Effect of Fe on the Superplastic Deformation of Zn-22 pct AI PRABIR K. CHAUDHURY, KYUNG-TAE PARK, and FARGHALLI A. MOHAMED In this investigation, the creep behavior of three grades of the superplastic Zn-22 pct A1 eutectoid was studied under identical conditions of grain size, temperature, and stress. These three grades were prepared from high-purity AI and Zn using the same procedure, but they have different Fe levels: grades A, B, and C contain 423, 120, and 40 ppm of Fe, respectively. The experi- mental results show that the creep behavior of the three grades exhibits a sigmoidal relationship between stress and strain rate, which is manifested by the presence of three regions: region I (low-stress region), region II (intermediate-stress region), and region III (high-stress region). In region II, the creep characteristics, including the stress exponent, n, and the activation energy for superplastic flow, Q, are insensitive to Fe level; n --- 2.5 and Q = Qsb, where Qgb is the activation energy for grain boundary diffusion. By contrast, the emergence of region I along with its stress exponent and activation energy is affected by Fe level; the higher the Fe level, the higher the stress exponent and the activation energy. The effect of Fe on region I behavior in Zn-22 pct A1 is attributed to a threshold stress for creep, r0, whose origin is related to Fe segregation at boundaries. An examination of the estimated values of threshold stresses in the three grades along with a comparison between these values and those reported earlier for two grades of Zn-22 pct AI containing 180 ppm and 100 ppm of impurities (120 and 50 ppm of Fe, respectively) reveals two findings. First, the threshold stress appears to approach a limiting value for Fe concentrations above 120 ppm with increasing Fe level. Second, for approximately the same Fe concentration, the presence of other impurities in Zn-22 pct AI leads to a higher value of %. These findings are discussed in terms of characteristics associated with grain boundary segregation (saturation and synergistic effects). I. INTRODUCTION THE ability of fine-grained materials (d < 10 /~m, where d is the grain size) to exhibit extensive plastic de- formation, often without the formation of a neck prior to fracture, is generally known as micrograin (structural) superplasticity. Micrograin superplastic behavior is in- dicated 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 super- plasticity is a diffusion-controlled process that can be de- scribed by a normalized equation of the following f o r m : I11 (;)(5 DGb - A [ 1 a] with D = Do exp (-Q/RT) [lb] 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, 796- 600, 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. constant, T is the absolute temperature, D is the diffu- sion 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 dif- fusion 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 sig- moidal 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 im- purity level in the alloy is reduced to about 6 ppm, and (b) the stress exponent, n, and the apparent activation energy for creep, Qa, in region I, unlike those in region II, are sensitive to impurity content. An analysis of the above results t2,31 on Zn-22 pct A1 has suggested that the origin of region I is most likely related to strong impurity segregation at boundaries. METALLURGICALAND MATERIALS TRANSACTIONSA VOLUME 25A, NOVEMBER 1994--2391