Influence of nonsteady state behavior on superplastic deformation of a 25.7 Cr-6.6 Ni stainless steel
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INTRODUCTION
INVESTIGATION of superplastic deformation has been of much scientific and technological interest in recent times. This has led to a large number of experimental results, and several theoretical models have been suggested to understand the mechanism of deformation.~ Most of the models proposed for superplastic deformation are based on the assumption of steady state. During steady state deformation the initial equiaxed microstructure is considered to be stable, and the flow stress is reported to be a unique function of strain rate, temperature, and grain size. Unlike conventional plastic deformation, the flow stress is believed to be independent of strain (e) during superplastic deformation. Experimental results are presumed to support the assumptions of stable grain size and strain insensitive flow stress during superplastic deformation. The parameters of the deformation mechanisms as deduced from experimental ~-k data are then compared with the predicted values from theoretical models. Thus, experimental results are used to assess the deformation models by comparing the stress (~r)strain rate (e) behavior in these two cases. However, in me as-worked state, many superplastic materials2 exhibit elongated grains. These elongated grains are found to be metastable during static annealing with a sluggish kinetics for the process of evolution toward equiaxed structures. The evolution toward equiaxed grains is significantly enhanced during deformation, but this occurs concurrently with substantial grain growth. These two kinds of microstructural instability have not been adequately explored, although their combined effect on flow behavior has received some attention. 2 The dominance of one kind of microstructural instability on flow behavior in preference to that of the other kind is suggested in some investigations. 3"4 Microstructural investigation over wide ranges of strain rates and temperatures suggests that grain growth occurs in preference to evolution toward equiaxed grains at lower strain rates and higher temperatures. As a consequence of this, strain hardening is noticed for such a combination of strain rate and temperature. Evolution toward equiaxed B.P. KASHYAP is a Postdoctoral Fellow and A.K. MUKHERJEE is Professor of Materials Science and Engineering. Both are with the Division of Materials Science, Department of Mechanical Engineering, University of California, Davis, CA 95616. Manuscript submitted December 20, 1982. METALLURGICALTRANSACTIONS A
grains is a dominating phenomenon at higher strain rates and lower temperatures. The evolution toward equiaxed grains under such combination of strain rate and temperature shows strain softening. The influence of microstructural instability on flow behavior gradually changes from strain hardening at strain rate-temperature combinations which have inherently lower flow stress for deformation to strain softening at strain rate-temperature combinations which have inherently higher flow stress for deformation. Under intermediate combination of strain r
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