Superplastic flow and cavitation in Zn-22 pct Al doped with Cu
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I.
INTRODUCTION
Superplasticity refers to the ability of fine-grained materials (d , 10 mm, where d is the grain size) to exhibit extensive neck-free elongations during deformation at elevated temperatures (T . 0.5Tm, where Tm is the melting point). An important characteristic of the deformation behavior of micrograin superplastic alloys is the experimental observation that the relationship between stress, t, and steadystate creep rate, gz , is often sigmoidal. Under creep conditions, such a sigmoidal behavior is characterized by the presence of three regions:[1–7] region I (the low-stress region), region II (the intermediate-stress region or the superplastic region), and region III (the high-stress region). Of these three regions, regions I and II have been the subject of many studies[8,9] that have aimed not only at establishing the deformation characteristics of both regions, but also at providing interpretations of these characteristics in terms of deformation mechanisms. Region II, where maximum ductility occurs, covers several orders of magnitude of strain rate and is characterized by a stress exponent, n 5 (] ln gz /] ln t)T,d, of 1.5 to 2.5, an apparent activation energy, Qa 5 2R[] ln gz /] ln (1/T)]t,d, that is close to that for boundary diffusion, and a grain-size sensitivity, s 5 (] ln gz /] ln d)t,T, of about 2 (R is the gas constant and T is the absolute temperature). These creep characteristics appear to be consistent with the predictions AHMADALI YOUSEFIANI, Graduate Research Assistant, and FARGHALLI A. MOHAMED, Professor, are with the Department of Chemical and Biochemical Engineering and Materials Science, University of California, Irvine, CA 92697-2575. Manuscript submitted November 10, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
of models that are based on boundary sliding accommodated by some form of dislocation motion.[10–15] Region I is characterized by[1–7,16–18] a stress exponent of 3 to 5, an apparent activation energy higher than that for grain-boundary diffusion, and a decrease in both ductility and contribution of boundary sliding to total strain. Early explanations for the origin of region I were centered around the following: (1) the operation of temperature-insensitive threshold stress processes,[13,19] (2) the emergence of a new deformation mechanism,[1–3,12] or (3) the occurrence of concurrent grain growth.[20,21] However, as concluded elsewhere,[22,23] these explanations are not entirely consistent with available experimental evidence. Recently, it has been demonstrated[22–30] that both deformation and cavitation behavior in region I are influenced by the purity level of the alloy. This finding is reflected in three primary observations: (1) Zn-22 pct Al[22,23] and Pb-62 pct Sn[24] do not exhibit region I when the level of impurities in both alloys is reduced to about 6 ppm (throughout this work ppm will refer to wt ppm, unless otherwise stated); (2) no cavitation is observed in high-purity Zn-22 pct Al[25]; and (3) increasing the impurity level at a constant initial strain
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