Superplastic behavior of thermomechanically treated P/M 7091 aluminum alloy
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INTRODUCTION
SUPERPLASTIC forming is attractive for production of complex components of aluminum alloys for aerospace applications, lt'2'3J A fine stable grain size of less than 10/~m is usually required for this purpose. Standard high-strength aluminum alloys (e.g., 7075 and 7475) have been made superplastic through application of intermediate thermomechanical treatments (ITMTs) to produce a fine grain size by discontinuous recrystallization, t4-7] To bring about the necessary grain refinement, the alloys are overaged following the solution treatment to produce a fine and uniform dispersion of precipitate particles of about 1.0 /xm in size, ta'9'l~ which subsequently act as nucleation sites for the new grains. The overaged alloy is heavily worked at a relatively low temperature and then recrystallized by heating it to a temperature near the solvus. Such processing results in a recrystallized grain size of about 10 /zm. The other class of thermomechanical treatments (TMTs) is capable of reducing the grain size of certain aluminum alloys during the t'n'st stages of high-temperature deformation, tn.121 In this process, warm rolling produces a complex, partially polygonized dislocation structure. The precipitate and solute distributions resist discontinuous recrystallization prior to the onset of deformation at high temperatures. Instead, the dislocation cell structure recovers continuously to form a network of subgrain boundaries that is stabilized by the particle dispersion. High-temperature deformation is necessary to complete the continuous recrystallization process. In an earlier investigation tt31 on the elevated temperature ductility of rapidly solidified 7091 P / M aluminum alloy, it was observed that, despite its starting fine grain H.N. AZARI, Postdoctoral Scientist, and G.S. MURTY and G.S. UPADHYAYA, Professors, are with the Department of Materials and Metallurgical Engineering, Indian Institute of Technology, Kanpur 208016, India. Manuscript submitted April 21, 1993. METALLURGICAL AND MATERIALS TRANSACTIONS A
size of the order of 3.0/zm, the alloy exhibited a maximum strain-rate sensitivity index m of 0.3 and maximum tensile ductility of 190 pct only. This was attributed to the partially recrystallized micmstructure of the alloy and the oxide particles at the grain boundaries limiting the grain-boundary sliding during elevated temperature deformation, t~3.~41In an attempt to completely recrystallize the microstructure while maintaining the fine grain size, the alloy was subjected to various ITMT schedules reported elsewhere./~5] It was observed that one of the thermomechanical schedules using an overaging treatment of 603 K/36 hours led to the formation of large ,/-phase (MgZn2) precipitate particles in the size range of 0.7 to 2.4 /zm (average size of 1.32 /zm). On discontinuous recrystallization subsequent to warm rolling, the alloy thus treated exhibited a completely recrystallized microstructure with grains in the size range of 0.5 to 10 /zm. However, due to slow heating rates of the order of 10
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