Effect of Fe on the superplastic deformation of Zn-22 pct Al
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I.
INTRODUCTION
THE ability of fine-grained materials (d < 10 /~m, where d is the grain size) to exhibit extensive plastic deformation, often without the formation of a neck prior to fracture, is generally known as micrograin (structural) superplasticity. Micrograin superplastic behavior is indicated in tension tests by large elongations, usually greater than 300 pct and sometimes in excess of 2000 pct, at small stresses and high temperatures above 0.5 TIn, where Tm is the melting point. It has been demonstrated that micrograin superplasticity is a diffusion-controlled process that can be described by a normalized equation of the following f o r m : I11
(;)(5
[ 1a]
D = Do exp (-Q/RT)
[lb]
DGb - A with
where ~, is the shear creep rate, k is the Boltzmann's PRABIR K. CHAUDHURY, formerly Graduate Research Assistant, Materials Section, Department of Mechanical and Aerospace Engineering, University of California, is Manager of Forming Department, Concurrent Technologies Corp., Johnstown, PA 15904. KYUNG-TAE PARK, formerly Research Associate, Materials Section, Department of Mechanical and Aerospace Engineering, University of California, is Researcher, Research Institute of Industrial Science & Technology (RIST), Pohang, 796600, Korea. FARGHALLI A. MOHAMED, Professor, is with the Materials Section, Department of Mechanical and Aerospace Engineering, University of California, Irvine, CA 92717. Manuscript submitted October 18, 1993. METALLURGICALAND MATERIALS TRANSACTIONSA
constant, T is the absolute temperature, D is the diffusion coefficient that characterizes the creep process, G is the shear modulus, b is the Burgers vector, A is a dimensionless constant, d is the grain size, s is the grain size sensitivity, r is the applied shear stress, n is the stress exponent, Q is the activation energy for the diffusion process that controls the creep behavior, and Do is the frequency factor for diffusion. The relationship between stress, z, and strain rate, ~/, in superplastic alloys is often sigmoidal. This sigmoidal behavior is manifested by the presence of three regions: region I (low-stress region), region II (intermediate-stress region), and region III (high-stress region). The division of the behavior into three regions is based on the value of the stress exponent, n (n = 1/m, where m is the strain rate sensitivity); in both regions I and III, the values of the stress exponent, n, are higher than that in region II, where maximum ductility occurs (the superplastic region). Very recent creep investigations t2,31on the superplastic Zn-22 pct AI eutectoid have revealed new insight into the origin of the sigmoidal relation between stress and strain rate at low stresses (region I). According to the results of these investigations, the emergence of region I at low stresses and its creep characteristics are controlled by the purity level of the alloy. This finding has been demonstrated by two main experimental observations: (a) Zn-22 pct AI does not exhibit region I when the impurity level in the alloy is reduced to about
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