Microstructural evolution and superplastic deformation behavior of fine grain 5083Al
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I.
INTRODUCTION
STIMULATED by an ongoing interest to develop new forming methods for materials that exhibit limited formability during conventional forming, research in the field of superplasticity has increased dramatically. This manufacturing process has also become of particular interest to the automotive industry because of the potential to form complex three-dimensional components in one step. In this respect, aluminum alloys, particularly Al-Mg alloys containing Mn as dispersoid formers, may have promise because of their extensive forming capabilities, good postformed tensile properties, and possibly good corrosion resistance.[1,2] The superplastic potential of aluminum alloys is critically dependent upon grain size, grain size stability, and cavitation.[3,4] For materials to exhibit superplastic response, a fine grain size of approximately 5 to 10 mm is necessary.[4] A significant amount of grain growth during superplastic deformation can lower a material’s strain rate sensitivity of flow stress and may result in premature fracture.[5] Another microstructural damage process that greatly affects the superplastic response of these alloys is cavitation.[3] Nucleation, growth, and eventual coalescence of cavities can limit the extent of superplasticity and may have deleterious effects on the postformed properties.[4] Cavity nucleation occurs at grain boundary heterogeneities such as dispersoid particles where the initial void quickly grows to a grain diameter in size. It is widely accepted that growth of these cavities is controlled by the local stress state in the vicinity P.A. FRIEDMAN, formerly Graduate Student, Department of Materials Science and Engineering, University of Michigan, is Project Engineer, Ford Motor Company, Dearborn, MI 48121. A.K. GHOSH, Professor, is with the Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI 48109. Manuscript submitted April 19, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A
of the cavity. Failure in these alloys is usually a result of cavity coalescence where very large cavities form. It has been found in several aluminum alloys that superplastic elongations to failure in uniaxial tensile tests are enhanced when a two-step strain-rate test is performed.[6–9] That is, by first straining at a very fast strain rate, a finer more stable grain structure can be maintained that could not be achieved by deforming at a slower strain rate. Large tensile elongations can then be achieved by lowering the strain rate into the superplastic forming regime before the material experiences any significant damage accumulation. This grain refinement has been attributed to a process of continuous dynamic recrystallization during the early prestraining period where subgrain boundaries are converted to high angle boundaries. This phenomenon has been reported in several alloys including aluminum-copper[6,7] and aluminum-lithium[8,9] alloys. However, more recently, this effect of enhanced superplasticity has also been observed in alloys in which dynamic rec
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