Failure mechanisms in superplastic AA5083 materials
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I. INTRODUCTION
COMMERCIAL aluminum alloy 5083 is the most commonly used material for superplastic forming (SPF) operations and is of particular importance to the transportation sector.[1,2] The recent commercial implementation of a proprietary version of SPF, designated “quick-plastic forming” (QPF), has brought SPF technology into large-scale, mass production for the automotive industry by lowering forming temperatures and shortening forming times compared to traditional SPF operations.[3] The deformation mechanism generally associated with superplasticity in fine-grained AA5083, such as commercially available superplastic-grade AA5083 materials, is grain-boundary-sliding (GBS) creep.[4–16] For the QPF process, however, temperatures are lower and strain rates are faster than in traditional SPF operations. It has been shown that the dominant deformation mechanism shifts toward solute-drag (SD) creep under these conditions,[12,17–19] as is observed in other commercial 5000-series alloys at elevated temperatures and slow strain rates.[20,21] The purpose of the present investigation is to examine the effects of this transition in deformation mechanisms on the tensile failure of commercial AA5083 materials. Several investigators have studied the failure of AA5083 materials under conditions for which deformation is domiMARY-ANNE KULAS, Graduate Student, is with the Materials Science and Engineering Program, The University of Texas at Austin, Austin, TX 787120292. W. PAUL GREEN, Engineer, is with Bell Helicopter Textron, Fort Worth, TX 76101. ERIC M. TALEFF, Associate Professor, is with the Materials Science and Engineering Program and the Department of Mechanical Engineering, The University of Texas at Austin. Contact e-mail: [email protected] PAUL E. KRAJEWSKI, Laboratory Group Manager, is with the Research and Development Center, General Motors Corp., Warren, MI 48090-9056. TERRY R. McNELLEY, Professor, is with the Department of Mechanical Engineering, Naval Postgraduate School, Monterey, CA 93943-5146. Manuscript submitted March 2, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS A
nated by GBS creep. These include studies of failure under uniaxial tension[12,22–24] and under more complex loading situations.[25,26,27] These investigations reveal that failure is controlled by cavity nucleation, growth, and coalescence, leading finally to specimen rupture, when deformation is controlled by GBS creep under conditions typical of SPF operations. A variety of mechanisms have been proposed for cavity nucleation,[22,28,29] and a recent study supports the idea that cavities form preferentially under GBS creep in AA5083 at high-angle grain boundaries.[30] Cavitation damage limits the useful forming strains in SPF and QPF processes, even when there is no material rupture; cavity concentrations in excess of 2 vol pct typically degrade mechanical properties beyond acceptable performance requirements.[27] There is currently a lack of data on the failure of AA5083 materials deformed under conditions favorable for SD creep, wh
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