The evolution of grain boundary character during superplastic deformation of an Al-6 pct Cu-0.4 pct Zr alloy
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SUPERPLASTICITY is the capacity of a material to exhibit large, neck-free tensile elongation under appropriate deformation conditions. This phenomenon was first reported more than 60 years ago and has been observed in many metallic, intermetallic, and ceramics systems as well as amorphous materials.[1] Superplastic flow by grain boundary sliding in polycrystaline materials requires highly refined grains and grain boundaries that resist tensile separation during elevated temperature deformation. However, grain refinement alone is not sufficient for superplasticity. Phenomena such as cavity formation at dispersed constituent particles or at other sites in the microstructure may limit ductility despite a fine grain size, and this has complicated the development of microstructure-property relationships for superplastic behavior.[2,3,4] In recent years, superplastic forming has experienced steady growth as a manufacturing process, although the mechanisms of microstructural transformation that enable superplastic response and govern superplastic ductility are not fully understood. There are two distinct thermomechanical processing (TMP) routes to achieve grain refinement for superplasticity in wrought aluminum-based materials. One of these routes involves overaging treatments to produce coarse precipitate particles, which become sites for particle-stimulated nucleation (PSN) of recrystallization after cold working.[5] In turn, this leads to a discontinuous, or primary, recrystallization reaction, which involves the formation of high-angle boundaries within the deformation zones that develop around the particles during straining, followed by migration of these M. EDDAHBI, Postdoctoral Fellow, and O.A. RUANO, Professor, are with the Department of Physical Metallurgy, Centro Nacional de Investigaciones Metalurgicas (CENIM), CSIC, 28040 Madrid, Spain. T.R. McNELLEY, Professor, is with the Department of Mechanical Engineering, Naval Postgraduate School, Monterey, CA 93943-5146. Manuscript submitted July 19, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
boundaries into the surrounding matrix. The material may exhibit superplastic flow by grain boundary sliding from the onset of deformation when such processing results in a finegrained, fully recrystallized initial state.[6] A recent investigation[7] into the elevated temperature mechanical behavior of a superplastic 5083 aluminum alloy has shown that asprocessed material had random grain orientations and grainto-grain disorientations that are consistent with PSN. The elevated temperature mechanical behavior was also consistent with predictions of accepted models for independent, additive contributions to the total deformation rate by slip creep and grain boundary sliding.[8] The other TMP route involves hot and cold working of as-cast material under conditions designed to retain a fine dispersion of second-phase particles.[9] The dispersed particles are intended to inhibit the migration of high-angle boundaries (HABs) during subsequent annealing treatments or elevated t
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