The influence of thermomechanical processing variables on superplasticity in a High-Mg, Al-Mg alloy
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INTRODUCTION
SUPERPLASTICITY in aluminum alloys has been the subject of numerous research, development, and application efforts in the past twenty years. Early research concentrated on eutectic or eutectoid alloys, 1'2'3 and the principal concerns of such studies were the mechanisms of superplastic deformation p e r se. More recent work has been directed toward existing alloys4'5 and also toward novel alloys6'7 with useful ambient temperature properties as well as elevated temperature superplasticity. Development of thermomechanical processing (TMP) methods to produce refined structures in such alloys4-7 has been central to attaining superplastic behavior in them. Such processing may be expected to influence various microstructure sensitive properties. Processing by TMP methods incorporating warm rolling has been shown to result in a particulate dispersion of intermetallic/3 (MgsA18) precipitates in high-Mg, A1-Mg alloys8 with strengthening as well as enhanced resistance to stress corrosion susceptibility as a result. Also, superplastic behavior results in these A1-Mg alloys at relatively low temperatures and high strain rates 9 when processed by warm roiling. Waldman et al. lo have shown that 7075 and other highstrength aluminum alloys can be processed to possess a fine, recrystallized grain structure by an appropriate TMP. Subsequently, Paton et al. 4.5 applied a similar TMP method and processed 7075 and 7475 aluminum alloys. These materials exhibited superplastic elongations of 500 pct at 793 K (520 ~ and a strain rate of 2 • 10 -4 S-1. Subsequent efforts by Wert II have extended this work by providing interpretation of the mechanisms of formation of the fine, recrystallized grain structures developed by the TMP. A dispersion of micron-sized precipitates is produced by E.-W. LEE, formerly with Materials Group, Mechanical Engineering Department, Naval Postgraduate School, Monterey, CA, is with the Naval Air Development Center, Warminster, PA 18974. T.R. McNELLEY is Associate Professor, Materials Group, Mechanical Engineering Department, Naval Postgraduate School, Monterey, CA 93943-5100. A. E STENGEL, formerly Graduate Student in Mechanical Engineering, Naval Postgraduate School, Monterey, CA,, is with the United States Navy. Manuscript submitted July 11, 1985. METALLURGICALTRANSACTIONS A
overaging of the alloy in .an initial step; subsequently, warm working is done to provide stored energy which is then released in recrystallization upon reheating to a temperature near the solvus for the strengthening phase. The precipitates formed in the initial overaging serve as nucleation sites, and the resultant grain size may be 5 /xm or less. A different approach to obtain superplasticity has been reported by McNelley, Lee, and Mills9 and applied to an Al-10.2 pct Mg-0.5 pct Mn alloy. The distinct feature of this method is development of a fine subgrain structure by a TMP which concludes with warm rolling at 573 K (300 ~ Here, the refined subgrain structure is stabilized by precipitation of/3 (MgsAIs) concurrently w
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