Effect of Fe on the superplastic deformation of Zn-22 pct Al

Abstract

In this investigation, the creep behavior of three grades of the superplastic Zn-22 pct Al eutectoid was studied under identical conditions of grain size, temperature, and stress. These three grades were prepared from high-purity Al 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 experimental 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 andQ =Qgb, whereQgb 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 Al is attributed to a threshold stress for creep, τ0, 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 Al 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 Al leads to a higher value of τ0. These findings are discussed in terms of characteristics associated with grain boundary segregation (saturation and synergistic effects).