Aluminum-based alloy with high strength and heat resistance

Abstract: An aluminum-based alloy which consists Al and 0.1 to 25 atomic % of at least two transition metal elements and has a structure in which at least quasicrystals are homogeneously dispersed in a matrix composed of Al or a supersaturated Al solid solution. The quasicrystals are preferably composed of an I-phase alone or a mixed phase of an I-phase and a D-phase and preferably has a volume nfraction of 20% or less. Specifically, the aluminum-based alloy has the composition represented by the general formula Al bal Ni a X b or Al bal Ni a X b M c wherein X is one or two elements selected between Fe and Co; M is at least one element selected from among Cr, Mn, Nb, Mo, Ta and W; 5 ≦ a ≦ 10; 0.5 ≦ b ≦ 10; and 0.1 ≦ c ≦ 5. The alloy is excellent in hardness and strength both at room temperature and high temperature and in heat resistance and has a high specific strength. It can retain the excellent characteristics even when affected by the heat of working.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an aluminum-based alloy having superior properties of high strength, high hardness and high heat resistance which comprises at least quasicrystals finely dispersed in a matrix composed of a principal metal element (aluminum). 2. Description of the Prior Art An aluminum-based alloy having high strength and high heat resistance has heretofore been produced by the rapid solidifying methods such as liquid quenching method. In particular, the aluminum-based alloy produced by the rapid solidifying method as disclosed in Japanese Patent Laid-Open No. 275732/1989 is amorphous or microcrystalline, and particularly the microcrystal as disclosed therein comprises a composite material that is constituted of a metallic solid solution composed of an aluminum matrix, a microcrystalline aluminum matrix phase and a stable or metastable intermetallic compound phase. Mat. Trans. JIM, 30 1989, 150-154 discloses that decagonal alloys consisting of large grains with a size of above 1 µm are obtained in the Al-Ni-Fe and Al Ni Co systems in the ranges of 9 to 16 at% Ni and 9 to 21 at% Fe or Co. Further, it is disclosed in said document, that replacement of Fe or Co by V, Cr, Mn or Ru may result in a structure consisting of decagonal, icosahedral and crystalline phases. In Phil. Mag. Lett. 61(1990) it is reported that icosahedral quasicrystaline phase are observed in rapidly solidified Al-Pd-Mn alloys with compositions ranging from 60 to 80 at% Al, 5 to 20 at% Pd and 10 to 20 at% Mn. Further, annealing induced ordering is observed in the said system. The aluminum-based alloy disclosed in the Japanese Patent Laid-Open No. 275732/1989 is an excellent alloy exhibiting high strength, high heat resistance and high corrosion resistance and further favorable workability as a high strength structural material but is deprived of the excellent characteristics as the rapidly solidified material in a temperature region as high as 300 °C or above, thereby leaving some room for further improvement with respect to heat resistance, especially heat-resisting strength. Moreover, there is some room also for improvement with regard to specific strength of the alloy, since the alloy is not sufficiently enhanced in specific strength because of its being incorporated with an element having a relatively high specific gravity. SUMMARY OF THE INVENTION It is an object of the present invention to provide an aluminum-based alloy having superior heat resistance, high strength at high temperatures, hardness and high specific strength by constituting a structure in which at least quasicrystals are finely dispersed in a matrix composed of aluminum and a method for producing the same. In order to solve the above problems, the present invention provides an aluminum-based alloy having high strength and high heat resistance which comprises aluminum as the principal element and at least two transition metal elements added thereto in the range of 0.1 to 25 atomic %, said alloy having a structure as specified in appended claim 1. The aforesaid quasicrystals consist of an icosahedral phase (I-phase) alone or a mixed phase of an I-phase and a regular decagonal phase (D-phase). The above structure is preferably such that the quasicrystals, various intermetallic compounds formed from aluminum and transition metal elements and/or various intermetallic compound formed from transition metal elements are homogeneously and finely dispersed in the matrix composed of aluminum. Specific examples of preferable compositions of the aluminum-based alloy include (I) one represented by the general formula Al bal Ni a X b wherein X is one or two elements selected between Fe and Co; and a and b are, in atomic percentages, 5 ≤ a ≤ 10 and 0.5 ≤ b ≤ 10, and (II) one represented by the general formula Al bal Ni a X b M c wherein X is one or two elements selected between Fe and Co; M is at least one element selected from among Cr, Mn, Nb, Mo, Ta and W; and a, b and c are, in atomic percentages, 5 ≤ a ≤ 10, 0.5 ≤ b ≤ 10 and 0.1 ≤ c ≤ 5. Of the alloys having the composition represented by the above general formulae, an alloy having a structure in which at least one intermetallic compound represented by Al 3 Ni is dispersed in a matrix composed of aluminum or a supersaturated solid solution of aluminum is more effective in reinforcing the matrix and controlling the growth of crystal grains. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a graph showing the relationship between the heat treatment temperature and the hardness of the test pieces in Example 2. FIG. 2 is a graph showing the result of X-ray diffraction profile of the test piece having the composition consisting of Al bal Ni 8 Fe 5 . FIG. 3 is a graph showing the result of X-ray diffraction profile of the test piece having the composition consisting of Al bal Ni 7 Co 4 . DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The aluminum-based alloy according to the present invention can be directly produced from a melt of the alloy having any of the aforesaid compositions by single-roller melt-spinning method, twin-roller melt-spinning method, in-rotating water melt-spinning method, any of various atomizing methods, liquid quenching method such as spraying method, sputtering method, mechanical alloying method, mechanical gliding method or the like. In these methods, the cooling rate varies somewhat depending on the alloy composition but is usually 10 2 to 10 4 K/sec. The aluminum-based alloy according to the present invention can possess a structure in which quasicrystals are precipitated from a solid solution by heat treating a rapidly solidified material obtained through the above-mentioned production method or by compacting a rapidly solidified material and thermal working the compact, through extrusion or the like, at a temperature preferably ranging from 360 to 600 °C. In the production of the aluminum-based alloy according to the present invention, it is easier of control and more useful than the aforestated direct production method to adopt a method wherein a rapidly solidified material is first produced and, then, heat treated or thermally worked to precipitate quasicrystals. Now, the reason for limiting the composition of the alloy of the present invention will be described in detail. In the present invention, quasicrystals can be homogeneously dispersed in an aluminum matrix or a supersaturated solid solution of aluminum by adding at least two transition metal elements in an amount of 0.1 to 25 atomic % to aluminum as the principal element, whereby an aluminum-based alloy excellent in strength, heat resistance and specific strength can be obtained. The total volume fraction of the quasicrystals, various intermetallic compounds formed from aluminum and transition metal elements and/or various intermetallic compounds formed by transition metals ranges from 2 to 40%. In this case, the volume fraction of the quasicrystals to be precipitated preferably ranges from 0 to 20% (exclusive of 0) as in the above case. A percentage less than 2% results in failure to sufficiently enhance the hardness, strength and rigidity of the material to be produced, whereas one exceeding 40% leads to an extreme lowering of the ductility of the material to be produced, thus making it impossible to sufficiently work the material to be produced. In the present invention, the matrix composed of aluminum or the matrix composed of a supersaturated solid solution of aluminum has an average crystal grain size of 40 to 2000 nm, and the quasicrystals and various intermetallic compounds have each an average particle size of 10 to 1000 nm. An average crystal grain size of the matrix smaller than 40 nm results in an alloy that is insufficient in ductility in spite of its high strength and high hardness, whereas one exceeding 2000 nm leads to a marked decrease in the strength of the alloy to be produced, thus failing to produce an alloy having high strength. The quasicrystals and various intermetallic compounds each having an average particle size of smaller than 10 nm cannot contribute to the reinforcement of the matrix and cause a fear of embrittlement when made to form excessive solid solution in the matrix, while those each having an average particle size of larger than 1000 nm cannot maintain the strength and function as the reinforcing components because of the excessively large particle size. Now, specific aluminum-based alloys represented by each of the general formulae will be described in detail. The atomic % a, b and c are limited to 5 to 10, 0.5 to 10 and 0.1 to 5, respectively, in the general formulae because the atomic % each in the above range can give the alloy higher strength and ductility withstanding practical working even at 300 °C or higher as compared with the conventional (marketed) high-strength and heat-resistant aluminum-based alloys. The Ni element in the aluminum-based alloy as represented by each of the general formulae has a relatively low diffusibility in the Al matrix and ineffective in reinforcing the matrix and suppressing the growth of crystal grains, that is, for markedly enhancing the hardness, strength and rigidity of the alloy, stabilizing the microcrystalline phase and giving heat resistance to the alloy. The X element(s) is(are) one or two elements selected between Fe and Co, capable of forming a quasicrystal in combination with a Ni element and indispensable for enhancing the heat resistance of the alloy. The M element is at least one element selected from among Cr, Mn, Nb, Mo, Ta and W, has a low diffusibility in the Al matrix, forms various metastable or stable quasicrystals together with Al and Ni and contributes to the stabilization of the microcrystalline structure and improvement in the characteristics of the alloy at an elevated temperature. Therefore, the alloy of the present invention can be further improved in Young's modulus, strength at room temperature, strength at an elevated temperature and fatigue strength when it has the composition represented by the general formula. It is possible to control the aluminum-based alloy of the present invention with regard to crystal grain size, particle sizes of the quasicrystal and intermetallic compounds, amount of the precipitate, dispersion state or the like by selecting proper production conditions of the alloy, and thus produce the objective alloy meeting various requirements such as strength, hardness, ductility, heat resistance, etc., thereby. Furthermore, excellent properties as the superplastic working material can be given to the alloy by regulating the average crystal grain size of the matrix to be in the range of 40 to 2000 nm. The present invention will now be described in more detail with reference to the following Examples. Example 1 Each aluminum-based alloy powder having the composition specified in Table 1 was produced by a gas atomizing apparatus, packed in a metallic capsule and degassed to form a billet for extrusion. The billet thus obtained was extruded on an extruder at a temperature of 360 to 600 °C. The mechanical properties (hardness at room temperature and hardness after holding at 400 °C for one hour) of the extruded material (consolidated material) obtained under the aforesaid production conditions were examined. The results are given in Table 1. Composition (at. %) Quasicrystal (vol%) Hardness (Hv) Al Nl X M at room temp. after holding at 400° for 1 hr Example 1 bal. 10 Fe=0.5 - 2 390 411 Example 2 bal. 9 Co=1.0 - 5 370 525 Example 3 bal. 9 Fe=2.0 - 7 365 423 Example 4 bal. 8 Co=2.5 - 8 357 398 Example 5 bal. 8 Fe=4.0 - 9 360 421 Example 6 bal. 7 Co=5.0 - 10 323 509 Example 7 bal. 6 Fe=1.0, Co=1.0 - 8 413 456 Example 8 bal. 5 Fe=2.0, Co=1.5 - 7 398 365 Example 9 bal. 5 Fe=2.5, Co=0.2 - 9 387 368 Example 10 bal. 10 Fe=0.7 - 2 389 425 Example 11 bal. 9 Co=1.5 Cr=0.2 4 402 526 Example 12 bal. 8 Fe=1.8 Mn=1.0 7 378 365 Example 13 bal. 8 Co=3.0 Nb=2.0 15 435 456 Example 14 bal. 7 Fe=4.5 Mo=3.0 13 422 398 Example 15 bal. 6 Co=5.0 Ta=4.0 9 412 412 Example 16 bal. 5 Fe=0.5, Co=1.2 W=1.0 8 488 377 Example 17 bal. 8 Fe=2.2, Co=1.3 Cr=1.0, Mn=1.2 8 412 456 Example 18 bal. 7 Fe=1.2, Co=2.2 Nb=3.0 9 432 555 Example 19 bal. 6 Fe=1.3, Co=3.0 Ta=2.5 7 433 565 Example 20 bal. 5 Fe=0.3, Co=0.2 Cr=3.0 5 478 486 It can be seen from the results in Table 1 that the alloy (consolidated material) has excellent characteristics in hardness at room temperature and in a hot environment (400 °C) and also has a high specific strength because of its high strength and low specific gravity. Examinations were made on the elongations at room temperature of each alloy (consolidated material) listed in Table 1 to reveal that it had an elongation not lower than a minimum value (2%) required for usual working. Test pieces for observation under a transmission electron microscopy (TEM) were cut off from the extruded materials obtained under the above-mentioned production conditions and subjected to observation of the crystal grain size of the matrix and particle sizes of the quasicrystals and intermetallic compounds. In each of the test pieces, the aluminum matrix or the matrix of a supersaturated aluminum solid-solution had an average crystal grain size of 40 to 2000 nm and besides, the particles composed of a stable or metastable phase of the quasicrystals and the various intermetallic compounds formed from the matrix element and other alloying elements and/or the various intermetallic compounds formed from at least two other alloying elements were homogeneously dispersed in the matrix, and the intermetallic compounds had each an average grain size of 10 to 1000 nm. Also the result of observation under a TEM revealed that the precipitated quasicrystals were composed of an icosahedral phase (I-phase) alone or a mixed phase of an I-phase with a regular decagonal phase (D-phase). In addition, the volume fraction of the precipitated quasicrystals ranged from 0 to 20% (exclusive of 0) and the total volume fraction of the quasicrystals and the intermetallic compounds ranged from 2 to 40%. In particular, Al 3 Ni precipitated as an intermetallic compound in the Example. It is conceivable that in the present Example, the control of the precipitation of the quasicrystals and intermetallic compounds, crystal grain size, particle sizes of the quasicrystals and intermetallic compounds, etc., was effected by thermal working during degassing (inclusive of compacting of powder during degassing) and extrusion. Example 2 Master alloys having compositions by atomic % of (a) Al 87 Ni 8 Fe 5 , (b) Al 87 Ni 8 Co 5 , (c) Al 87 Ni 8 Fe 4 Mo 1 and (d) Al 87 Ni 8 Fe 4 W 1 , respectively, were melted in an arc melting furnace and formed into thin strips with 20 µm thickness and 1.5 mm width by a conventional single-roll liquid quenching apparatus (melt spinning apparatus) having a copper roll with 200 mm diameter at 4,000 rpm in an atmosphere of argon at 10 -3 Torr. The thin strips of alloys having respective compositions as stated above were obtained in the above way, and each of them was examined for the relationship between the hardness of the alloy and heat treatment temperature at a heat treatment time of 1 hour. The results are given in FIG. 1. As can be seen from FIG. 1, an alloy exhibiting a high hardness is obtained by the heat treatment at a high temperature (500 to 700 °C). The above-mentioned test pieces of thin strips were observed under a TEM before and after the heat treatment to reveal that the matrix of aluminum or a supersaturated solid solution of aluminum in the thin strips before the heat treatment had an average crystal grain size of smaller than 400 nm, and some intermetallic compounds having an average particle size of smaller than 10 nm were precipitated. On the other hand, the result of observation of the thin strips after the heat treatment revealed that the aluminum matrix or the matrix of a supersaturated aluminum solid solution had an average crystal grain size of 40 to 2000 nm and besides, the particles composed of a stable or metastable phase of quasicrystals and various intermetallic compounds formed from the matrix element and other alloying elements and/or various intermetallic compounds formed from at least two other alloying elements were homogeneously dispersed in the matrix, and the intermetallic compounds had each an average grain size of 10 to 1000 nm. The volume fraction of the precipitated quasicrystals in each of the samples (a) to (d) was 2% after the heat treatment at 300 °C and 10% after the heat treatment at 700 °C, that is, increased from 2% to 10% with an increase in the heat treatment temperature from 300 °C to 700 °C. However, the percentage remained constant at 10% at the heat treatment temperature exceeding 700 °C. The total volume fraction of the quasicrystals and the intermetallic compounds was 2 to 40%. It was seen from the results of observation under a TEM that the quasicrystals and the intermetallic compounds increased with an increase in the heat treatment temperature. Example 3 In a similar manner to that of Example 2, thin strips having the compositions of Al 87 Ni 8 Fe 5 and Al 87 Ni 7 Co 4 , respectively, were prepared and heat treated at 550 °C for 1 hour to prepare thin strip test pieces, which were subjected to X-ray diffraction profile. The results are given in FIG. 2 and FIG. 3, wherein the peaks as marked with O, and □ and ▿ refer to those of Al, Al 3 Ni and quasicrystal (I-phase), respectively. It can be seen from FIG. 2 and FIG. 3 that the alloy according to the present invention has a matrix composed of aluminum or a supersaturated aluminum solid solution and quasicrystals and an intermetallic composed consisting of Al 3 Ni. In a similar manner to that of Examples 1 and 2, thin strip test pieces were observed under a TEM to reveal that the aluminum matrix or the matrix of a supersaturated aluminum solid solution had an average crystal grain size of 40 to 2000 nm, the quasicrystals (I-phase) and Al 3 Ni had each an average particle size of 10 to 1000 nm, the volume fraction of the precipitated I-phase ranged from 0 to 20% (exclusive of 0) and the total volume fraction of the I-phase and Al 3 Ni ranged from 2 to 40%. As described hereinbefore, the alloy according to the present invention is excellent in hardness and strength at room temperature and at high temperature and also in heat resistance and is useful as a material having a high specific strength because of its being constituted of the elements having high strength and low specific gravity. Being excellent in heat resistance, the alloy according to the present invention can retain the characteristics obtained through the rapid solidification method, heat treatment or thermal working even when affected by the heat of working.

Claims: An aluminum-based alloy having high strength and heat resistance which comprises aluminum as the principal element and at least two transition metal elements added thereto in the range of 0.1 to 25 atomic %, said alloy having a structure in which at least quasicrystals are homogeneously dispersed in a matrix composed of aluminum or a supersaturated solid solution of aluminum wherein said quasicrystals have an average particle size of 10 to 1000 nm, said matrix has an average crystal grain size of 40 to 2000 nm and said quasicrystals are dispersed in the matrix in a volume fraction of 2 to 20 %. An aluminum-based alloy according to claim 1 wherein various intermetallic compounds formed from aluminum and said transition metal elements and/or various intermetallic compounds formed by said transition metals are further homogeneously and finely dispersed in the matrix, in a total volume fraction combined with the quasicrystals being from 2 to 40 % and the intermetallic compounds have an average particle size of 10 to 1000 nm. The alloy according to claim 1 wherein the quasicrystal is composed of an icosahedral phase (I-phase) or a mixed phase of an I-phase and a regular decagonal phase (D-phase). The alloy according to claim 1 wherein the alloy has a composition represented by the general formula: Al bal Ni a X b, wherein X is one or two elements selected between Fe and Co; a and b are, in atomic percentages, 5 ≤ a ≤ 10 and 0.5 ≤ b ≤ 10. The alloy according to claim 1 wherein the alloy has a composition represented by the general formula: Al bal Ni a X b M c wherein X is one or two elements selected between Fe and Co; M is at least one element selected from among Cr, Mn, Nb, Mo, Ta and W; a, b and c are, in atomic percentages, 5 ≤ a ≤ 10, 0.5 ≤ b ≤ 10 and 0.1 ≤ c ≤ 5. The alloy according to any of claims 1 through 4 wherein the alloy is in the form of a rapidly solidified material, a heat treated material of the rapidly solidified material, or a compacted and consolidated material formed from the rapidly solidified material. A process of producing an aluminum-based alloy according to any of the preceding claims comprising subjecting a melt of said alloy which comprises aluminum as the pricipal element and at least two transition elements added thereto in the range of 0.1 to 25 atomic % to rapid solidification at a cooling rate of 10 2 to 10 4 K/sec and heat treating the thus obtained rapidly solidified material at a temperature ranging form 360 to 700 °C.

Inventors: KITA KAZUHIKO, KITA, KAZUHIKO
Applicants: YKK CORP, YKK CORPORATION
Application year: 1993
Main classification: C22C21
IPC codes: C22C45, C22C1, C22C21