High-strain-rate superplasticity in bulk cryomilled ultra-fine-grained 5083 Al
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TRA-FINE-GRAINED (UFG) materials refer to materials with an average grain size, d, in the 100- to 500-nm range. The UFG materials have been the subject of considerable interest for two reasons: (1) as a result of grain refinement, UFG materials exhibit superior properties over their coarse-grained counterparts; and (2) techniques for the fabrication of bulk UFG materials have been successfully developed.[1–4] In this article, experimental data obtained on the creep behavior of bulk UFG 5083 Al are reported and analyzed. The alloy in this case was processed by gas atomization followed by cryomilling, hot isostatic pressing (‘‘hipping’’), and extrusion. The investigation was motivated by several considerations. First, despite considerable research over the past decade on the mechanical behavior of UFG metals and alloys prepared by severe plastic deformation such as equal-channel angular pressing (ECAP),[5,6] the creep behavior of Al alloys processed by a cryomilling route is not studied in detail. Tensile deformation behavior in cryomilled Al-4 pct Mg has been studied[7] in the temperature range 573 to 644 K and at stresses in the range of 27.5 to 58.5 MPa. However, the results were preliminary in nature and provided very limited information on deformation processes in cryomilled Al alloys. Second, UFG alloys have grains that fall between the macro (.1 mm) and the nano (,100 nm) size ranges. Accordingly, studying the creep behavior of UFG 5083 Al can provide guiding information regarding rate-controlling processes in the transition region between these two ranges. MANISH CHAUHAN and INDRANIL ROY, Graduate Research Assistants, and FARGHALLI A. MOHAMED, Professor, are with the Department of Chemical Engineering and Materials Science, University of California, Irvine, CA 92697-2575. Contact e-mail: [email protected] Manuscript submitted November 2, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS A
Third, well-documented information on the creep behavior of metallic systems shows that alloys having a fine, stable, and equiaxed grain size (less than 10 mm) would exhibit micrograin superplasticity,[8,9] which is indicated by large tensile elongations (usually greater than 300 pct and sometimes in excess of 2000 pct). This behavior has been plausibly explained in terms of dislocation-accommodated grain boundary sliding.[10,11,12] In addition, as the grain size of the alloy is reduced to the submicron grain range, the optimum strain rate for superplastic deformation (104– 103 s1) is expected to shift to faster strain rates (102– 101 s1), leading to high-strain-rate (HSR) superplasticity. This expectation has been verified in metal matrix composites, mechanically alloyed materials prepared by powder metallurgy,[13] and Al-base alloys after ECAP.[14] At present, there are no data that show whether the characteristics of the creep behavior of UFG 5083 Al prepared by cryomilling are consistent with those associated with superplastic flow. Finally, the preparation of UFG 5083 Al by a cryomilling route leads to the introduction of o
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