Superplastic deformation behavior in commercial and high purity Zn- 22 Pct Al
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I.
INTRODUCTION
MICROGRAINsuperplasticity refers to the ability of finegrained materials (d -< 10/zm, where d is the grain size) to exhibit, upon deformation at elevated temperatures (T -> 0.5 TIn, where T m is the melting point), extremely large elongations of several hundreds of percent. Over the past two decades, the mechanical behavior of micrograin superplastic alloys has been extensively studied. tj'2,3] As a result of these studies, .two findings are well documented. First, micrograin superplasticity is a diffusion-controlled process that can be represented by the following dimensionless equation: t4J g/kT = A DGb
[11
where g/is the steady-state strain rate, k is Boltzmann's constant, T is the absolute temperature, D is the appropriate diffusion coefficient, G is the shear modulus, b is the Burgers vector, A is a dimensionless constant, s is the grain size sensitivity, "r is the applied stress, and n is the stress exponent. Second, the relationship between stress and strain rate in micrograin superplastic alloys, IS-l~ especially the Zn-22 pct A1 eutectoid and the Pb-62 pct Sn eutectic, is often sigmoidal, with a high stress exponent at low and high stresses (regions I and III, respectively) and a low stress exponent at intermediate stresses (region II or the superplastic region) where the superplastic alloys exhibit maximum ductility, t~2,~3j
PRABIR K. CHAUDHURY, Graduate Research Assistant, and FARGHALLI A. MOHAMED, Professor, are with the Department of Mechanical Engineering, University of California, Irvine, CA 92717. V. SIVARAMAKRISHNAN, formerly Graduate Student, Department of Mechanical Engineering, University of California, Irvine, is with The Arizona State University, Tempe, AZ. Manuscript submitted January 4, 1988. METALLURGICALTRANSACTIONS A
Considerable attentionI5-~31has been focused on the creep behavior of superplastic alloys in region I (low stresses), and experimental data obtained in this region show several distinct characteristics. These include (a) a stress exponent of 3 to 5, (b) an activation energy higher than the activation energy for grain boundary diffusion, Qgb, that characterizes region II, and (c) a decrease in ductility. In addition to these characteristics, it was demonstrated that region I represents a true region of flow and is not a consequence of the occurrence of concurrent grain grOWth; [14'15] a well-defined region I, with high values of the stress exponent and the activation energy for creep, was observed in creep experiments involving negligible grain growth. [9"l~ In earlier analyses of superplastic flow, the presence of region I at low stresses was attributed to (a) the operation of threshold stress processes ttr'~7'~sl that are insensitive to temperature, t~9'2~ or (b) the emergence of a new deformation mechanism [5- 7921922] such as pure grain boundary sliding controlled by barriers inherent in the grain boundary structure. However, these two explanations are not consistent with the increase in the activation energy in region I; for example, the deformation
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