Enhanced Ductility in Coarse-Grained AI-Mg Alloys ERIC M. TALEFF, DONALD R. LESUER, and JEFFREY WADSWORTH Enhanced ductilities, i.e., values of tensile ductility exceeding those normally expected in metallic alloys, have been observed at warm temperatures in coarse-grained A1-Mg alloys which exhibit viscous-glide controlled creep. Numerous tests have been conducted in order to quantify this phe- nomenon over wide ranges of temperature and magnesium concentration. The contributions of strain-rate sensitivity and strain hardening have been analyzed in relation to the observed tensile ductilities. It is shown that an analysis based only on flow instability in tension cannot be used to predict failure in a unique manner. L INTRODUCTION ENHANCED tensile ductilities, of up to 300 pct, have been observed previously at warm temperatures in several coarse-grained, solid solution alloys of aluminum contain- ing magnesiumY~] Such high tensile elongations are es- pecially remarkable, because none of these materials exhibit the significant grain-boundary sliding behavior that is typ- ical of most fine-grained, traditional superplastic metallic alloys. The capability of coarse-grained materials to exhibit high tensile ductilities is of great interest from the view- point of creating materials which do not require expensive processing to achieve high formabilities and are therefore economical to fabricate. High formability is obviously im- portant in forming operations, because it permits a cost- effective approach to the manufacturing of complex components through a reduction of required stamping, ma- chining, and joining operations. The enhanced tensile ductilities observed in these coarse- grained AI-Mg alloys are evidently not the result of a clas- sic superplastic deformation mechanism but rather the result of a solute-drag controlled creep mechanism,tg,~~ Alloys which exhibit such solute-drag or viscous-glide controlled creep behavior, referred to here as Class I alloys after the original definition by Sherby and Burke, tt~] exhibit an in- herent high strain-rate sensitivity value of m ~- 0.33. By contrast, pure aluminum and aluminum alloys with a low magnesium content do not exhibit high tensile ductilities at warm temperatures; instead, they exhibit either pure metal or Class II alloy behavior with a low strain-rate sensitivity of m ~ 0.2.[H] Increasing values of m generally produce increases in tensile ductility for many materials,t12] By contrasting the tensile and torsional behavior of several aluminum alloys with differing magnesium concentrations, McQueen and Kassner showed that the enhanced tensile ductility observed in the Class I AI-Mg alloys was primarily a result of the high strain-rate sensitivity values of these alloys,tg] This is Table L Alloy Compositions in Weight Percent Alloy Mg Mn Si Fe Cu Zn Ti I 1.02 0.50 0.028 0.022 0.001 0.015 0.001 II 2.52 0.46 0.034 0.022 0.002 0.017 0.001 III 4.05 0.46 0.039 0.023 0.002 0.024 0.001 IV 5.51 0.47 0.043 0.023 0.002 0.021 0.001 V 6.64 0.48 0.043 0.023 0.002 0.022 0.001 in contrast with the case for torsional ductility, in which recovery processes, especially dynamic and geometric-dy- namic recrystallization, were controlling and pure alumi- num exhibited higher ductility than its alloys with magne- sium. The unusual tensile elongations (up to 300 pet) observed in Class I AI-Mg alloys have been the origin of confusion in the literature in that this behavior has been assumed to be classical superplasticity, but in anomalously coarse- grained alloys.t6] Whereas superplastic materials (m > 0.5) can be expected to show higher elongations than Class I alloys, the grain-boundary sliding mechanism which yields superplasticity requires a very fine, stable grain size to be developed, often necessitating expensive processing and relatively complex two-phase or multiphase alloys, both to create fine grain sizes and to inhibit subsequent grain growth. By contrast, Class I alloys require no such special processing, because the high value of m = 0.3 is an inher- ent result of the solute-drag creep mechanism which has no observed grain size dependence.till For this reason, utilizing enhanced ductility in Class I alloys for commercial forming operations is economically attractive. McQueen and Kassner defined the characteristics of ma- terials exhibiting extended tensile ductility which distin- guish them from classical superplastic materials.[9] The two primary characteristics of enhanced tensile ductility are re- defined here as follows: ERIC M. TALEFF, Assistant Professor, is with the Department of Aerospace Engineering and Engineering Mechanics, The University of Texas at Austin, Austin, TX 78712-1085. DONALD R. LESUER, Group Leader/Materials Engineer, Manufacturing and Materials Engineering Division, and JEFFREY WADSWORTH, Associate Director, Chemistry and Materials Science Directorate, are with the Lawrence Livermore National Laboratory, Livermore, CA 94550. Manuscript submitted October 25, 1994. (1) intermediate tensile elongations of 100 to 300 pct that result from a strain-rate sensitivity of approximately 0.33, which is a result of a viscous-glide controlled creep process; and (2) grain sizes that are greater than about 20 ~m, reducing the likelihood of the occurrence of a grain-boundary sliding creep mechanism as a major contributing factor to deformation. U.S. GOVERNMENT WORK METALLURGICAL AND MATERIALSTRANSACTIONS A VOLUME 27A, FEBRUARY 1996--343 NOT PROTECTED BY U.S. COPYRIGHT