The effect of grain size and temperature on the superplastic deformation behavior of a 7075 Al alloy
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I. INTRODUCTION
MOST of the studies on the superplastic deformation behavior have, so far, attempted to analyze the superplastic flow behavior based on a single power-law relation between the flow stress (s) and the strain rate («˙ ), with the strainrate sensitivity (m) as its power index.[1–4] The strain-rate sensitivity parameter has been widely used as a characteristic parameter of superplasticity, requiring m to have a large value, although there exists no clearly defined critical value of m for structural superplasticity (SSP).[5] Furthermore, there have been some unresolved issues in the superplasticity community, such as the existence and origin of the so-called threshold stress and the so-called region I in flow curves,[6,7,8] as well as the more fundamental issues regarding the major accommodation mechanism for grain-boundary sliding (GBS),[3,4] which is generally accepted to be the major physical process of SSP.[8,9] A new internal-variable theory has recently been proposed to address these unresolved issues and, hence, to provide a more comprehensive understanding of superplastic deformation behavior.[9] The theory is based on the concept of internal variables, contrary to the external-variable approach of conventional studies, assuming the dislocation glide processes within and across the grain boundaries to be the major accommodation mechanism for GBS. Hence, it is possible to separate the overall flow curves into two parts, viz., a GBS curve and a grain-matrix deformation (GMD) curve, depending on their corresponding physical processes, in contrast to the single power-law relation used in most of the YONG NAM KWON, Graduate Student, Department of Materials Science and Engineering, and YOUNG WON CHANG, Professor, Center for Advanced Aerospace Materials, are with the Pohang University of Science and Technology, Pohang 790-784, Korea. Manuscript submitted July 29, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
conventional studies. It is believed possible to extract more valuable information from this approach, such as the effects of grain size and test temperature on the flow behavior, which have not been possible to obtain from the conventional approaches. The present experimental study has been planned and carried out precisely within the aforementioned context to examine the role of grain size and test temperature on the flow behavior of a 7075 Al alloy. Load relaxation tests were then carried out to obtain flow curves for the alloy, with various grain sizes, at temperatures ranging from 445 8C to 515 8C. These overall flow curves were then separated into the GMD curve and the GBS curve, as prescribed by the theory and described in detail in Section IV. By analyzing these individual curves, the precise role of grain size and test temperature on the GMD and GBS processes was quantitatively identified for the first time. Finally, the results obtained from the analysis of each curve are discussed in Section V in relation to the overall flow behavior and the critical issues of SSP mentioned previousl
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