High-strength amorphous magnesium alloy and method for producing the same

Abstract: An amorphous magnesium alloy has a composition of Mg a M b X c (M is Zn and/or Ga, X is La, Ce, Mm (misch metal), Y, Nd, Pr, Sm and Gd), a is from 65 to 96.5 atomic %, b is from 3 to 30 atomic %, and c is from 0.2 to 8 atomic %). The magnesium alloy has a high specific strength and does not embrittle at room temperature.

Description: Background of Invention 1. Field of Invention The present invention relates to a method for producing an amorphous magnesium alloy having improved specific strength and ductility, and to the alloy produced thereby. 2. Description of Related Arts Magnesium alloys have tensile strength of approximately 24kg/mm 2 and specific gravity of 1.8, as is stipulated in JIS H5203, MC2. Magnesium alloys have therefore a high specific strength and are promising materials to reduce weight of automotive vehicles, which weight reduction is required for conserving fuel consumption. Japanese Unexamined Patent Publication No. 3-10041 proposes an amorphous magnesium alloy having a composition of Mg-rare earth element-transition element. The proposed amorphous magnesium alloy has a high strength; however, when a large amount of the rare-earth element is added to vitrify the Mg alloy, enhancement of the specific strength is less than expected. The proposed Mg alloy would therefore not be as competitive as other high specific strength materials. It is also known that the ternary Mg-Al-Ag magnesium alloy can be vitrified. The Mg-Al-Ag amorphous alloy has a low crystallization temperature and has the disadvantage of embrittlement when exposed at room temperature in ambient atmosphere for approximately 24 hours. The Mg-rare earth element-transition metal alloy has a higher specific weight than the Mg-Al-Ag alloy and hence does not have a satisfactorily high specific strength. In addition, since not a few compositions of the Mg-rare earth element-transition metal alloy embrittle when exposed as described above, the properties of this alloy are unstable. Under the circumstances described above, development of the practical application of Mg alloys has lagged behind Al alloys. Summary of the Invention It is therefore an object of the present invention to provide a method for producing an amorphous magnesium alloy, which has a sufficiently high Mg content and high strength so as to attain high specific strength, which has a sufficiently high crystallization temperature so as to attain improved heat-resistance, and which does not embrittle when exposed at room temperature. It is another object of the present invention to provide the amorphous magnesium alloy produced by said method. The invention relates to two methods as given by claims 1 and 2. The present inventors discovered that specific elements added to an Mg-rich composition can provide an amorphous Mg alloy which has a high strength. A high-strength amorphous magnesium alloy produced by a first method of the present invention has a composition (I) of Mg a M b X c (M is at least one element selected from the group consisting of Zn and Ga, X is at least one element selected from the group consisting of La, Ce, Mm (misch metal), Y, Nd, Pr, Sm and Gd, a is from 65 to 96.5 atomic %, b is from 3 to 30 atomic %, and c is from 0.2 to 8 atomic %), and has at least 50% of amorphous phase. Another high-strength amorphous magnesium alloy produced by a second method of the present invention has a composition (II) of Mg d M e X f T g (M is at least one element selected from the group consisting of Zn and Ga, X is at least one element selected from a group consisting of La, Ce, Mm (misch metal), Y, Nd, Pr, Sm and Gd, T is at least one element selected from the group consisting of Ag, Zr, Ti and Hf, d is from 65 to 96.5 atomic %, e is from 2 to 30 atomic %, f is from 0.2 to 8 atomic %, and g is from 0.5 to 10 atomic %), and has at least 50% of amorphous phase. Said first and second methods for producing the high-strength amorphous magnesium alloys according to the present invention are characterized by cooling, at a cooling speed of from 10 2 to 10 5 °C/s, a magnesium-alloy melt having said composition (I) or (II) and by subsequently heat treating at a temperature lower than the crystallisation temperature of the alloy to provide a structure which comprises at least 50% amorphous phase providing a matrix in which are dispersed hcp magnesium particles of a size in the range 1 to 100 nm. Mg is a major element for providing light weight. M (Zn and/or Ga), and X (La, Ce, Mm, Y, Nd, Pr, Sm and/or Gd) are vitrifying elements. T (Ag, Zr, Ti and/or Hf) is/are element(s) for attaining improved ductility. A part of T is a solute of the crystalline Mg. The other part of T becomes a component of the amorphous phase and enhances the crystallization temperature. In the light of attaining high strength Ce, La and Mn are preferred, because these elements can enhance the tensile strength as higher as or higher than the other X element at an identical atomic %. When M is added in an amount greater than 30 atomic %, an Mg-M compound precipitates in a great amount and also the specific weight increases. On the other hand, when M is added in an amount smaller than 3 atomic %, vitrification becomes difficult. When X is added in an amount smaller than 0.2 atomic %, vitrification becomes difficult. On the other hand, when X is added in an amount greater than 8 atomic %, not only does embrittlement occur but also specific weight increases. When T is added in an amount smaller than 0.5 atomic %, neither heat-resistance nor strength is enhanced effectively. On the other hand, when T is added in an amount greater than 10 atomic %, vitrification becomes difficult. The amorphous phase must be 50% or more, because embrittlement occurs at a smaller amorphous phase. The above mentioned alloys can be vitrified at least 50% by cooling the alloy melt at a cooling rate of from 10 2 to 10 5 °C/s which is the normal cooling rate. A 100% amorphous structure can be obtained by increasing the cooling speed. The phase other than the amorphous phase is a crystalline α-Mg (M, X and T are solutes) having hcp structure. This crystalline Mg phase is from 1 to 100 nm in size and disperses in the amorphous phase as particles and strengthens the Mg alloy. When the magnesium particles are uniformly dispersed in the amorphous matrix, the strength is exceedingly high. The melt-quenched amorphous alloy is then heat-treated at a temperature lower than the crystallization temperature (Tx) which is in the range of from 120 to 262°C. Then, the magnesium particles are separated and precipitate in the amorphous matrix. Strength is enhanced usually by approximately 100MPa, but elongation decreases as compared with the melt-quenched state. The present invention is hereinafter described with reference to the drawings. Brief Description of Drawings Fig. 1 illustrates a single-roll apparatus. Fig. 2 shows X-ray diffraction patterns. Figs. 3A and C show the dark-field and bright-field of electronic microscope images of a ribbon material, respectively. Fig. 3B shows an electron-diffraction pattern of the ribbon material. Examples Example 1 A magnesium alloy, whose composition is given in Table 1, was prepared as mother alloy by a high-frequency melting furnace. The mother alloy was melt-quenched and solidified by the single-roll method which is well known as a method for producing the amorphous alloys. A ribbon was thus produced. A quartz tube 2, with an orifice 0.1mm in diameter at the front end, was filled with the mother alloy in the form of an ingot. The mother alloy was then heated and melted. The quartz tube 2 was then positioned directly above the roll 2 made of copper. The resultant molten alloy 4 in the quartz tube 4 was ejected through the orifice 2 under argon gas pressure and was brought into contact with the surface of roll 3. An alloy ribbon 5 was thus produced by melt quenching and solidification at a cooling speed of 10 3 °C/s. The alloy ribbon 5 had a composition of Mg 85 Zn 12 Ce 3 and was 20µm thick and 1mm wide. The alloy ribbon was subjected to X-ray diffraction by a diffractometer. The result is shown in Fig. 2 as "A". In the diffraction pattern, a halo pattern of amorphous alloy and a peak of Mg are recognized. The proportion of crystalline Mg was 12%. The alloy ribbon was heat-treated at a temperature lower by 1°C than the crystallization temperature (Tx) for 20 seconds. X-ray diffraction pattern of the heat-treated ribbon is shown in Fig. 2 as "B". Peaks of the hcp Mg are clear as compared with the diffraction pattern of the non heat-treated alloy. Structure of the heat-treated alloy was observed by an electronic microscope. It was revealed that particles 10 nm or finer were dispersed in the amorphous matrix in a proportion of 20% (Fig.3). The proportion of amorphous phase is 80%. Mg 85 Zn 12 Ce 3 Melt-Quenched Material Heat-treated Material Structure Amorphous+Crystalline Amorphous+Crystalline Tensile Strength 670MPa 980MPa Elongation 7% 3% Hardness (Hv) 175 210 The crystalline phase of the molt-quenched material is an hcp Mg. Example 2 Magnesium alloys, whose compositions are given in Table 2, were prepared as mother alloys by a high-frequency melting furnace. The mother alloys were melt-quenched and solidified by the single roll to produce the ribbons. The results of X-ray diffraction of the ribbons are given in Table 2. The ribbons were allowed to stand at room temperature for 24 hours and then subjected to bend test and tensile test. The results of a 180° tight bend test and tensile test are given in Table 2. Composition Structure 180° tight bending Tensile Strength (MPa) Tx (°C) Inventive 1 Mg 80 Zn 15 Mm 5 Amorphous+Crystalline Possible 680 170 2 Mg 80 Zn 15 Y 5 Amorphous+Crystalline Possible 590 167 3 Mg 80 Zn 15 Ce 5 Amorphous+Crystalline Possible 630 173 4 Mg 80 Zn 15 La 5 Amorphous+Crystalline Possible 650 167 Comparative 5 Mg 97 Zn 2 La 1 Crystalline Brittle - 77 6 Mg 64 Zn 35 Ce 1 Amorphous Possible 500 87 Inventive 7 Mg 84 Zn 10 La 5 Ag 1 Amorphous+ Crystalline Possible 680 158 8 Mg 73 Zn 20 La 5 Ti 1 Ag 1 Amorphous+ Crystalline Possible 690 162 9 Mg 74 Zn 20 Ce 5 Ag 1 Amorphous+ Crystalline Possible 650 168 10 Mg 74 Zn 20 Y 5 Ag 1 Amorphous+ Crystalline Possible 630 172 11 Mg 79 Zn 20 Y 0.5 Hf 0.5 Amorphous+ Crystalline Possible 645 158 12 Mg 79 Ga 15 Nd 5 Ag 1 Amorphous+ Crystalline Possible 620 207 13 Mg 79 Ga 15 Mm 5 Ag 1 Amorphous+ Crystalline Possible 595 207 14 Mg 79 Zn 15 Gd 5 Ag 1 Amorphous+ Crystalline Possible 580 226 Inventive 15 Mg 79 Zn 15 Ce 5 Ag 1 Amorphous+ Crystalline Possible 590 177 Inventive 16 Mg 79 Ga 15 Ce 5 Ag 1 Amorphous+ Crystalline Possible 620 208 Comparative 17 Mg 58 Ga 35 Ce 5 Ti 2 Amorphous Possible 490 217 18 Mg 58 Zn 35 La 5 Ti 2 Amorphous+ Possible 500 157 19 Mg 92 Ga 1 La 5 Ti 2 Crystalline Brittle - - 20 Mg 89 Zn 1 La 5 Ag 5 Crystalline Brittle - - The above ribbons were heat-treated for 0.1 hour at a temperature 10°C lower than the crystallization temperature (Tx). The bend and tensile tests were then carried out. The results are given in Table 3. Composition Structure 180° tight bending Tensile Strength (MPa) Inventive 1 Mg 80 Zn 15 Mm 5 Amorphous+Crystalline Possible 780 2 Mg 80 Zn 15 Y 5 Amorphous+Crystalline Possible 800 3 Mg 80 Zn 15 Ce 5 Amorphous+Crystalline Possible 780 4 Mg 80 Zn 15 La 5 Amorphous+Crystalline Possible 790 Comparative 5 Mg 97 Zn 2 La 1 Crystalline Brittle - 6 Mg 64 Zn 35 Ce 1 Amorphous Possible 650 Inventive 7 Mg 84 Zn 10 La 5 Ag 1 Amorphous+ Crystalline Possible 780 8 Mg 73 Zn 20 La 5 Ti 1 Ag 1 Amorphous+ Crystalline Possible 820 9 Mg 74 Zn 20 Ce 5 Ag 1 Amorphous+ Crystalline Possible 780 10 Mg 74 Zn 20 Y 5 Ag 1 Amorphous+ Crystalline Possible 790 11 Mg 79 zn 20 Y 0.5 Hf 1 Amorphous+ Crystalline Possible 780 12 Mg 79 Ga 15 Nd 5 Ag 1 Amorphous+ Crystalline Possible 780 13 Mg 79 Ga 15 Mm 5 Ag 1 Amorphous+ Crystalline Possible 690 14 Mg 79 Zn 1.5 Gd 5 Ag 1 Amorphous+ Crystalline Possible 720 15 M 79 Zn 15 Ce 5 Ag 1 Amorphous Possible 680 16 Mg 79 Ga 15 Ce 5 Ag 1 Amorphous+ Crystalline Possible 780 Comparative 17 Mg 58 Ga 35 Ce 5 Ti 2 Amorphous Possible 530 18 Mg 58 Zn 35 La 5 Ti 2 Amorphous+ Possible 490 19 Mg 58 Ga 1 La 5 Ti 2 Crystalline Brittle - 20 Mg 88 Zn 1 La 5 Ag 5 Crystalline Brittle - As is clear from the above experimental results, the Mg alloy according to the present invention has a high strength and can be vitrified even at an Mg rich composition. The Mg alloy according to the present invention is tough and does not embrittle so that it can be bent at an angle of 180°. The specific gravity of the Mg alloy according to the present invention is approximately 2.4. The specific strength in terms of tensile strength (kg/mm 2 )/specific gravity is approximately 14kg/mm 2 and hence very high.

Claims: A method for producing a high-strength amorphous magnesium alloy, having a composition of formula (I) Mg a M b X c : where M is at least one of Zn and Ga; X is at least one of La, Ce, Mm (misch metal), Y, Nd, Pr, Sm and Gd; a is from 65 to 96.5 atomic %; b is from 3 to 30 atomic %; and c is from 0.2 to 8 atomic %; and having a structure which comprises at least 50% amorphous phase providing a matrix in which are dispersed hcp magnesium particles of a size in the range 1 to 100nm, wherein an alloy melt having a composition of formula (I) is cooled at a cooling rate of from 10 2 to 10 5 °C/s and is subsequently heat treated at a temperature lower than the crystallization temperature of the alloy to provide the said structure. A method for producing a high-strength amorphous magnesium alloy, having a composition of formula (II): Mg d M e X f T g where M is at least one of Zn and Ga; X is at least one of La, Ce, Mm (misch metal), Y, Nd, Pr, Sm and Gd; T is at least one of Ag, Zr, Ti and Hf; d is from 65 to 96.5 atomic %; e is from 2 to 30 atomic %; f is from 0.2 to 8 atomic %; and g is from 0.5 to 10 atomic %; and having a structure which consists of at least 50% amorphous phase providing a matrix in which are dispersed hcp magnesium particles, of a size in the range 1 to 100nm, wherein an alloy melt having a composition of formula (II) is cooled at a cooling rate of from 10 2 to 10 5 °C/s and is subsequently heat-treated at a temperature lower than the crystallization temperature of the alloy to provide the said structure. A method according to claim 1 or claim 2 wherein said alloy comprises at least 50% amorphous phase after cooling to ambient temperature. A high strength amorphous magnesium alloy when produced by a method as claimed in any one of the preceding claims.

Inventors: INOUE AKIHISA, KATO AKIRA, MASUMOTO TSUYOSHI, NISHIYAMA NOBUYUKI, SHIBATA TOSHISUKE, INOUE, AKIHISA, KATO, AKIRA, MASUMOTO, TSUYOSHI, NISHIYAMA, NOBUYUKI, SHIBATA, TOSHISUKE, INOUE, AKIHISA, 11-806, KAWAUCHI JUTAKU, Nishiyama, Nobuyuki, c/o Teikoku Piston Ring
Applicants: MASUMOTO TSUYOSHI, TEIKOKU PISTON RING CO LTD, TOYOTA MOTOR CO LTD, YKK CORP, MASUMOTO, TSUYOSHI, TEIKOKU PISTON RING CO. LTD., TOYOTA JIDOSHA KABUSHIKI KAISHA, YKK CORPORATION
Application year: 1992
Main classification: C22C1
IPC codes: C22C23, C22C45, C22C1