Amorphous magnesium alloy and method for producing the same

Abstract: A bulky amorphous magnesium alloy having heat-resistance and toughness is provided by setting the alloy composition as: Mg a M b Al c h d Z e (M is at least one element selected from the group consisting of La, Ce, Mm (misch metal) and Y, X is at least one element selected from the group consisting of Ni and Cu, and Z is at least one element selected from the group consisting of Mn, Zn, Zr, and Ti, and, a=70 ∼ 90at%, b=2 ∼ 15at%, c=1 ∼ 9at%, d=2 ∼ 15at%, e=0.1∼8at%, a+b+c+d+e=100at%).

Description: BACKGROUND OF INVENTION 1. Field of Invention The present invention relates to amorphous magnesium alloy having high specific strength and the method for producing the same. 2. Description of Related Arts Crystalline magnesium alloy exhibits a high specific strength and hence can attain weight reduction of automobiles parts, which leads to savings in fuel. Representative crystalline magnesium alloys are based on Mg-Mn, Mg-Al, Mg-Zn, and Mg-rare earth elements. The representative properties are 19∼ 23kg/mm 2 of tensile strength and 10∼ 13 of specific strength for the Mg-2wt%Mn alloy and 16∼18kg/mm 2 of tensile strength and 10 ∼ 12 of specific strength for Mg-2 ∼ 3.5wt% Zn-0.5wt% Zr-2.5 ∼ 4.5wt% R.E. (rare earth element) alloy. The application development of the magnesium alloy is not as advanced as that of aluminum alloys which have already been employed for weight reduction of automobile parts, because the price of magnesium alloy is high, the specific weight is low, and further there is a problem with corrosion in the ambient air. It is known that aluminum-alloy, which is one of the light alloys, enhances strength, by vitrification, thus leading to further enhancement of the specific strength as compared with crystalline alloys. One example of an amorphous aluminum alloy is Al-R.E.-transition element alloy, whose tensile strength amounts to 100kg/mm 2 . It is known that the composition of magnesium alloys which can be vitrified are limited to Mg-Al-Ag and Mg-R.E.-transition metal. However, the former amorphous Mg-Al-Ag alloy has a low crystallizing temperature and hence low heat resistance. In addition, this alloy embrittles after production and shelving at room temperature in ambient air. The latter Mg-R.E.-transition metal alloy is such a brittle material that it is destroyed by bending at room temperature in most cases. Since the specific weight of magnesium is 1.7, which is lower than that of aluminum (2.7), when any amorphous magnesium alloy attains tensile strength of 50kg/mm 2 or more, and incurs neither post-heating embrittlement due to heating at high temperature nor transformation from an amorphous state to crystals during holding at normal temperature, the so-provided amorphous magnesium alloy could be used, in practice, for lightweight parts. Conventionally, the amorphous alloys have been produced by a single-roll apparatus for melt-quenching, which can impart a cooling speed of 10 4 K/sec or more and which can provide a thickness of from 10 to 30µm and width of 100mm. Amorphous alloys with a wider area are produced by the gas-phase deposition method. Their thickness is a few micron meters. The amorphous alloys produced by these method are very thin. In order to produce thicker or bulky amorphous alloys, a ribbon produced by the single-roll method is mechanically crushed and then the crushed powder is hot-consolidated by means of for example extrusion and pressing. Alternatively, the amorphous powder produced by gas atomizing is consolidated by explosion bonding. However, it is difficult to produce bulky amorphous materials with 100% of amorphous structure by these methods, because the pressing and forming conditions for holding the amorpous structure are strict. In addition, since the extrusion, pressing and the like must be carried out at a temperature less than the crystallizing temperature, the required forming force is so great, that the production cost becomes impractically high. EP-A-0361136 discloses high strength amorphous alloys defined by four different compositions. The general composition of formula (IV) is Mg a X c M d Ln e wherein X = Cu, Ni, Sn and/or Zn, M = Aℓ, Si and/or Ca and Ln = Y, La, Ce, Nd, Sm and/or misch metal and a,c,d,e are atomic percentages falling within the following ranges : 40 ≤ a ≤ 90, 4 ≤ c ≤ 35, 2 ≤ d ≤ 25 and 4 ≤ e ≤ 25. SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide an amorphous magnesium alloy which can attain tensile strength of 50kg/mm 2 or more, and incurs neither post-heating embrittlement due to heating at high temperature nor transformation from an amorphous state to crystals during holding at normal temperature, and which could be used, in practice, for lightweight parts. It is another object of the invention to provide a method for producing various shapes of bulky amorphous magnesium alloy relatively easily, and an inexpensive method, with said amorphous magnesium alloy exhibiting high specific strength. The invention provides an amorphous magnesium alloy having the composition Mg a M b Al c X d Z e Zn f , where: M is at least one element selected from La, Ce, Mm (misch metal) and Y; X is at least one element selected from Ni and Cu; Z is at least one element selected from Mn, Zr and Ti; Zn is optionally present; 70 ≤ a ≤90 at%; 2 ≤ b ≤ 15 at%; 1 ≤ c ≤ 9 at%; 2 ≤ d ≤ 15 at%; 0.1 ≤ e; 0.1 ≤ (e+f) ≤ 8 at%; and a + b + c + d + e + f = 100 at%. The invention also provides a method of producing an amorphous magnesium alloy characterised in that an alloy melt having a composition as defined in claim 1 and a value ΔT ≥ 10K, where ΔT = Tx - Tg, Tx is the crystallisation temperature and Tg is the glass transition temperature, is subjected, while flowing, to a primary cooling (13) at a cooling speed Vc, thereby cooling the alloy melt to a temperature in the vicinity of the melting point, and is then subjected to a secondary cooling (14) in which the alloy melt is fed into a mould and cooled to the glass transition temperature Tg at a secondary cooling speed higher than the initial cooling speed Vc, where the value of Vc is such that if used in the secondary cooling it would give rise to partial crystallisation. Preferred features of the alloy and method of the invention can be identified by reference to the following more detailed description and the appended claims. The present invention is described hereinafter in detail. The alloy composition is first described. Mg is the basic metal which is indispensable for weight reduction. When its content (a) is less than 70 atomic %, the specific weight of the alloy becomes high. On the other hand, when the Mg content (a) is more than 90%, it becomes difficult to vitrify the alloy. M and X are elements necessary for vitrification. When the content (b) of M is more than 15 at%, the mixed structure of amorphous phase and crystalline (compound) phase are formed and the strength is decreased. On the other hand, when the content (b) of M is less than 2 at%, the structure becomes totally crystalline. The content (d) of X may be low, in a case where the content (b) of M is high. However, vitrification becomes easy when the content (d) of M is 2 at% or more. On the other hand, when the content (d) of M is more than 15 at%, a brittle amorphous structure is formed. Al element forms a strong oxide film on the surface of the magnesium and enhances the corrosion resistance of magnesium against water, air and the like. When the content (c) of aluminum is less than 1 at%, its effect for enhancing the corrosion resistance is slight. On the other hand, when the content (c) of aluminum is more than 9 at%, the toughness of the amorphous alloy is lessened. Zr, Ti and/or Mn elements in an amount (e) of 0.1 at% or more, are necessary for imparting heat-resistance. On the other hand, when the content (e) is more than 8 at%, vitrification is impeded. Zn in an amount (f) of 0.1 at% or more is effective for enhancing the strength. Zn in a content of 8 at% or more impedes the vitrification. Preferably, Zn is added together with Zr, Ti and/or Mn. BRIEF DESCRIPTION OF DRAWINGS Fig. 1 is a graph illustrating continuous transformation. Fig. 2 is a graph illustrating continuous transformation in two-stage cooling. Fig. 3 illustrates a single-roll cooling apparatus. Fig. 4 illustrates a casting apparatus using a pressing method. Fig. 5 illustrates a centrifugal casting apparatus. DESCRIPTION OF PREFERRED EMBODIMENTS When a thick amorphous alloy is to be produced by means of casting at a relatively slow cooling speed, the magnesium alloy must have a glass-transition temperature (Tg), and, the difference of the absolute temperature (ΔT) between the glass-transition temperature (Tg) and the crystallization temperature (Tx) must be 10K or more (c.f. Fig. 1). Crystals are formed in a range rightside of the curve denoted by AB in Fig. 1. As is shown in Fig. 1, at ΔT>10K, the crystal-forming region shifts toward a longer time span. As is shown in Fig. 2, the cooling at the initial stage is carried out at approximately the melting temperature of the alloy at such a cooling speed that if the alloy were cooled at this rate down to Tg, partial crystallization would occur. Subsequent to the initial cooling stage, the secondary cooling stage is carried out at a higher cooling speed than the initial cooling stage. The two-stage cooling is carried out to produce a relatively thickly cast amorpous magnesium alloy, while avoiding passing through the crystallizing area (N). If great heat-withdrawal is made in the primary cooling stage as shown by the dotted line in Fig. 2, transit through the crystallizing region can be avoided, but such a cooling speed is difficult to achieve by casting. In order to mitigate the cooling load imposed in the secondary cooling stage to vitrify the magnesium alloy, the cooling speed in the primary cooling stage is preferably 10 2 K/sec or more. According to the preferred primary cooling stage, the magnesium alloy is caused to flow from a melt reservoir to a passage, which is drawn in the form of a nozzle or an orifice, and, the temperature of the melt issuing out of the passage is lowered down in proximity of the melting point of the magnesium alloy. This preferable cooling enables the easy attainment of a cooling speed of >10 2 K/sec. Thorough cooling can be carried out in the subsequent secondary cooling by means of forcing close contact between the melt and the cooling metal-mold, hence increasing the heat conduction between them. The mold is made of metal or other material with good heat-conductivity. The mold is preferably water-cooled. The magnesium-alloy melt, which is sufficiently super-cooled in the primary cooling stage, is preferably pressure cast or centrifugally cast at 50G or more, G being the acceleration of gravity. A high cooling speed is thus obtained. The bulky material, which can be produced by the method of the present invention, is from 1 to 5mm in thickness. In addition, amorphous magnesium-alloy having various shapes can be produced by changing the shape of the mold. The bulky material can be used for reinforcing aluminum alloy to provide a composite material. The casting method according to the present invention is described with reference to Figs. 1 and 2. The primary cooling zone corresponds to a region between 1 and 2 shown in Fig. 2. The secondary cooling zone corresponds to a region between 2 and 3 shown in Fig. 2. In the primary cooling zone, the temperature should be lowered below Tm (melting point) as soon as possible. That is, the end point of the primary cooling should be lowered into the proximity of Tm 2 . However, for a smaller product, whose heat capacity is low, primarily cooling can be carried out in such a manner that the secondary cooling starts at Tm 1 . If the primary cooling is not carried out, the cooling speed varies as shown schematically by A-B in Fig. 2. The end point of the primary cooling may be Tm±20k. The secondary cooling may not be intensified, because heat of the melt has been withdrawn in the primary cooling zone. This line A-B indicates that, crystallization occurs if only primary cooling is carried out to cool a cast product with a great volume, because the heat-emission speed from the mold usually slows with the lapse of time after casting, and, hence the cooling pattern crosses the crystallization nose. Since the heat emission in the secondary cooling zone can be reduced, the cooling speed in the secondary cooling zone can be made so high that the cooling does not cross the nose where crystallization takes place. Even a thick product can therefore be vitrified. The present invention is described hereinafter with reference to the drawings. Example 1 Magnesium alloys, whose compositions are given in Table 1, were preliminarily prepared and then heated and melted in a high-frequency induction furnace, which was equipped with a melting crucible 2 made of quartz and a high-frequency heater (Fig. 3). The melt was then injected by means of pressure of argon gas through a slot 1 (0.5mm in diameter) in the quartz melting crucible 2 onto the roll 4 made of copper, which was installed directly beneath the crucible 2. The alloy melt was brought into direct contact with the surface of the roll 4 and was rapidly solidified to obtain an alloy foil strip 5. This method is the single roll method which is generally well known for producing amorphous alloys. The results of X-ray diffraction are shown in "Structure" in Table 2. In order to test the toughness directly after production, the foil strips were subjected to 180° tight contacting and bending around a round frame having a diameter of 0.5mm. The test results are shown in "Toughness" in Table 2. In addition, after heating at 150°C for 100 hours, the same tightly contacting and bending test was carried out. The test results are shown in "Post-heating Toughness" in Table 2. From Table 2, it is apparent that the properties of the inventive alloys are superior to those of the comparative crystalline and amorphous alloys. Compositions of Inventive and Comparative alloys Chemical Composition (at%) Mg La Ce Mm Y Al Ni Cu Mn Zn Zr Inventive 1 Bal 10 - - - 3 - 15 3 - - " 2 " - 10 - - 3 - 10 2 - - " 3 " - - 10 - 3 - 12 2 - - " 4 " - - - 10 3 - 14 2 - - " 5 " - - - 10 3 - 10 2 - - " 6 " 10 - - - 3 10 - 2 - - Comparative 7 " - - 8 - 5 - 10 - 5 - Inventive 8 " - - 8 - 5 - 10 - 4 0.5 Comparative 9 " - 5 - - 3 - 12 - 5 - Comparative 10 " - - - 5 5 10 - - 1 - Inventive 11 " 5 - - - 4 5 - - - 1 Comparative 1 Bal 1 - - - 5 - 10 8 - - " 2 " - 10 - - - - 1 - 5 - " 3 " - - 8 - 6 - - - 5 - " 4 " - 10 - - 15 - 10 - 5 - " 5 " - - 15 - 5 - 20 - - 3.0 Properties of Inventive and Comparative Alloys Structure Toughness Tensile Strength (Kg/mm 2 ) Post-heating Toughness Inventive 1 Amorphous Possible 85 Possible " 2 Amorphous Possible 98 Possible " 3 Amorphous Possible 76 Possible " 4 Amorphous Possible 65 Possible " 5 Amorphous Possible 75 Possible " 6 Amorphous Possible 82 Possible Comparative 7 Amorphous Possible 78 Possible Inventive 8 Amorphous Possible 93 Possible Comparative 9 Amorphous Possible 88 Possible Comparative 10 Amorphous Possible 102 Possible Inventive 11 Amorphous Possible 75 Possible Comparative 1 Crystalline Impossible 65 Impossible " 2 Amorphous Possible 65 Impossible " 3 Crystalline Possible 32 Impossible " 4 Amorphous Impossible 75 Impossible " 5 Crystalline Impossible 38 Impossible Example 2 (Comparative) A 2mm thick, 30mm wide and 30mm long amorpous magnesium alloy having a composition of Mg 79 Ni 10 Y 5 Al 5 Zn 1 was produced in this example by using a metallic-mold casting apparatus shown in Fig. 4. The magnesium alloy melt 10 was prepared by the heater coil 3 in the crucible 1. The magnesium-alloy melt was injected through the nozzle 13 into the die-cavity 15 of the metallic mold 14. The entire metallic-mold casting apparatus was placed in a box so as to optionally prepare the vacuum and inert atmosphere. The respective raw materials were measured and then charged in the crucible 1 made of calcia, and were high-frequency melted by the heater coil 3. The alloy melt 10 was held at a temperature 100°C higher than the melting point of the alloy. Gas was introduced above the alloy melt 10 from a nozzle opened above the crucible 12 so as to apply 0.5kg/cm 2 of pressure onto the alloy melt 1 and then introduce it into the melt reservoir 11. Subsequently, the melt was pressed by the plunger 12 at a pressure of 300kg/cm 2 to introduce it into the die cavity 15 of the metallic mold 14. The nozzle 13 was 10mm long and is longer than the length (5mm) of the ordinary die-casting nozzle so as to increase the temperature fall in the nozzle. A thermocouple was inserted into the metallic mold to measure the temperature, which revealed that the temperature of the magnesium melt in the metallic mold was virtually in the proximity of the melting point. This indicates that the primary cooling is completed at the outlet of the nozzle 13. The melt was then subjected to the secondary cooling in the metallic mold to solidify the melt. Heat-exchange between the metallic mold and the melt was continued in the secondary cooling zone. After thorough cooling, the product was withdrawn out of the metallic mold. The withdrawal could be facilitated by means of thinly applying on the metallic mold a mineral oil or the like. as parting agent. Samples were cut from the products to investigate the structure by means of X-ray diffraction, which showed a halo pattern peculiar to the amorphous alloy. In addition, the strength and hardness were the same as the ribbon materials. Example 3 The respective elements were charged in the crucible shown in Fig. 5, so as to provide the Mg 85 Ni 5 La 5 Al 4 Zr 1 composition. The melt having 100°C higher than the melting point was caused to flow through the nozzle 13 and then poured into the metallic mold 14 102mm in diameter, which rotated at 300rpm. A cylindrical product having a cross section of 2mm x 2mm and central diameter of 100mm was the result. Example 4 Alloys having the compositions given in Table 3 were cast by the method of Example 3. The glass-transition temperature (Tg) and the crystallization temperature (Tx) were measured. The values ΔT (=Tx-Tg) shown in Table 4 were obtained. Since the cast product of the comparative examples were crystallized, ribbons were prepared by the single-roll method, which could provide the cooling speed of 10 5 K/sec or more, and were subjected to measurement of the vitrifying temperature (Tg) and the crystallization temperature (Tx). The results indicate that, when the value of ΔT (=Tx-Tg) is 10K or more, an amorphous cast product can be obtained. Tx-Tg= Δ T (K) Structure Inventive 1 45 Amorphous " 2 45 ↑ " 3 45 ↑ " 4 45 ↑ " 5 50 ↑ " 6 45 ↑ Comparative 7 34 ↑ Inventive 8 38 ↑ Comparative 9 40 ↑ Comparative 10 35 ↑ Inventive 11 23 ↑ Comparative 1 < 5 Amorphous + Crystal " 2 <10 Crystal " 3 < 5 ↑

Claims: An amorphous magnesium alloy having the composition Mg a M b Al c X d Z e Zn f , where: M is at least one element selected from La, Ce, Mm (misch metal) and Y; X is at least one element selected from Ni and Cu; Z is at least one element selected from Mn, Zr and Ti; Zn is optionally present; 70 ≤ a ≤90 at%; 2 ≤ b ≤ 15 at%; 1 ≤ c ≤ 9 at%; 2 ≤ d ≤ 15 at%; 0.1 ≤ e; 0.1 ≤ (e+f) ≤ 8 at%; and a + b + c + d + e + f = 100 at%. An amorphous magnesium alloy according to claim 1 in the form of a ribbon. An amorphous magnesium alloy according to claim 1 or claim 2 wherein said alloy has a thickness of from 1 to 5 mm. A method of producing an amorphous magnesium alloy characterised in that an alloy melt having a composition as defined in claim 1 and a value ΔT ≥ 10K, where ΔT = Tx - Tg, Tx is the crystallisation temperature and Tg is the glass transition temperature, is subjected, while flowing, to a primary cooling (13) at a cooling speed Vc, thereby cooling the alloy melt to a temperature in the vicinity of the melting point, and is then subjected to a secondary cooling (14) in which the alloy melt is fed into a mould and cooled to the glass transition temperature Tg at a secondary cooling speed higher than the initial cooling speed Vc, where the value of Vc is such that if used in the secondary cooling it would give rise to partial crystallisation. A method according to claim 4 wherein the initial cooling speed is at least 10 2 K/sec. A method according to claim 4 or claim 5 wherein said primary cooling of the flowing alloy occurs in a nozzle located directly before the mould. A method according to any one of claims 4 to 6 wherein the flowing alloy is caused to flow into the mould under pressure applied from a plunger. A method according to any one of claims 3 to 7 wherein said mould is rotated so as to apply to the alloy melt in the mould a centrifugal force of 50G or more, where G is acceleration of gravity. A method according to any one of claims 3 to 8 wherein said alloy in the mould has a thickness of from 1 to 5 mm.

Inventors: INOUE AKIHISA, KATO AKIRA, MASUMOTO TSUYOSHI, SHIBATA TOSHISUKE, YAMAGUCHI HITOSHI, INOUE, AKIHISA, KATO, AKIRA, MASUMOTO, TSUYOSHI, SHIBATA, TOSHISUKE, YAMAGUCHI, HITOSHI
Applicants: MASUMOTO TSUYOSHI, YKK CORP, MASUMOTO, TSUYOSHI, YKK CORPORATION
Application year: 1992
Main classification: C22C1
IPC codes: C22C45