HIGH-STRENGTH, HIGH-TOUGHNESS ALUMINUM ALLOY AND PROCESS FOR PREPARING THE SAME

Abstract: An aluminum alloy which is industrially producible and has both higher strength and toughness than the prior art and a preparation method thereof are provided. The high-strength high-toughness aluminum alloy comprises a phase of α-aluminum consisting of crystal grains whose mean crystal grain size is within the range of 60 to 1000 nm and a phase of at least two types of intermetallic compounds consisting of crystal grains whose mean crystal grain sizes are within the range of 20 to 2000 nm, and the crystal grains of the intermetallic compounds are so dispersed that linkage between the crystal grains of the intermetallic compounds is intermittent.

Description: Technical Field The present invention relates to an aluminum alloy, which is applicable to a part or a structural material to which toughness is required, has high strength, and is excellent in toughness, and a method of preparing the same. Background Art As to aluminum alloys of high strength with start materials of alloys containing amorphous phases or quasi-crystal phases, many studies have heretofore been made. According to the technique disclosed in Japanese Patent Laying-Open No. 1-275732, for example, an amorphous substance or a complex of amorphous and microcrystalline substances having tensile strength of 853 to 1009 MPa (87 to 103 kg/mm 2 ) and yield strength of 804 to 941 MPa (82 to 96 kg/mm 2 ) is obtained by rapidly solidifying a ternary alloy consisting of a general formula: Al a M b X c (where M: at least one or two metallic elements selected from V, Cr, Mn, Fe, Co, Ni, Cu, Zr, Ti, Mo, W, Ca, Li, Mg and Si, X: at least one or two metallic elements selected from Y, La, Ce, Sm, Nd, Hf, Nb, Ta and Mm (misch metal), and a, b and c are a: 50 to 95 at.%, b: 0.5 to 35 at.% and c: 0.5 to 25 at.% in atomic %). As to an amorphous or microcrystalline high-strength aluminum alloy of low specific gravity and high strength, it is disclosed in Japanese Patent Laying-Open No. 6-316738. The aluminum alloy is expressed in a general formula: Al a X b Mm c (Mm: misch metal), where X is at least one or two elements selected from Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zr, a, b and c are atomic %, a: 95.2 to 97.5 at.%, and b and c are values satisfying 2.5 < b + c < 5 and b > 0.5 and c > 1. Due to having such a composition, obtained is an aluminum alloy of low specific gravity and high strength in which an amorphous phase or a microcrystal phase is properly homogeneously dispersed in a microcrystal phase of a matrix while suppressing the amount of addition of alloy elements and the microcrystal phase of the matrix is solution-strengthened with Mm and the transition metal such as Ti, V, Cr, Mn, Fe, Co, Ni, Cu or Zr. As hereinabove described, an amorphous alloy or an alloy consisting of a complex of amorphous and microcrystalline substances, or a microcrystalline alloy having a matrix of Al has tensile strength of at least twice as compared with a conventional aluminum crystalline alloy. However, the Charpy impact value of the aforementioned aluminum alloy is so low that it does not satisfy about 1/5 as compared with a conventional aluminum ingot material. Thus, there has been such a problem that it is difficult to use the aluminum alloy as the material for a mechanical part or an automobile part to which reliability is required. In Japanese Patent Laying-Open No. 6-184712, on the other hand, a method of preparing a high-strength aluminum alloy is disclosed. The aluminum alloy is expressed in a general formula: Al a Ln b M c , where Ln in the formula is at least one metallic element selected from Mm (misch metal), Y, La, Ce, Sm, Nd, Hf, Nb and Ta, M is at least one metallic element selected from V, Cr, Mn, Fe, Co, Ni, Cu, Zr, Ti, Mo, W, Ca, Li, Mg and Si, and a, b and c are atomic % in the ranges of a: 50 to 97.5 at.%, b: 0.5 to 30 at.% and c: 0.5 to 30 at.%. A preparation method of performing plastic working on a rapidly solidified aluminum alloy having such a composition and having such a cellular diploid structure that an amorphous phase of 5 to 50 volume % encloses a microcrystal phase at a temperature exceeding the amorphous crystallization temperature and obtaining such a structure that an intermetallic compound consisting of at least two of the aforementioned Al, Ln and M is dispersed in a microcrystal matrix is disclosed in the aforementioned gazette. In such an aluminum alloy, relatively high toughness is obtained such that the tensile strength is 760 to 890 MPa and elongation is 6.0 to 9.0 %. In the preparation method of the aluminum alloy disclosed in the aforementioned gazette, however, it requires a high cooling rate at the time of rapid solidification for obtaining the amorphous phase of 5 to 50 volume %, and hence there is such a problem that the preparation cost increases in actual industrial production. In Japanese Patent Laying-Open No. 7-179974, further, an aluminum alloy comprising high strength and high toughness is disclosed. The aluminum alloy is characterized in that, in a dispersion-strengthened aluminum alloy having a complex structure including a matrix of α-aluminum and a precipitation phase of an intermetallic compound with a volume ratio of not more than 35 volume % of the intermetallic compound, the aspect ratio of the precipitation phase of the intermetallic compound is not more than 3.0, the ratio of the crystal grain size of the α-aluminum to the grain size of the precipitation phase of the intermetallic compound is at least 2.0, and the crystal grain size of the α-aluminum is not more than 200 nm. In the aforementioned gazette, further, it is disclosed that the aluminum alloy having the aforementioned limited structure is obtained by performing a first heating treatment and a second heating treatment on gas-atomized powder containing an amorphous phase by at least 10 volume % or a green compact thereof and thereafter performing hot plastic working. Also in the preparation method of the aluminum alloy disclosed in the aforementioned gazette, it still requires a high cooling rate at the time of rapid solidification for obtaining the amorphous phase of 10 volume %, and hence there is such a problem that the preparation cost therefor increases in actual industrial production. The problems of the aforementioned conventional techniques are summarized as in the following Table 1. Alloy Structure Problem Japanese Patent Laying-Open No. 1-275732 amorphous substance or complex of amorphous and microcrystalline substances low toughness Japanese Patent Laying-Open No. 6-316738 microcrystal or microcrystal with amorphous substance dispersed therein low toughness Japanese Patent Laying-Open No. 6-184712 microcrystal with intermetallic compound dispersed therein requirement for high quenching degree Japanese Patent Laying-Open No. 7-179974 microcrystal with intermetallic compound dispersed therein requirement for high quenching degree Accordingly, an object of the present invention is to solve the aforementioned problems and provide an industrially producible aluminum alloy having both strength and toughness higher than general ones and a method of preparing the same. Disclosure of the Invention In order to overcome the aforementioned problems, the inventors of this application have made thoroughgoing evaluation and study as to submicron level microstructures of aluminum alloys and mechanical properties thereof. On that occasion, they have regarded the aluminum alloys as composite materials of α-aluminum crystals and intermetallic compounds of Al-added elements, and evaluated the same as grain dispersion-strengthened composite materials by returning to the relations between the material structures and the mechanical properties thereof. Consequently, the following matters have been proved: Assume that consideration is given to a grain dispersion-strengthened composite material consisting of a matrix of a ductile material and grains of a brittle material. It is assumed that the aspect ratio of the grains of the brittle material is close to 1 on that occasion. When the grains of the brittle material are gradually added to random positions from the state of the matrix of the ductile material of 100 %, the spaces between the grains of the brittle material which have been separately present at first gradually narrow, so that clusters in which a plurality of grains of the brittle material are linked with each other occur in places. Further, when the grains of the brittle material are so increased that the volume ratio thereof exceeds 30 to 40 %, the grains of the brittle material are linked with each other throughout the sample. If the volume ratio of the grains of the brittle material is less than 30 %, the toughness of the composite material is at a degree loosely reducing following increase of the brittle material grains. When the volume ratio of the grains of the brittle material exceeds 30 to 40 %, however, the toughness remarkably reduces. When the aspect ratio of the grains of the ductile material is sufficiently larger than 1 and the grains of the brittle material exist on random positions toward random directions, for example, the grains of the brittle material are linked with each other throughout the sample even in places where the volume ratio of the grains of the brittle material is lower than 30 %, and the critical volume ratio of toughness reduction reduces. Even if the volume ratio of the grains of the brittle material is higher than 40 % to the contrary, it can happen that the linkage between the grains of the brittle material does not range over the sample and the toughness may be maintained when the grains of the brittle material take regular arrangement. As hereinabove described, the toughness of the grain dispersion-strengthened composite material is not evenly regulated by only the volume ratio of the strengthening grains (the grains of the brittle material here) as having been considered in general, but to be regulated by the linkage between the strengthening grains. When such recognition is applied to an aluminum alloy of an Al-TM-Ln (TM: transition metallic element, Ln: rare earth element) system or the like, an α-aluminum crystal can be regarded as the matrix of the ductile material, crystal grains of an intermetallic compound or fine amorphous regions can be regarded as the grains of the brittle material, and the aforementioned relation as to the volume ratio of the grains of the brittle material can be applied. When the aforementioned recognition is thus applied, it is necessary that the crystal grains of the intermetallic compound are not linked with each other throughout the sample, in order to obtain sufficient toughness. On the basis of the aforementioned recognition, the high-strength high-toughness aluminum alloy according to the present invention is defined by the appended claims. The limitation reasons for the mean crystal grain size of the α-aluminum and the mean crystal grain sizes of the intermetallic compounds are described below. If the mean crystal grain size of the α-aluminum is less than 60 nm, it requires a high cooling rate in preparation of the aluminum alloy and the preparation cost increases. If the mean crystal grain size of the α-aluminum is larger than 1000 nm, on the other hand, strengthening by refinement of the crystal grains does not effectively act but the strength reduces on the contrary. For such reasons, the range of the mean crystal grain size of the α-aluminum is limited. If the mean crystal grain sizes of the intermetallic compounds are less than 20 nm, it requires a high cooling rate in preparation of the aluminum alloy, and the preparation cost increases. If the mean crystal grain sizes of the intermetallic compounds are larger than 2000 nm, on the other hand, composition strengthening action between the same and the matrix does not effectively act but the strength reduces on the contrary. The range of the mean crystal grain sizes of the intermetallic compounds is limited for such a reason. The aluminum alloy of the present invention is characterized in that it contains a first intermetallic compound consisting of crystal grains whose crystal grain sizes are 20 to 900 nm in the interior of the crystal grains of the α-aluminum, and at least one type of second intermetallic compound of a type different from the first intermetallic compound, consisting of crystal grains whose crystal grain sizes are 400 to 2000 nm, is dispersed along the crystal grain boundary of the α-aluminum, in addition to the aforementioned characteristics. As hereinabove described, it is possible to suppress grain growth of the α-aluminum crystal under a high temperature for improving heat resistance by the geometrical configuration of the first and second intermetallic compounds, i.e., at least two types of intermetallic compounds. In the aluminum alloy of the present invention, further, the first intermetallic compound existing in the interior of the crystal grains of the α-aluminum contains Al and Zr, and the second intermetallic compound distributed along the crystal grain boundary of the α-aluminum contains Al and Z (Z is at least one metallic element selected from the group consisting of Y, La, Ce, Sm, Nd and Mm (misch metal)). The first intermetallic compound existing in the α-aluminum crystal grains thus contains Al and Zr, whereby the heat resistance can be improved due to the fact that diffusion of Zr in the aluminum matrix is slow. Due to the fact that the second intermetallic compound distributed along the α-aluminum crystal grain boundary contains Al and Z (Z is at least one metallic element selected from the group consisting of Y, La, Ce, Sm, Nd and Mm (misch metal)), further, the dispersiveness of the second intermetallic compound in the crystal grain boundary improves so that the toughness of the aluminum alloy can be improved. Preferably, the first intermetallic compound existing in the α-aluminum crystal grains has an L1 2 type or D0 23 type crystal structure. Due to the fact that the first intermetallic compound is of the L1 2 type, matching of the grating with the α-aluminum crystal improves and the heat resistance can be improved. If the first intermetallic compound is of the DO 23 type, on the other hand, an intermetallic compound excellent in stability of the crystal structure can be obtained. Further preferably, the shape of the second intermetallic compound distributed along the α-aluminum crystal grain boundary has a limited shape as described below, on a ground section of the aluminum alloy of the present invention: It is preferable that the mean value of the peripheral length of the second intermetallic compound is 7 to 15 µm, the mean value of the roundness of the second intermetallic compound is 0.15 to 0.45, the mean value of the acicular ratio of the second intermetallic compound is 1 to 5, the standard deviation of the second intermetallic compound in the major axis direction is at least 40°, and the volume ratio of the second intermetallic compound is 12 to 25 %. The second intermetallic compound can effectively exhibit a grain boundary pinning effect for the α-aluminum crystal for heat resistance improvement with no linkage by distributing the grains of the second intermetallic compound having the shape thus limited along the α-aluminum crystal grain boundary. In the aforementioned limitation related to the shape of the intermetallic compound, the roundness is defined as 4 x π x (sectional area of intermetallic compound)/ (peripheral length of section of intermetallic compound) 2 . The acicular ratio is defined as a2/a1 (absolute maximum length of section of intermetallic compound)/(distance between two straight lines at the time of holding outer periphery of section of intermetallic compound with two straight lines parallel to straight line extending along the absolute maximum length a2) on a section of an intermetallic compound shown in Fig. 1. Further, the standard deviation of the intermetallic compound in the major axis direction is expressed in dispersion of an angle  formed by an X-axis and the direction of the major axis of an intermetallic compound grain expressed by a dotted line on a section of the intermetallic compound shown in Fig. 2, i.e., the standard deviation. The composition of the aluminum alloy of the present invention is expressed in a general formula: Al a Zr b X c Z d . Here, X is at least one metallic element selected from the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni and Cu, Z is at least one metallic element selected from the group consisting of Y, La, Ce, Sm, Nd and Mm (misch metal), a, b, c and d are atomic %, a is within the range of 90 to 97 at.%, b is within the range of 0.5 to 4 at.%, and c and d are atomic % within the range enclosed with points A, B, C and D in Fig. 3. Fig. 3 takes the atomic % of the metallic element X on the horizontal axis and the atomic % of the metallic element Z on the vertical axis, the coordinates are expressed in sets of the atomic % of the metallic element X and the atomic % of the metallic element Z, the coordinates of the point A are (0.1, 4), the coordinates of the point B are (0.1, 1), the coordinates of the point C are (2.5, 1), and the coordinates of the point D are (1.5, 3). The values of the atomic % of c and d have values within a region of a slant part enclosed with the points A, B, C and D shown in Fig. 3. The reasons why the roles of the elements added to the aluminum alloy and the contents thereof are limited as described above are now described. Al forms a homogeneous and fine structure as an α-aluminum crystal, and contributes to improvement of the strength due to a crystal grain refinement effect. Zr becomes a crystal nucleus of α-aluminum crystallization as Al 3 Zr in rapid solidification. Homogeneous fine dispersion of α-aluminum crystal grains becomes possible by homogeneous dispersion of this crystal nucleus in a sample. It is necessary that the content of Zr is in the range of 0.5 to 4 atomic %. The effect of becoming a crystal nucleus is not sufficient if the content of Zr is less than 0.5 atomic %. If the content of Zr is larger than 4 atomic %, on the other hand, the volume ratio of Al 3 Zr as an intermetallic compound becomes too large and the toughness reduces. The content of Zr is limited for such reasons. X (at least one metallic element selected from the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni and Cu) increases the viscosity of an alloy melt, and increases the number density of the crystal nucleus of α-aluminum crystallization. The effect of increasing the number density of the crystal nucleus is not sufficient if the content of the metallic element X is less than 0.1 atomic %. If the content of the metallic element X is larger than 2.5 atomic %, on the other hand, the volume ratio of Al-X as an intermetallic compound becomes too large and the toughness reduces. The range of the content of the metallic element X is limited for such reasons. Z (at least one metallic element selected from Y, La, Ce, Sm, Nd and Mm (misch metal)) increases the viscosity of the alloy melt, and increases the number density of the crystal nucleus for α-aluminum crystallization. Further, the metallic element Z is dispersed and precipitated along the grain boundary of the α-aluminum crystal grains in crystallization as the intermetallic compound with Al, and contributes to strength improvement by dispersion strengthening. If the content of the metallic element Z is less than 1 atomic %, the effect of increasing the number density of the crystal nucleus is not sufficient. If the content of the metallic element Z is larger than 4 atomic %, on the other hand, the volume ratio of Al-X as the intermetallic compound becomes too large and the toughness reduces. The range of the content of the metallic element Z is limited for such reasons. The aluminum alloy of the present invention can be obtained by rapidly solidifying a melt of an alloy consisting of at least two types of added elements which are strong in affinity for Al and mutually weak in affinity for each other and Al by a liquid quenching method and performing a heat treatment thereon at need. It is particularly preferable that the cooling rate in this case is 10 3 to 10 5 K/sec. According to a method of preparing an aluminum alloy according to the present invention, further, a high-strength high-toughness aluminum alloy limited as described above is obtained by heat-treating a rapidly solidified aluminum alloy having such a cellular diploid structure that an intermetallic compound phase having Al as one of its elements, which is different from a crystal nucleus, encloses an α-aluminum microcrystal phase with the crystal nucleus of an intermetallic compound having Al as one of its elements to a temperature of at least 593 K at a temperature rising rate of at least 1.5 K/sec. The method thus employs the aforementioned rapidly solidified crystalline aluminum alloy as the starting material, whereby the starting material can be prepared at a lower cooling rate as compared with the prior art. Further, the intermetallic compound distributed along the α-aluminum crystal grain boundary, which has been linked in the stage of the starting material, is not linked by heat-treating this starting material to the temperature of at least 593 K at the temperature rising rate of at least 1.5 K/sec., and high toughness can be obtained as a result. If the heat treatment at this time is performed at a temperature of less than 593 K, linkage of the intermetallic compound distributed along the α-aluminum crystal grain boundary cannot be cut. If the heat treatment is performed at a temperature rising rate of less than 1.5 K/sec., on the other hand, the α-aluminum crystal grains become coarse and the strength of the obtained alloy reduces as a result. It is preferable that the rapid solidification at the time of preparing the aforementioned aluminum alloy as the starting material is performed by a gas atomizing method or a liquid atomizing method. Further, it is preferable to perform hot plastic working after the aforementioned heat treatment. In this case, it is preferable that the hot plastic working is performed by powder forging. According to the present invention, as hereinabove described, it is possible to obtain an aluminum alloy having both high strength and toughness at a low cost by an industrially producible method. Brief Description of the Drawings Fig. 1 is a diagram, employed for defining the acicular ratio of an intermetallic compound distributed along an α-aluminum crystal grain boundary in a preferable aluminum alloy according to the present invention, typically showing a section of the intermetallic compound. Fig. 2 is a diagram, employed for defining the standard deviation of the intermetallic compound distributed along the α-aluminum grain boundary in the preferable aluminum alloy according to the present invention in the direction of the major axis, typically showing a section of the intermetallic compound. Fig. 3 is a diagram showing the composition range of metallic elements X and Z in the preferable aluminum alloy according to the present invention. Best Mode for Carrying Out the Invention Example A Aluminum alloys having alloy compositions shown in Table 2 were worked into ingots by arc melting, and thereafter these ingots were worked into ribbon-like samples with a single-roll type liquid quencher. In Table 2, the compositions of the respective alloys are shown in values of atomic % of the contained elements, and "Al-bal" indicates that the balance is aluminum. Preparation of the ribbon-like samples was performed by setting a quartz nozzle comprising pores of 0.5 mm in diameter on its forward end on a position of 0.5 mm immediately above a copper roll rotating at 2000 rpm, high-frequency melting the ingot aluminum alloys introduced into the quartz nozzle and injecting melts of the aluminum alloys under injection pressure of 78 kPa for making ribboning. Observing the structure of the ribbon-like sample thus obtained as to each Example, it was confirmed that the same has such a cellular diploid structure that an intermetallic compound phase having Al as one of its elements, which is different from a crystal nucleus, encloses an α-aluminum crystal phase with the crystal nucleus of an intermetallic compound having Al as one of its elements. Further, these ribbons were heat-treated under conditions in Table 2. In Table 2, e.g. "773K30sec" means that it was heat-treated at the temperature of 773 K for 30 seconds. The temperature rising rate was at least 1.5 K/sec. in each heat treatment. In order to confirm the cooling rate at the time of ribboning, a ribbon of a 2014 Al alloy composition was prepared under similar preparation conditions, and the actual cooling rate was estimated by measuring the dendrite arm space in its structure. According to this, the cooling rate was 3 x 10 4 K/sec. The microstructures were observed with a scanning electron microscope (SEM) of high resolution as to the obtained ribbons of respective Examples and respective comparative examples. According to the results of the observation, it was observed that intermetallic compounds (IMC) were finely dispersed without being linked with each other in Examples, as shown in Table 2. On the other hand, it was observed that intermetallic compounds were linked with each other in comparative examples. Further, a tensile test was performed with an Instron tensile tester by employing the ribbons obtained in respective Examples and respective comparative examples. The results thereof are also shown in Table 2. UTS indicates the values of tensile strength. It is understood that each one of Examples has both high tensile strength and high elongation as compared with comparative examples. Example B Aluminum alloy powder materials having alloy compositions shown in Table 3 were prepared with a gas atomizer. Atomization was performed by pressurizing nitrogen gas to 10 kgf/cm 2 and colliding the same against melts of the aluminum alloys dropped from a nozzle whose hole diameter was 2 mm. Observing the structure of the aluminum alloy powder thus obtained, it was confirmed that the same has such a cellular diploid structure that an intermetallic compound phase having Al as one of its elements, which is different from the aforementioned crystal nucleus, encloses an α-aluminum crystal phase with the crystal nucleus of an intermetallic compound having Al as one of its elements, similarly to Example A. Powder of the 2014 Al alloy composition was prepared under atomization conditions similar to the above, and the actual cooling rate was estimated from measurement of the dendrite arm space in its structure. According to this, the cooling rate was 2 x 10 4 K/sec. when aluminum alloy powder whose grain size is 65 µm was obtained. Then, each aluminum alloy powder prepared as described above was sieved to less than 65 µm, the treated powder was press-molded, thereafter a heating and degassing treatment was performed, and powder forging was performed at a temperature in the range of 593 to 873 K. Ultimate temperatures and temperature rising rates of heating conditions for the respective press-molded bodies are shown in Table 3. The microstructures of the aluminum alloys of respective Examples and respective comparative examples thus obtained were observed with an SEM of high resolution similarly to Example A. According to this, it was observed that intermetallic compounds (IMC) were finely dispersed without being linked with each other in each of Examples. In comparative examples, on the other hand, it was observed that intermetallic compounds were linked with each other. Further, sections of the respective powder-forged bodies were mirror-ground and microstructural photographs were taken with an SEM of high resolution with 50,000 magnifications. Thereafter the respective photographs were loaded into a personal computer, for performing image analysis by the computer. The shapes of second intermetallic compounds distributed along α-aluminum crystal grain boundaries were measured by this analysis. Data related to the shapes of the intermetallic compounds shown in Table 4 show mean values of data measured in three fields. In Table 4, direction standard deviation shows the standard deviation in the direction of the major axes of the intermetallic compounds. The intermetallic compounds and α-aluminum are different in contrast on the microstructural photographs from each other, whereby it was possible to perform measurement of the shapes of the intermetallic compounds by making the computer recognize only the second intermetallic compounds distributed on the α-aluminum crystal grain boundaries. As to the volume ratio of the intermetallic compound, it comes to that the area ratio on a section is equal to the volume ratio as such, assuming that spatial distribution of the intermetallic compound is completely isotropic. Data obtained by calculating area ratios and regarding the values as the volume ratios are shown in Table 4 here. It is understood that the data related to the shape of the intermetallic compound prepared in the aforementioned manner is within the range defined in the present invention in each Example. Further, a tensile test was performed with an Instron tensile tester similarly to Example A, for measuring tensile strength (UTS) and elongation of each powder-forged body. The Charpy impact value of each powder-forged body was also measured. These results are also shown in Table 4. As obvious from these data related to the mechanical properties too, it is understood that the powder-forged bodies according to Examples have both high tensile strength and elongation as compared with those of comparative examples, and Charpy impact values thereof are also high. Powder-Forged Body Mean Peripheral Length (µm) Mean Roundness Mean Acicular Ratio Direction Standard Deviation Volume Ratio (%) UTS (MPa) Elongation (%) Charpy Impact Value (J) Example 31 11 0.31 1.8 45 18 810 5.1 11 Example 32 12 0.33 1.6 48 17 760 4.8 10 Example 33 11 0.3 1.8 46 22 820 4.6 10 Example 34 13 0.25 1.7 43 19 700 5.9 9 Example 35 11 0.35 1.8 45 18 770 6.2 10 Example 36 10 0.32 1.8 48 23 730 4.5 7 Example 37 12 0.22 1.7 47 18 730 5.2 8 Example 38 12 0.35 1.7 43 24 800 4.5 10 Example 39 11 0.31 1.9 48 17 740 4.7 10 Example 40 10 0.31 1.9 46 23 760 4.4 10 Comparative Example 41 18 0.11 2.2 42 19 740 0.2 1 Comparative Example 42 20 0.1 2.3 41 36 790 0.3 2 Comparative Example 43 20 0.1 2.3 40 37 720 0.5 1 Comparative Example 44 25 0.08 2.5 41 38 700 0.6 1 Examples disclosed above must be considered as being not restrictive but illustrative in all points. The scope of the present invention is decided not by the aforementioned Examples but by the appended claims. Industrial Applicability As hereinabove described, the aluminum alloy according to the present invention is suitable for employment for a part or a structural material to which toughness is required. Further, the aluminum alloy of the present invention is at a low cost, and industrially producible.

Claims: A high-strength, high-toughness aluminium alloy having a composition expressed as a general formula of: Al a Zr b X c Z d where X is at least one metallic element selected from Ti, V, Cr, Mn, Fe, Co, Ni and Cu, Z is at least one metallic element selected from Y, La, Ce, Sm, Nd and Mm (misch metal), a is within the range of 90 to 97 atomic %, b is within the range of 0.5 to 4 atomic %, and c and d are expressed in atomic % within a range enclosed by points A (0.1, 4), B (0.1, 1), C (2.5, 1) and D (1.5, 3) shown in Figure 3; said alloy comprising a phase of α-aluminium consisting of crystal grains whose mean crystal grain size is within the range of 60 to 1000 nm and a phase of at least two types of intermetallic compounds consisting of crystal grains whose mean crystal grain sizes are within the range of 20 to 2000 nm, the crystal grains of said intermetallic compounds being so dispersed that linkage between the crystal grains of said intermetallic compounds is intermittent; and wherein said phase of at least two types of intermetallic compounds contains a first intermetallic compound whose crystal grain sizes are 20 to 900 nm in the interior of the crystal grains of said α-aluminium, and at least one type of a second intermetallic compound whose crystal grains sizes are 400 to 2000 nm, said at least one type of a second intermetallic compound being different from said first intermetallic compound and being distributed along the crystal grain boundary of said α-aluminium. An aluminium alloy in accordance with claim 1, wherein said first intermetallic compound contains Al and Zr, and said second intermetallic compound contains Al and Z (Z is at least one metallic element selected from Y, La, Ce, Sm, Nd and Mm (misch metal)). An aluminium alloy in accordance with claim 2, wherein said first intermetallic compound has an L1 2 type or D0 23 type crystal structure. An aluminium alloy in accordance with claim 3, wherein the mean value of the peripheral length of said second intermetallic compound is 7 to 15 µm, the mean value of the roundness of said second intermetallic compound is 0.15 to 0.45, the mean value of the acicular ratio of said second intermetallic compound is 1 to 5, the standard deviation of said second intermetallic compound in the major axis direction is at least 40° and the volume ratio of said second intermetallic compound is 12 to 25%, on a ground section of said aluminium alloy, said roundness is defined as 4 x π x (sectional area of intermetallic compound)/(peripheral length of section of intermetallic compound) 2 , and said acicular ratio is defined as (absolute maximum length of section of intermetallic compound)/(distance between two straight lines at the time of holding outer periphery of section of intermetallic compound with two straight lines parallel to a straight line extending along the absolute maximum length). A method of preparing a high-strength, high-toughness aluminium alloy as defined in any one of claims 1 to 5 by heat-treating a rapidly solidified aluminium alloy having such a cellular diploid structure that an intermetallic compound phase having Al as one of its elements, being different from a crystal nucleus, encloses an α-aluminium crystal phase having an intermetallic compound having Al as one of its elements as said crystal nucleus to a temperature of at least 593 K at a temperature rising rate of at least 1.5 K/sec. A method in accordance with claim 5, wherein said rapid solidification is performed by a gas atomizing method or a liquid atomizing method, and hot plastic working is performed after said heat treatment. A method in accordance with claim 6, wherein said hot plastic working is powder forging.

Inventors: HASHIKURA MANABU, HATTORI HISAO, KAJI TOSHIHIKO, TAKANO YOSHISHIGE, HASHIKURA, MANABU, HATTORI, HISAO, KAJI, TOSHIHIKO, TAKANO, YOSHISHIGE, HASHIKURA, Manabu, Itami Works,, HATTORI, Hisao, Itami Works,, Kaji, Toshihiko, Itami Works,, TAKANO, Yoshishige, Itami Works,
Applicants: JAPAN SCIENCE & TECH CORP, SUMITOMO ELECTRIC INDUSTRIES, JAPAN SCIENCE AND TECHNOLOGY CORPORATION, SUMITOMO ELECTRIC INDUSTRIES, LTD
Application year: 1997
Main classification: C22C21
IPC codes: C22C21, C22F1, C22C45, C22C33, C22C1, C22C38