Patent Publication Number: US-6334911-B2

Title: High-strength, high-ductility aluminum alloy

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an aluminum alloy excellent in mechanical properties such as high-temperature strength, ductility, impact strength and tensile strength. 
     2. Description of the Prior Art 
     Aluminum alloys known hitherto include, for example, Al—Cu, Al—Si, Al—Mg, Al—Cu—Si, Al—Cu—Mg and Al—Zn—Mg alloys. They are widely used as members of aircrafts, vehicles, seacrafts, etc., exterior materials, sashes, roof materials, etc. for buildings, members of marine equipment, or members of nuclear reactors depending on the characteristic properties thereof. However, the hardness and thermal resistance of these aluminum alloys are generally insufficient. Under these circumstances, it has been attempted recently to solidify an aluminum alloy material by quenching in order to make the structure thereof fine and also to improve the mechanical properties such as strength thereof and chemical properties such as corrosion resistance (refer to Japanese Patent Laid-Open Nos. 275732/1989, 256875/1994 and 199317/1996). Although these materials are excellent in strength and thermal resistance, they still have room for improvement in ductility and formability so as to improve the practical use thereof. 
     SUMMARY OF THE INVENTION 
     The present invention has been completed after intensive investigations made under the above-described circumstances. An object of the present invention is to provide an aluminum alloy excellent in strength, hardness, ductility and formability and having a high specific strength by forming a structure comprising quasi-crystals or crystals close to them which are finely dispersed in an aluminum matrix having a specified composition. 
     The present invention provides a high-strength, high-ductility aluminum alloy having a composition represented by the general formula: 
     
       
         Al bal Cu a M b  or Al bal Cu a M b TM c   
       
     
     wherein M represents one or two elements selected between Mn and Cr; TM represents at least one element selected from the group consisting of Ti, V, Fe, Co, Ni and Zr; and a, b and c each represent an atomic percentage of 0&lt;a≦3, 2&lt;b≦5 and 0&lt;c≦2, and containing quasi-crystals in the structure thereof. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The single FIGURE is a graph showing the test results of the high-temperature strength of the alloy of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In the present invention, the quasi-crystal particles are composed of three indispensable elements of Al, Cu and M. The combination of elements Al and M is indispensable for the formation of quasi-crystals. When the amount of M is 2 atomic % or less, no quasi-crystals can be formed and the extent of strengthening is insufficient. When a combination of Mn and Cr is used as M even in a small amount, the formation of the quasi-crystal phase becomes possible by the synergistic effect of them and the quasi-crystal phase thus obtained is stable. When the amount of M exceeds 5 atomic %, the quasi-crystal particles become coarse and the volume ratio thereof becomes excess and lowers the ductility. TM as the constituent element of the quasi-crystals contributes to the strengthening and, when it is dissolved in a matrix to form a solid solution, it strengthens the matrix. Further, TM can be in the form of an intermetallic compound effective in strengthening the alloy. When the amount of TM exceeds 2 atomic %, no quasi-crystals can be formed and a coarse intermetallic compound is formed to seriously reduce the ductility. Under the conditions of b≧a and b≧a+c, the quasi-crystals can be further stabilized and the matrix and the intermetallic compound can be made in more useful forms. 
     The particles of the quasi-crystals are desirably not larger than 1 μm, more desirably not larger than 500 nm. Copper is an element which forms a solid solution in the matrix and which is precipitated to strengthen the matrix. When no copper is contained in the matrix, the strength of the matrix is insufficient. When the amount of the copper exceeds 3 atomic %, it is precipitated in the form of coarse Al 2 Cu in the matrix to reduce the ductility. 
     The quasi-crystals are in an icosahedral phase (I phase) or decagonal phase (D phase) or a crystal phase close to these crystal phases (hereinafter referred to as an “approximate crystal phase”). The structure thereof comprises the quasi-crystal phase and an aluminum phase or the quasi-crystal phase and a supersaturated solid solution phase of aluminum. If necessary, the structure may contain various intermetallic compounds formed from aluminum and other elements and/or other intermetallic compounds formed from other elements. The intermetallic compounds are particularly effective in strengthening the matrix and also in controlling the crystal particles. 
     The amount of the quasi-crystals contained in the alloy structure is preferably 20 to 80% by volume. When it is below 20% by volume, the object of the present invention cannot be perfectly attained and, on the contrary, when it exceeds 80% by volume, the embrittlement of the alloy might be caused to make the sufficient processing of the obtained material impossible. The amount of the quasi-crystals contained in the alloy structure is still preferably 50 to 80% by volume. The average particle size in the aluminum phase or the phase of the supersaturated solid solution of aluminum is preferably 40 to 2,000 nm in the present invention. When the average particle size is below 40 nm, the obtained alloy will have an insufficient ductility though it has a high strength and a high hardness. When it exceeds 2,000 nm, the strength is sharply lowered to make the production of the high-strength alloy impossible. 
     The aluminum alloy of the present invention can be directly obtained from the molten alloy having the above-described composition by the single-roller melt-spinning method, twin-roller melt-spinning method, in-rotating-water melt-spinning method, various atomizing methods, liquid-quenching method such as spray method, sputtering method, mechanical alloying method or mechanical gliding method. In these methods, the cooling rate which varies to some extent depending on the composition of the alloy is about 10 2  to 10 4  K/sec. The aluminum alloy of the present invention precipitate the quasi-crystals from the solid solution by heat-treating the material rapidly solidified by the above-described method or by consolidating the rapidly-solidified material and subjecting it to thermal processing such as compaction or extrusion. The temperature in this step is preferably 320 to 500° C. 
     The elongation of the alloy obtained by the present invention is at least 10% and the Young&#39;s modulus thereof is at least 85 GPa. 
     The following Examples will further illustrate the present invention. 
     An aluminum alloy powder having a composition shown in the left column in Table 1 was prepared with a gas atomizer. The aluminum alloy powder thus obtained was fed into a metal capsule and then -degassed to obtain a billet to be extruded. The billet was extruded with an extruder at a temperature of 320 to 500° C. 
     The strength, elongation, modulus of elasticity (Young&#39;s modulus) and hardness of the extruded material (consolidated material) obtained under the above-described production conditions were determined at room temperature. Further, as for Samples Nos. 15 and 17, the Charpy impact values thereof were also determined. The results are given in the right columns in Table 1. 
     
       
         
           
               
               
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                   
                   
                   
                 Young&#39;s 
                   
               
               
                   
                   
                 Strength 
                 Elongation 
                 Charpy 
                 modulus 
                 Hardness 
               
               
                   
                 Alloy (at %) 
                 (MPa) 
                 (%) 
                 (J/cm 2 ) 
                 (GPa) 
                 (Hv) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 1 
                 Al bal Mn 5 Cu 2   
                 658 
                 10 
                   
                 87 
                 188 
               
               
                 2 
                 Al bal Mn 4 Cu 3   
                 675 
                 11 
                   
                 86 
                 191 
               
               
                 3 
                 Al bal Mn 4 Cu 2 Co 1   
                 690 
                 12 
                   
                 92 
                 195 
               
               
                 4 
                 Al bal Mn 5 Cu 1   
                 574 
                 11 
                   
                 88 
                 168 
               
               
                 5 
                 Al bal Mn 4 Cu 1   
                 551 
                 20 
                   
                 88 
                 161 
               
               
                 6 
                 Al bal Mn 3 Cu 2   
                 566 
                 20 
                   
                 87 
                 166 
               
               
                 7 
                 Al bal Cr 4 Cu 1   
                 567 
                 18 
                   
                 88 
                 160 
               
               
                 8 
                 Al bal Cr 1 Mn 3 Cu 1   
                 505 
                 16 
                   
                 85 
                 140 
               
               
                 9 
                 Al bal Cr 1 Mn 3 Cu 3   
                 571 
                 14 
                   
                 92 
                 164 
               
               
                 10 
                 Al bal Cr 1 Mn 3 Cu 2 Ti 1   
                 600 
                 12 
                   
                 92 
                 175 
               
               
                 12 
                 Al bal Cr 1 Mn 2 Cu 2 V 1   
                 560 
                 15 
                   
                 90 
                 161 
               
               
                 13 
                 Al bal Cr 1 Mn 2 Cu 3   
                 500 
                 21 
                   
                 90 
                 147 
               
               
                 14 
                 Al bal Cr 1 Mn 2 Cu 1 Co 2   
                 570 
                 15 
                   
                 93 
                 175 
               
               
                 15 
                 Al bal Cr 1 Mn 2 Cu 2   
                 520 
                 20 
                 16 
                 88 
                 145 
               
               
                 16 
                 Al bal Cr 1 Mn 2 Cu 1.5 Zr 0.5   
                 572 
                 16 
                   
                 91 
                 165 
               
               
                 17 
                 Al bal Cr 1 Mn 3 Cu 1   
                 515 
                 18 
                 8.8 
                 88 
                 147 
               
               
                 18 
                 Al bal Cr 1 Mn 2 Cu 1 Fe 1   
                 560 
                 14 
                   
                 90 
                 163 
               
               
                 19 
                 Al bal Mn 3 Cu 1 Ni 1   
                 545 
                 15 
                   
                 87 
                 159 
               
               
                 20 
                 Al bal Cr 1 Mn 3 Cu 1 Ni 1   
                 558 
                 12 
                   
                 86 
                 163 
               
               
                 21 
                 Al bal Cr 1 Mn 2 Cu 1 Ni 2   
                 553 
                 14 
                   
                 89 
                 162 
               
               
                 22 
                 Al bal Cr 2 Mn 1 Cu 1 Co 1   
                 543 
                 16 
                   
                 89 
                 154 
               
               
                   
               
            
           
         
       
     
     The results given in Table 1 indicate that the alloys (consolidated materials) of the present invention are excellent in strength, elongation, modulus of elasticity (Young&#39;s modulus), hardness, etc. at room temperature, and in particular, they have an elongation of as high as at least 10% and a modulus of elasticity (Young&#39;s modulus) of as high as 85 GPa. It was apparent that although the properties of each alloy were changed by heating in the step of preparing the consolidated material, the properties were still excellent. 
     The extruded material obtained under the above-described production conditions was cut to obtain test pieces for the TEM observation, and the structure of the alloy and the particle sizes in the respective phases were examined. The results of the TEM observation indicated that the quasi-crystals comprised the icosahedral phase alone or a mixture of the icosahedral phase and the decagonal phase. An approximant crystal phase thereof was recognized depending on the kind of the alloy. The amount of the quasi-crystals in the structure was 20 to 80% by volume. 
     The alloy structure comprised a mixture of an aluminum phase and the quasi-crystal phase or a supersaturated solid solution phase of aluminum and the quasi-crystal phase. Particularly in an alloy containing the TM elements, such a structure further comprised a phase of various intermetallic compounds (intermetallic compound phase of aluminum and TM elements). The average particle size in the aluminum phase or supersaturated solid solution phase of aluminum was 40 to 2,000 nm, and that in the quasi-crystal phase was 10 to 1,000 nm and mostly not larger than 500 nm. When the alloy contained the intermetallic compound phase, the average particle size thereof was 10 to 1,000 nm. In a composition wherein the intermetallic compound phase was precipitated, the intermetallic compound phase was homogeneously and finely dispersed in the alloy structure. Supposedly, the control of the alloy structure, particle sizes in each phase, etc. was effected by the degassing (including compaction in the degassing step) and the heat processing in the extrusion step. 
     The high-temperature strength of Al 95 C 1 Mn 2 Cu 2  alloy (No. 15 in Table 1) was determined. The high-temperature strength was determined at a predetermined temperature (373 K, 473 K, 573 K or 673K) after keeping the sample at that temperature for one hour. The results are shown in the figure. It is apparent from the figure that the high-temperature strength of the alloy of the present invention was as high as 423 MPa at 373 K, 307 MPa at 473 K and 183 MPa at 573 K, while that of Extra Super Duralumin (7075) which is a commercially available high-strength aluminum alloy was 397 MPa at 373 K, 245 MPa at 473 K and 83 MPa at 573 K. The strength is particularly high at 473 K (200° C.) and 573 K (300° C.). 
     As described above, the alloy of the present invention is excellent in strength, elongation, modulus of elasticity (Young&#39;s modulus), hardness, etc. at room temperature, and in particular, it has an elongation of as high as at least 10% and a modulus of elasticity (Young&#39;s modulus) of as high as at least 85 GPa. Although the properties of the alloy are changed by heating in the step of preparing the consolidated material, the properties are still excellent.