Characteristics of the Transition from Grain-Boundary Sliding to Solute Drag Creep in Superplastic AA5083
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TRODUCTION
INCREASING the use of aluminum in automobiles can contribute to reduced mass and, thereby, improve vehicle fuel economy and performance. However, the relatively low formability of aluminum alloys in conventional stamping operations is an impediment to the expanded use of aluminum in vehicle applications.[1,2] The superplastic forming (SPF) of aluminum sheet materials offers an alternative approach, but optimum superplastic ductility generally requires deformation at the slow strain rates of the grain-boundary sliding (GBS) regime. This limitation generally precludes the use of conventional SPF techniques for the high-rate production of vehicle components.[2–5] Quick plastic forming (QPF) technology is a proprietary variation on SPF and involves the hot blow forming of aluminum alloys, such as fine-grained AA5083, at higher rates and lower temperatures than are employed in conventional SPF.[3,5] During QPF, deformation takes TERRY R. MCNELLEY, Professor, and SRINIVASAN SWAMINATHAN, NRC Postdoctoral Associate, are with the Center for Materials Science and Engineering, Department of Mechanical and Astronautical Engineering, Naval Post Graduate School, Monterey, CA 93943-5146, USA. KEIICHIRO OH-ISHI, Postdoctoral Fellow, is with the National Institute for Materials Science, Tsukuba, Ibaraki 305-0047, Japan. ALEXANDER P. ZHILYAEV, RyC Research Associate, is with the Department of Physical Metallurgy, Centro Nacional de Investigaciones Metallurgicas (CENIM), 28040 Madrid, Spain. PAUL E. KRAJEWSKI, Laboratory Group Manager, is with the Research and Development Center, General Motors Corporation, Warren, MI 48090-9056, USA. ERIC M. TALEFF, Professor, is with the Department of Mechanical Engineering, University of Texas at Austin, Austin, TX 78712-0292, USA. Contact e-mail: [email protected] Manuscript submitted May 24, 2007. Article published online December 6, 2007 50—VOLUME 39A, JANUARY 2008
place in a regime of strain rate and temperature in which both GBS and solute drag creep (SDC) contribute together.[6–8] For the deformation of Al-Mg alloys such as AA5083 in the combined GBS and SDC regime, values of the strain-rate sensitivity coefficient, m d log r=d log e_ , of 0.25 through 0.33 were reported, where r is the true flow stress and e_ is the true strain rate.[6] In general, slip creep processes involving diffusion-controlled dislocation climb in coarse-grained polycrystalline materials give m ’ 0:2[9,10] and ductility 100 pct.[11,12] Resistance to localized necking and ductility increase as m values increase, and so the strain-rate sensitivity coefficients observed in AA5083 during deformation in the combined GBS and SDC regime may facilitate QPF. A more complete understanding of GBS and solute-drag-controlled slip creep during deformation in the transition regime will facilitate the broader application of the QPF of aluminum alloys. Conventional SPF relies on the capability of fine-grained materials to undergo large, neck-free tensile elongations prior to failure during deformation at temperatures and strain
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