to coarsen the size of Mg parent phase, each kind of the Mg-based alloy extruded materials was held in a muffle furnace kept at 200 degree Celsius in an air atmosphere for one hour such that the heat treatment (strain annihilation via annealing) was performed. Then, the room temperature tensile and compression tests were performed 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 by the heat treatment. In the case where o is shown in the heat treatment column in Table 1, it means that the heat treatment as mentioned here was performed while in the case where x is shown, it means that the heat treatment as mentioned here was not performed.

Comparative Embodiment

(26) The room temperature tensile and compression tests were performed with the extruded material of the commercially available magnesium alloy (Mg-3 mass % Al-1 mass % Zn: commonly know 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 observed with the optical microscope is shown in FIG. 5. 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. A typical example of the crystal grain is enclosed with a black bold line and shown in the figure. It should be understood that the crystal grain size is at least 20 micrometer.

(27) 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

(28) 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

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