High-strength magnesium-based alloy

Abstract: A high-strength magnesium-based alloy possessing a microcrystalline composition represented by the general formula: Mg a Al b Zn c (wherein a, b, and c stand for atomic percents respectively in the ranges of 80 ≦ a ≦ 92, 4 ≦ b ≦ 12, and 4 < c ≦ 12). This alloy can be advantageously produced by rapidly solidifying the melt of an alloy of the composition shown above by the liquid quenching method. It is useful as high-strength materials and highly refractory materials owing to its high hardness, strength, and heat-resistance. It is also useful as materials with high specific strength because of light weight and high strength.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention: This invention relates to high-strength magnesium-based alloys obtained by the rapid solidification method or quench solidifying method. 2. Description of the Prior Art: The magnesium-based alloys heretofore known to the art include those of the compositions of Mg-Al, Mg-Al-Zn, Mg-Th-Zr, Mg-Th-Zn-Zr, Mg-Zn-Zr, and Mg-Zn-Zr-RE (rare earth element). Depending on their material characteristics, these magnesium-based alloys have been finding extensive utility as light-weight structural materials for aircraft and vehicles, as materials for storage batteries, and as sacrifice electrodes, for example. The conventional magnesium-based alloys of varying types cited above, however, are generally deficient in hardness and strength. As materials obtainable by the rapid solidification method, magnesium-based alloys of varying compositions have been developed. For example, Japanese Patent Application laid open to public inspection, KOKAI (Early Publication) No. 3-87339 (87,339/ 1991) discloses a magnesium-based alloy of Mg-M-X [wherein M stands for Al, Si, Ca, Cu, Ni, Sn, or Zn and X for Y, La, Ce, Sm, Nd, or Mm (misch metal)] and Japanese Patent Application, KOKAI No. 3-10041 (10,041/1991) discloses magnesium-based alloys of Mg-X, Mg-X-M, Mg-X-Ln, and Mg-X-M-Ln (wherein X stands for Cu, Ni, Sn, or Zn, M for Al, Si, or Ca, and Ln for Y, La, Ce, Nd, Sm, or Mm). These magnesium-based alloys, however, are amorphous alloys containing at least 50% by volume of an amorphous phase. As respects crystalline magnesium-based alloys, Japanese Patent Application, KOKAI No. 3-47941 (47,941/ 1991) discloses magnesium-based alloys of Mg-X, Mg-X-M, Mg-X-Ln, and Mg-X-M-Ln (wherein X stands for Cu, Ni, Sn, or Zn, M for Al, Si, or Ca, and Ln for Y, La, Ce, Nd, Sm, or Mm). Though the magnesium-based alloys reported in said Japanese Patent Application, KOKAI No. 3-47941 are excellent in hardness and tensile strength, they are imperfect in terms of thermal stability and specific strength and have room for improvement. Further, said Japanese Patent Application, KOKAI No. 3-47941 has no concrete mention anywhere about magnesium-based alloys of Mg-Al-Zn. SUMMARY OF THE INVENTION An object of this invention, therefore, is to provide a magnesium-based alloy which possesses high hardness, high strength, and high heat-resistance, exhibits high specific strength, and proves to be useful as high-strength material, highly heat-resistant material, and a light, strong material of high specific strength. Another object of this invention is to provide a magnesium-based alloy which excels in such characteristic properties as elongation at room temperature and Young's modulus and, therefore, endures working by extrusion and forging, for example. To accomplish the objects mentioned above, in accordance with this invention, there is provided a high-strength magnesium-based alloy possessing a microcrystalline composition represented by the general formula: Mg a Al b Zn c wherein a, b, and c stand for atomic percents falling respectively in the ranges, 80 ≦ a ≦ 92, 4 ≦ b ≦ 12, and 4 < c ≦ 12 and (a + b + c) = 100 and having an intermetallic compound of Mg and said other alloy elements uniformly and finely dispersed in a Mg matrix of a hexagonal close-packed structure. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is an explanatory diagram schematically illustrating the construction of an example of the apparatus for the production of a magnesium-based alloy of this invention. Fig. 2 is a graph showing changes in tensile strength due to changes in the amount of Zn and the amount of Al in a Mg-Al-Zn magnesium-based alloy according to this invention. DETAILED DESCRIPTION OF THE INVENTION The magnesium-based alloy of this invention possesses a composition of Mg a Al b Zn c and has the intermetallic compounds of Mg and other alloy elements mentioned above dispersed homogeneously and finely in a magnesium matrix of a hexagonal close-packed structure (hereinafter referred to briefly as "h.c.p."). In the magnesium-based alloy of this invention mentioned above, a is limited to the range of 80 to 92 atomic %, b to that of 4 to 12 atomic %, and c to that of 4 to 12 atomic % (but not equal to 4 atomic %) respectively for the purpose of ensuring formation of a supersaturated solid solution surpassing the limit of equilibrium solid solution and production of the alloys of the microcrystalline phases by the rapidly solidifying means on a commercial basis by utilizing the liquid quenching technique, for example. Another important reason for fixing the ranges mentioned above resides in ensuring precipitation of fine h.c.p. Mg and further uniform precipitation of intermetallic compounds of Mg and other alloy elements. By enabling the intermetallic compounds containing at least Mg as one of the components thereof, particularly the intermetallic compounds of Mg and Zn, to be uniformly and finely dispersed in the Mg matrix of h.c.p. mentioned above, the supersaturated Mg matrix can be reinforced and the strength of the alloy can be enhanced conspicuously. Even if the amount of Mg is less than 80 atomic %, the alloy containing an amorphous phase in a certain proportion can be obtained and the amorphous phase can be decomposed by heating this amorphous alloy at a prescribed temperature. When a crystalline alloy is produced by thermal decomposition as described above, however, this crystalline alloy suffers from unduly low toughness because the intermetallic compound is precipitated simultaneously with or preferentially over the precipitation of the h.c.p. Mg during the thermal decomposition. If the amount of Mg is less than 80 atomic %, the alloys similar to that just described can be obtained by decreasing the cooling rate. The alloy thus produced only betrays deficiency in ductility because it fails to acquire a supersaturated solid solution in the cooled state and the coarse compound phases precipitate with coarse Mg matrix. In the magnesium-based alloy of this invention, the element Al manifests an excellent effect of forming a supersaturated solid solution or metastable intermetallic compound with magnesium and other additive elements and, at the same time, of stabilizing a microcrystalline phase, and enhances strength of the alloy without any sacrifice of ductility. The element Zn forms a stable or metastable intermetallic compound with magnesium and other additive elements, causes this intermetallic compound to be uniformly and finely dispersed in the magnesium matrix (α phase), conspicuously enhances hardness and strength of the alloy, suppresses the otherwise inevitable coarsening of the microcrystalline phase at elevated temperatures, and imparts heat-resistance to the alloy. The element Zn, particularly in the alloy of this invention, can form Mg 7 Zn 3 , an intermetallic compound capable of improving mechanical properties. The alloy of the composition represented by the general formula mentioned above, therefore, is desired to possess a texture having at least the intermetallic compound of Mg 7 Zn 3 uniformly and finely dispersed in a Mg matrix of h.c.p. Besides, the element Zn has an effect of improving resistance to corrosion of the alloy. The magnesium-based alloy of this invention can be advantageously produced by preparing the alloy of the prescribed composition and using rapidly solidifying process such as the liquid quenching method. The cooling in this case is effected advantageously at a rate in the range of from 10 2 to 10 6 K/sec. The magnesium-based alloy of this invention is useful as high-strength materials and highly refractory materials owing to its high hardness, strength, and heat-resistance. It is also useful as materials with high specific strength because of light weight and high strength. Since this alloy excels in elongation at room temperature and Young's modulus, it can be worked by extrusion and forging. Furthermore, this alloy endures the sharp bending work. The shaped articles produced by working this alloy, therefore, enjoy the outstanding mechanical properties which are inherent in the alloy as the starting material. Now, this invention will be described more specifically below with reference to working examples. As a matter of course, this invention is not limited to the following examples. It ought to be easily understood by any person of ordinary skill in the art that this invention allows various modifications within the scope of the spirit of this invention. Example 1: A molten alloy 3 of a prescribed percentage composition was prepared by the use of a high-frequency blast furnace. This molten alloy 3 was introduced into a quartz tube 1 provided at the leading terminal thereof with a small hole 5 (0.5 mm in diameter) as illustrated in Fig. 1 and thermally melted by means of a high-frequency heating coil 4 wound around the quartz tube 1. Then, the quartz tube 1 was set in place directly above a roll 2 made of copper. The roll 2 was kept rotated at a high speed in the range of from 3,000 to 5,000 r.p.m. and the molten alloy 3 in the quartz tube 1 was spouted under the pressure of argon gas (0.7 kg/cm 2 ) through the small hole 5 of the quartz tube 1. A thin alloy strip 6 was obtained by bringing the spouted alloy into contact with the surface of the roll 2 in rotation and rapidly solidifying the alloy. Nine thin alloy strips (1 mm in width and 20 µm in thickness) varying in composition as shown in Table 1 were produced under the conditions mentioned above. The thin alloy strips were each subjected to X-ray diffraction and tested for such mechanical properties as hardness (Hv), tensile strength (σ f ), elongation at break (ε f ), Young's modulus (E), and specific strength (σ f /ρ). The results are shown in Table 1. The hardness (Hv) is the magnitude (DPN) measured with a micro-Vickers hardness tester operated under a load of 25 g, the specific strength is the magnitude obtained by dividing the tensile strength by the density. When the alloys indicated in Table 1 were examined under a transmission electron microscope (TEM), they were found to have crystal grain sizes of not more than 1.0 µm and have intermetallic compounds of Mg with Zn or Al (Mg 7 Zn 3 , Al 2 Mg 3 ) uniformly and finely dispersed in a Mg matrix of h.c.p. No. C. (at%) Phase Hv (DPN) σ f (MPa) ε f (%) E (GPa) σ f /ρ Mg Al Zn 1 90 4 6 Mg+Mg 7 Zn 3 121 371 1.3 37 186 2 86 8 6 Mg+Mg 7 Zn 3 142 427 1.4 36 211 3 80 12 8 Mg+Mg 7 Zn 3 185 485 1.1 44 228 4 92 4 4 Mg+Mg 7 Zn 3 97 360 2.8 36 188 5 88 8 4 Mg+Mg 7 Zn 3 120 330 1.0 37 170 6 84 12 4 Mg+Mg 7 Zn 3 155 328 1.0 36 166 7 88 4 8 Mg+Mg 7 Zn 3 160 427 1.3 33 207 8 82 12 6 Mg+Mg 7 Zn 3 166 530 1.7 38 258 9 86 4 10 Mg+Al 2 Mg 3 178 333 1.1 30 156 As shown in Table 1, all the alloys showed outstanding mechanical properties, i.e. hardnesses Hv exceeding 97 (DPN), tensile strengths exceeding 328 (MPa), elongations at break exceeding 1.0%, Young's moduluses exceeding 30 (GPa), and specific strengths exceeding 156. The change in tensile strength due to the change in the amount of Zn was investigated on the basis of the results of the test described above. The results are shown in Fig. 2. It is noted from Fig. 2 that the Mg-Al-Zn alloys showed a peak of tensile strength between the Zn contents of 6 and 8 atomic % and that this strength decreased according to increasing or decreasing of Zn content from the peak. It is further noted from Fig. 2 that the strength increased with increasing Al content.

Claims: A high-strength magnesium-based alloy possessing an entirely microcrystalline composition represented by the general formula Mg a Al b Zn c wherein a, b and c stand for atomic percents respectively in the ranges of 80 ≤ a ≤ 92, 4 ≤ b ≤ 12, and 4 < c ≤ 12 and (a + b + c) = 100 and having an intermetallic compound of Mg and said other alloy elements uniformly and finely dispersed in a Mg matrix of a hexagonal close-packed structure. A magnesium-based alloy according to claim 1, which exhibits a hardness Hv exceeding 97 (DPN), a tensile strength exceeding 328 (MPa), an elongation at break exceeding 1.0%, a Young's modulus exceeding 30 (GPa), and a specific strength exceeding 156. A magnesium-based alloy according to claim 1 or 2, having the microcrystalline phase of an intermetallic compound of at least Mg 7 Zn 3 or Al 2 Mg 3 uniformly and finely dispersed in a Mg matrix of a hexagonal close-packed structure. A magnesium-based alloy according to claim 1 or 2, which is obtained by rapidly solidifying the melt of said alloy at a cooling rate of from 10 2 to 10 6 K/sec.

Inventors: INOUE AKIHISA, MASUMOTO TSUYOSHI, SHIBATA TOSHISUKE, INOUE, AKIHISA, MASUMOTO, TSUYOSHI, SHIBATA, TOSHISUKE
Applicants: INOE AKIHISA, MASUMOTO TSUYOSHI, YKK CORP, INOUE, AKIHISA, MASUMOTO, TSUYOSHI, YKK CORPORATION
Application year: 1992
Main classification: C22C1
IPC codes: C22C23, C22C45