EFFECT OF FE ON THE SUPERPLASTIC DEFORMATION OF ZN-22 PCT AL

In this investigation, the creep behavior of three grades of the super plastic Zn-22 pet Al eutectoid was studied under identical conditions of grain size, temperature, and stress. These three grades were prepar ed 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 40ppm of Fe, respectively. The experi mental results show that the creep behav ior of the three grades exhibits a sigmoidal relationship between stre ss and strain rate, which is manifested by the presence of three regio ns: region I (low-stress region), region II (intermediate-stress regio n), and region III (high-stress region). In region II, the creep chara cteristics, including the stress exponent, n, and the activation energ y for superplastic flow, Q, are insensitive to Fe level; n congruent t o 2.5 and Q = Q(gh), where Q(gh) is the activation energy for grain bo undary diffusion. By contrast, the emergence of region I along with it s stress exponent and activation energy is affected by Fe level; the h igher the Fe level, the higher the stress exponent and the activation energy. The effect of Fe on region I behavior in Zn-22 pet Al is attri buted to a threshold stress for creep, tau(o), whose origin is related to Fe segregation at boundaries. An examination of the estimated valu es of threshold stresses in the three grades along with a comparison b etween these values and those reported earlier for two grades of Zn-22 pet Al containing 180 ppm and 100 ppm of impurities (120 and 50 ppm o f Fe, respectively) reveals two findings First, the threshold stress a ppears to approach a limiting value for Fe concentrations above 120 pp m with increasing Fe level. Second, for approximately the same Fe conc entration, the presence of other impurities in Zn-22 pct Al leads to a higher value of tau(o). These findings are discussed in terms of char acteristics associated with grain boundary segregation (saturation and synergistic effects).