New aspects on the superplasticity of fine-grained 7475 aluminum alloys
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INTRODUCTION
R E C E N T L Y , thermomechanical processes have been developed to produce fine grain microstructure in high-strength aluminum alloys which makes the aluminum alloys superplastic, t~-7] Previous work t7] has indicated that the superplasticity of the 7475 A1 alloy could be enhanced by a slight modification of the process by Wert et al. t~] In the previous report, tT] there was a particular feature, that is, the dispersoid-free zone (DFZ), adjacent to certain grain boundaries. After superplastic tests, several specimens revealed large zones free of dispersoid particles--occasionally, as large as 5/~m across. This fact suggests that the occurrence of DFZ may be related to the superplastic deformation mechanism of this alloy. As a result of extensive studies on microstructural superplasticity, grain boundary sliding (GBS) has been suggested as a principal mode of superplastic deformation. Since GBS is associated with high strain-rate sensitivity, it can often result in superplastic flow. The main differences among various models are the descriptions of the accommodation process at triple points of grain boundaries, t8] Grain boundary sliding cannot occur continuously on all interfaces without some accommodation in the interface or within the grains themselves. The specific micromechanism that accommodates the stresses generated at triple points due to GBS is the rate-controlling step. Two principal modes of deformation have been considered in explaining the deformation behavior of finestructure superplastic materials: GBS accommodated by DONG H. SHIN, Associate Professor, and KI S. KIM, Graduate Research Assistant, are with the Department of Metallurgy and Materials Science, Hanyang University, Ansan, Kyunggi-do, 425-791, Korea. DONG W. KUM, Senior Research Scientist, is with the Korea Institute of Science and Technology (KIST), P.O. Box 131, Cheongryang, Seoul, Korea. SO0 W. NAM, Professor, is with the Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology (KAIST), P.O. Box 150, Cheongryang, Seoul, Korea. Manuscript submitted March 5, 1990. METALLURGICALTRANSACTIONSA
diffusional flow (the Ashby-Verrall tgl model) and GBS accommodated by slip. t8] The Ashby-Verrall model of grain switching has been criticized by Spingarn and Nix [10l on the grounds that symmetry between adjacent grains is violated in the process envisioned by Ashby and Verrall. [9] Optimum strain rates for superplasticity reported in the 7000 series aluminum alloys are achieved at strain rates of about 2 • 10 -4 s-1. [2-6] Obtaining superplasticity at higher strain rates has been one of the persistent goals for many researchers, tHa2] The purposes of this paper are (1) to investigate the microstructural aspects ( e . g . , DFZ) of superplastic 7475 A1 alloy, (2) to study superplastic materials with a possibility that free-grained and coarse-grained materials may be governed by different mechanisms, and (3)to produce a fine and stable structure in 7475 A1 in such a way that optimum superplastici
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