High strength magnesium-based alloys

Abstract: The present invention provides high strength magnesium-based alloys which are composed a fine crystalline structure, the alloys having a composition represented by the general formula (I) Mg a X b ; (II) Mg a X c M d , (III) Mg a X c Ln e ; or (IV) Mg a X c M d Ln e (wherein X is one or more elements selected from the group consisting of Cu, Ni, Sn and Zn; M is one or more elements selected from the group consisting of Al, Si and Ca; Ln is one or more elements selected from the group consisting of Y, La, Ce, Nd and Sm or a misch metal of rare earth elements; and a, b, c, d and e are atomic percentages falling within the following ranges: 40 ≦ a ≦ 95, 5 ≦ b ≦ 60, 1 ≦ c ≦ 35, 1 ≦ d ≦ 25 and 3 ≦ e ≦ 25). Since the magnesium-­ based alloys have a superior combination of properties of high hardness, high strength and good processability, they are very useful in various industrial applications.

Description: The present invention relates to magnesium-based alloys which have a superior combination of high hardness and high strength and are useful in various industrial applications. As conventional magnesium-based alloys, there have been known Mg-Al, Mg-Al-Zn, Mg-Th-Zr, Mg-Th-Zn-Zr, Mg-Zn-Zr, Mg-Zn-Zr-RE (rare earth element), etc. and these known alloys have been extensively used in a wide variety of applications, for example, as light-weight structural component materials for aircrafts and automobiles or the like, cell materials and sacrificial anode materials, according to their properties. The following pieces of prior art have been published after the priority date of the present application. WO-89/08154 dicloses magnesium alloys having a high breaking load and the following composition, by weight: Al 2-11%, Zn 0-12%, Mn 0-0.6%, Ca 0-7%, but at least Zn and/or Ca being present. These alloys may be produced by a rapid solidification process. WO-89/11552 discloses magnesium alloys having good formability and mechanical properties having a composition represented by the formula Mg bal Al a Zn b X c , wherein X is at least one element selected from the group consisting of Mn, Ce, Nd, Pr, and Y , "a" ranges from 0 to 15 atom%, "b" ranges from 0 to 4 atom%, "c" ranges from 0.2 to 3 atom%, with the proviso that the sum of aluminum and zinc ranges from 2 to 15 atom%. These alloys are rapidly solidified into ribbon or powder at a cooling rate of 10 5 to 10 7 °C/s. EP-0 361 136 which has been published after the priority date of the present application discloses high strength magnesium-based alloys produced by rapid solidification of similar composition as the alloys of the present application but which are characterized by an amorphous or mixed amorphous and crystalline structure. However, conventional magnesium-based alloys, as set forth above, have a low hardness and strength and also are poor in corrosion resistance. In view of the foregoing, it is an object of the present invention to provide novel magnesium-based alloys at a relatively low cost which have an advantageous combination of properties of high hardness and strength and which are readily processable, for example, by extrusion. According to the present invention, there is provided a high strength magnesium-based alloy having a composition represented by one of the general formulae (I) to (IV): Mg a X b wherein: X is at least two elements selected from the group consisting of Cu, Ni, Sn and Zn; and a and b are atomic percentages falling with the following ranges: 40 ≤ a ≤ 95 and 5≤ b ≤ 60; Mg a X c M d wherein: X is one ore more elements selected from the group consisting of Cu, Ni, Sn and Zn; M is one or more elements selected from the group consisting of Al, Si and Ca; and a, c and d are atomic percentages falling within the following ranges: 40 ≤ a ≤ 95, 1 ≤ c ≤ 35 and 1 ≤ d ≤25; with the proviso that Mg-Zn-Al alloys and Mg-Zn-Al-Ca alloys are excluded from said alloys of formula (II) ; Mg a X c Ln e wherein: X is one or more elements selected from the group consisting of Cu, Ni, Sn and Zn; Ln is one or more elements selected from the group consisting of Y, La, Ce, Nd and Sm or a misch metal (Mm) which is a combination of rare earth elements; and a, c and e are atomic percentages falling within the following ranges 40 ≤ a ≤ 95, 1 ≤ c ≤ 35 and 3 ≤ e ≤25; Mg a X c M d Ln e wherein: X is one or more elements selected from the group consisting of Cu, Ni, Sn and Zn; M is one or more elements selected from the group consisting of Al, Si and Ca; Ln is one or more elements selected from the group consisting of Y, La, Ce, Nd and Sm or a misch metal (Mm) which is a combination of rare earth elements; and a, c, d and e are atomic percentages falling within the following ranges; 40 ≤ a ≤ 95, 1 ≤ c ≤ 35, 1 ≤ d ≤25 and 3 ≤ e ≤ 25; with the proviso of e > 3 when said alloy of formula (IV) includes both Al and Zn; said alloy being composed of a fine crystalline structure with the exclusion of a composite structure of an amorphous phase and a fine crystalline phase. The expression "fine crystalline structure" is used herein to mean an alloy structure consisting of a supersaturated solid solution, a stable or metastable intermetallic phase or mixed phases thereof. Among the elements included in the above-defined alloy compositions, La, Ce, Nd and/or Sm may be replaced with a misch metal (Mm) which is a composite containing those rare earth elements as main components. The Mm used herein consists of 40 to 50 atomic % Ce and 20 to 25 atomic % La with other rare earth elements and acceptable levels of impurities (Mg, Al, Si, Fe, etc). Mm may be replaced for the other Ln elements in an about 1 : 1 ratio (by atomic %) and provides an economically advantageous effect as a practical source of the Ln element because of its low cost. The single figure is a schematic illustration of a single-roller melt-spinning apparatus employed to prepare thin ribbons from the alloys of the present invention by a rapid solidification process. The magnesium-based alloys of the present invention can be obtained by rapidly solidifying a melt of an alloy having the composition as specified above by means of liquid quenching techniques. The liquid quenching techniques involve rapidly cooling a molten alloy and, particularly, single-roller melt-spinning , twin-roller melt-spinning and in-rotating-water melt-spinning are mentioned as especially effective examples of such techniques. In these techniques, a cooling rate of 10 3 to 10 5 K/sec can be obtained. In order to produce thin ribbon materials by single-roller melt-spinning, twin-roller melt-spinning or the like, the molten alloy is ejected from the opening of a nozzle onto a roll of, for example, copper or steel, with a diameter of 30 - 3000 mm, which is rotating at a constant rate of 300 - 10000 rpm. In these techniques, various thin ribbon materials with a width of 1 - 300 mm and a thickness of 5 - 500 µm can be readily obtained. Alternatively, in order to produce fine wire materials by the in-rotating-water melt-spinning technique, a jet of the molten alloy is directed, under application of a back pressure of argon gas, through a nozzle into a liquid refrigerant layer with a depth of 1 to 10 cm which is held by centrifugal force in a drum rotating at a rate of 50 to 500 rpm. In such a manner, fine wire materials can be readily obtained. In this technique, the angle between the molten alloy ejecting from the nozzle and the liquid refrigerant surface is preferably in the range of 60° to 90° and the ratio of the relative velocity of the ejecting molten alloy to the liquid refrigerant surface is preferably in the range of 0.7 to 0.9. The alloys of the present invention are prepared at a cooling rate on the order of 10 3 to 10 5 K/sec. When the cooling rate is lower than 10 3 K/sec, it is impossible to obtain fine crystalline structure alloys having the properties contemplated by the present invention. On the other hand, cooling rates exceeding 10 5 K/sec provides an amorphous structure or a composite structure of an amorphous phase and a fine crystalline phase. For this reason, the above specified cooling rate is employed in the present invention. However, the fine crystalline structure alloy of the present invention may be also prepared by forming first an amorphous alloy in the same procedure as described above, except employing cooling rates of 10 4 to 10 6 K/sec, and, then, heating the amorphous alloy to the vicinity of its crystallization temperature (crystallization temperature ± 100 °C), thereby causing crystallization. In some alloy compositions, the intended fine crystalline structure alloys can be produced at temperatures lower than 100°C less than their crystallization temperature. Besides the above techniques, the alloy of the present invention can also be obtained in the form of a thin film by a sputtering process. Further, rapidly solidified powder of the alloy composition of the present invention can be obtained by various atomizing processes such as, for example, high pressure gas atomizing or spray deposition. In the magnesium-based alloys of the present invention represented by the above general formula (I), a is limited to the range of 40 to 95 atomic % and b is limited to the range of 5 to 60 atomic %. The reason for such limitations is that when the content of Mg is lower than the specified lower limit, it is difficult to form a supersaturated solid solution containing solutes therein in amounts exceeding their solid solubility limits. Therefore, fine crystalline structure alloys having the properties contemplated by the present invention can not be obtained by industrial rapid cooling techniques using the above-mentioned liquid quenching, etc. On the other hand, if the content of Mg exceeds the specified upper limit, it is impossible to obtain fine crystalline structure alloys having the properties intended by the present invention. In the magnesium-based alloys of the present invention represented by the above general formula (II), a, c and d are limited to the ranges of 40 to 95 atomic %, 1 to 35 atomic % and 1 to 25 atomic %, respectively. The reason for such limitations is that when the content of Mg is lower than the specified lower limit, it becomes difficult to form the supersaturated solid solution with the solutes dissolved therein in amounts exceeding solid solubility limits. Therefore, the fine crystalline structure alloys having the properties contemplated by the present invention can not be obtained by industrial rapid cooling techniques using the above-mentioned liquid quenching, etc. On the other hand, if the content of Mg exceeds the specified upper limit, it is impossible to obtain the fine crystalline structure alloys having the properties intended by the present invention. In the magnesium-based alloys of the present invention represented by the above general formula (III), a is limited to the range of 40 to 95 atomic %, c is limited to the range of 1 to 35 atomic % and e is limited to the range of 3 to 25 atomic %. As described above, the reason for such limitations is that when the content of Mg is lower than the specified lower limit, it becomes difficult to form the supersaturated solid solution with the solutes dissolved therein in amounts exceeding their solid solubility limits. Therefore, fine crystalline alloys having the properties contemplated by the present invention can not be obtained by industrial rapid cooling techniques using the above-mentioned liquid quenching, etc. On the other hand, if the content of Mg exceeds the specified upper limit, it is impossible to obtain fine crystalline structure alloys having the properties intended by the present invention. Further, in the magnesium-based alloys of the present invention represented by the above general formula (IV), a, c, d and e should be limited within the ranges of 40 to 95 atomic %, 1 to 35 atomic %, 1 to 25 atomic % and 3 to 25 atomic %, respectively. The reason for such limitations is, as described above, that when the content of Mg is lower than the specified lower limit, it becomes difficult to form the supersaturated solid solution with the solutes dissolved therein in amounts exceeding their solid solubility limits. Therefore, the fine crystalline structure alloys having the properties contemplated by the present invention can not be obtained by industrial rapid cooling techniques using the above-mentioned liquid quenching, etc. On the other hand, if the content of Mg exceeds the specified upper limit, it is impossible to obtain fine crystalline structure alloys having the properties intended by the present invention. The X element is one or more elements selected from the group consisting of Cu, Ni, Sn and Zn and these elements provide a superior effect in stabilizing the resulting crystalline phase, under the conditions of the preparation of the fine crystalline structure alloys, and improve the alloy's strength while retaining its ductility. The M element is one or more elements selected from the group consisting of Al, Si and Ca and forms stable or metastable intermetallic compounds in combination with magnesium and other additive elements under the production conditions of the fine crystalline structure alloys. The formed intermetallic compounds are uniformly distributed throughout a magnesium matrix (α-phase) and, thereby, considerably improve the hardness and strength of the resultant alloys. Further, the M element prevents coarsening of the fine crystalline structure at high temperatures and provides a good heat resistance. Among the above elements, Al element and Ca element have the effect of improving the corrosion resistance and Si element improves the fluidity of the molten alloy. The Ln element is one or more elements selected from the group consisting of Y, La, Ce, Nd and Sm or a misch metal (Mm) consisting of said rare earth elements and the Ln element is effective to provide a more stable, fine crystalline structure, when it is added to the Mg-X system or the Mg-X-M system. Further, the Ln element provides a greatly improved hardness. Further, since the magnesium-based alloys of the present invention show superplasticity at a high temperature range, permitting the presence of a stable fine crystalline phase, they can be readily subjected to extrusion, press working, hot forging, etc. Therefore, the magnesium-based alloys of the present invention, obtained in the form of thin ribbon, wire, sheet or powder, can be successfully consolidated into bulk materials by way of extrusion, press working, hot-forging, etc., at the high temperature range for a stable, fine crystalline phase. Further, some of the magnesium-based alloys of the present invention are sufficiently ductile to permit a high degree of bending. Example Molten alloy 3, having a predetermined composition, was prepared using a high-frequency melting furnace and charged into a quartz tube 1 having a small opening 5 (diameter: 0.5 mm) at the tip thereof, as shown in the drawing. After being heated to melt the alloy 3, the quartz tube 1 was disposed right above a copper roll 2. Then, the molten alloy 3 contained in the quartz tube 1 was ejected from the small opening 5 of the quartz tube 1 under the application of an argon gas pressure of 0.7 kg/cm 2 and brought into contact with the surface of the copper roll 2 rapidly rotating at a rate of 5,000 rpm. The molten alloy 3 was rapidly solidified and an alloy thin ribbon 4 was obtained. According to the processing conditions as described above, there were obtained 21 different alloy thin ribbons (width: 1 mm, thickness: 20 µm) having the compositions (by at.%) as shown in the Table. Hardness (Hv) and tensile strength were measured for each test specimen of the thin ribbons and the results are shown in a right column of the Table. The hardness (Hv) is indicated by values (DPN) measured using a Vickers micro hardness tester under a load of 25 g. As shown in the Table, all test specimens showed a high level of hardness Hv (DPN) of at least 240 which is about 2.5 to 4.0 times the hardness Hv (DPN), i.e., 60 - 90, of the conventional magnesium-based alloys. Further, the test specimens of the present invention all exhibited a high tensile-strength level of not less than 850 MPa and such a high strength level is approximately 2 times the highest strength level of 400 MPa achieved in known magnesium-based alloys. It can be seen from such results that the alloy materials of the present invention are superior in hardness and strength. In addition, for example, specimen Nos. 3, 7 and 12 shown in the Table exhibited a superior ductility permitting a large degree of bending and a good formability. No. Specimen Hv(DPN) δf (MPa) 1. Mg 65 Ni 25 La 10 325 1150 2. Mg 90 Ni 5 La 5 295 1010 3. Mg 90 Ni 5 Ce 5 249 920 4. Mg 75 Ni 10 Y 15 346 1280 5. Mg 75 Ni 10 Si 5 Ce 10 302 1100 6. Mg 75 Ni 10 Mm 15 295 1120 7. Mg 90 Ni 5 Mm 5 270 920 8. Mg 60 Ni 20 Mm 20 357 1150 9. Mg 70 Ni 10 Ca 5 Mm 15 313 1180 10. Mg 70 Ni 5 Al 5 Mm 20 346 1260 11. Mg 55 Ni 20 Sn 10 Y 15 355 1215 12. Mg 90 Cu 5 La 5 246 872 13. Mg 80 Cu 10 La 10 266 935 14. Mg 50 Cu 20 La 10 Ce 20 327 1160 15. Mg 75 Cu 10 Zn 5 La 10 346 1195 16. Mg 75 Cu 15 Mm 10 265 877 17. Mg 80 Cu 10 Y 10 274 901 18. Mg 75 Cu 10 Sn 5 Y 10 352 1150 19. Mg 70 Cu 12 Al 8 Y 10 307 1180 20. Mg 80 Sn 10 La 10 291 1087 21. Mg 70 Zn 15 La 10 Ce 5 304 1125 As described above, the magnesium-based alloys of the present invention have a high hardness and a high strength which are respectively, at least 2.5 times and at least 2 times greater than those of a similar type of magnesium-based alloy which has been heretofore evaluated as the most superior alloy and yet also have a good processability permitting extrusion or similar operations. Therefore, the alloys of the present invention exhibit advantageous effects in a wide variety of industrial applications.

Claims: A high strength magnesium-based alloy having a composition represented by one of the general formulae (I) to (IV): Mg a X b wherein: X is at least two elements selected from the group consisting of Cu, Ni, Sn and Zn; and a and b are atomic percentages falling with the following ranges: 40 ≤ a ≤ 95 and 5≤ b ≤ 60; Mg a X c M d wherein: X is one or more elements selected from the group consisting of Cu, Ni, Sn and Zn; M is one or more elements selected from the group consisting of Al, Si and Ca; and a, c and d are atomic percentages falling within the following ranges: 40 ≤ a ≤ 95, 1 ≤ c ≤ 35 and 1 ≤ d ≤25; with the proviso that Mg-Zn-Al alloys and Mg-Zn-Al-Ca alloys are excluded from said alloys of formula (II); Mg a X c Ln e wherein: X is one or more elements slected from the group consisting of Cu, Ni, Sn and Zn; Ln is one or more elements selected from the group consisting of Y, La, Ce, Nd and Sm or a misch metal (Mm) which is a combination of rare earth elements; and a, c and e are atomic percentages falling within the following ranges: 40 ≤ a ≤ 95, 1 ≤ c ≤ 35 and 3 ≤ e ≤25; Mg a X c M d Ln e wherein: X is one or more elements selected from the group consisting of Cu, Ni, Sn and Zn; M is one or more elements selected from the group consisting of Al, Si and Ca; Ln is one or more elements selected from the group consisting of Y, La, Ce, Nd and Sm or a misch metal (Mm) which is a combination of rare earth elements; and a, c, d and e are atomic percentages falling within the following ranges; 40 ≤ a ≤ 95, 1 ≤ c ≤ 35, 1 ≤ d ≤25 and 3 ≤ e ≤ 25; with the proviso of e > 3 when said alloy of formula (IV) includes both Al and Zn; said alloy being composed of a fine crystalline structure with the exclusion of a composite structure of an amorphous phase and a fine crystalline phase. A process for making a magnesium-based alloy having a hardness (Hv) of at least 240 and a tensile strength of not less than 850 MPa, said alloy having a composition represented by one of the general formulae (I) to (IV): Mg a X b wherein: X is at least two elements selected from the group consisting of Cu, Ni, Sn and Zn; and a and b are atomic percentages falling within the following ranges: 40 ≤ a ≤ 95 and 5≤ b ≤ 60; Mg a X c M d wherein: X is one or more elements selected from the group consisting of Cu, Ni, Sn and Zn; M is one or more elements selected from the group consisting of Al, Si and Ca; and a, c and d are atomic percentages falling within the following ranges: 40 ≤ a ≤ 95, 1 ≤ c ≤ 35 and 1 ≤ d ≤ 25; Mg a X c Ln e wherein: X is one or more elements slected from the group consisting of Cu, Ni, Sn and Zn; Ln is one or more elements selected from the group consisting of Y, La, Ce, Nd and Sm or a misch metal (Mm) which is a combination of rare earth elements; and a, c and e are atomic percentages falling within the following ranges: 40 ≤ a ≤ 95, 1 ≤ c ≤ 35 and 3 ≤ e ≤25; and Mg a X c M d Ln e wherein: X is one or more elements selected from the group consisting of Cu, Ni, Sn and Zn; M is one or more elements selected from the group consisting of Al, Si and Ca; Ln is one or more elements selected from the group consisting of Y, La, Ce, Nd and Sm or a misch metal (Mm) which is a combination of rare earth elements; and a, c, d and e are atomic percentages falling within the following ranges; 40 ≤ a ≤ 95, 1 ≤ c ≤ 35, 1 ≤ d ≤25 and 3 ≤ e ≤ 25; said process comprising rapidly solidifying a melt of the alloy at a cooling rate of from 10 3 to 10 5 K/sec, whereby said alloy is composed of a fine crystalline structure with the exclusion of a composite structure of an amorphous phase and a fine crystalline phase. A process for making a magnesium-based alloy having a hardness Hv of at least 240 and a tensile strength of not less than 850 MPa, said alloy having a composition represented by one of the general formulae (I) to (IV): Mg a X b wherein: X is at least two elements selected from the group consisting of Cu, Ni, Sn and Zn; and a and b are atomic percentages falling within the following ranges: 40 ≤ a ≤ 95 and 5≤ b ≤ 60; Mg a X c M d wherein: X is one or more elements selected from the group consisting of Cu, Ni, Sn and Zn; M is one or more elements selected from the group consisting of Al, Si and Ca; and a, c and d are atomic percentages falling within the following ranges: 40 ≤ a ≤ 95, 1 ≤ c ≤ 35 and 1 ≤ d ≤ 25; Mg a X c Ln e wherein: X is one or more elements slected from the group consisting of Cu, Ni, Sn and Zn; Ln is one or more elements selected from the group consisting of Y, La, Ce, Nd and Sm or a misch metal (Mm) which is a combination of rare earth elements; and a, c and e are atomic percentages falling within the following ranges: 40 ≤ a ≤ 95, 1 ≤ c ≤ 35 and 3 ≤ e ≤25; and Mg a X c M d Ln e wherein: X is one or more elements selected from the group consisting of Cu, Ni, Sn and Zn; M is one or more elements selected from the group consisting of Al, Si and Ca; Ln is one or more elements selected from the group consisting of Y, La, Ce, Nd and Sm or a misch metal (Mm) which is a combination of rare earth elements; and a, c, d and e are atomic percentages falling within the following ranges; 40 ≤ a ≤ 95, 1 ≤ c ≤ 35, 1 ≤ d ≤25 and 3 ≤ e ≤ 25; said process comprising rapidly solidifying a melt of the alloy to obtain an alloy having an amorphous or a mixed amorphous and crystalline structure, and thereafter heating said alloy to a temperature ranging from -100 °C to +100 °C of its crystallization temperature, whereby said alloy is composed of a fine crystalline structure with the exclusion of a composite structure of an amorphous phase and a fine crystalline phase.

Inventors: AIKAWA KAZUO, TAKETANI KATSUYUKI, AIKAWA, KAZUO, TAKETANI, KATSUYUKI
Applicants: YKK CORP, YKK CORPORATION
Application year: 1990
Main classification: C22C23
IPC codes: C22C45, C22C23