Creep at low stress levels in the superplastic Zn-22% Al eutectoid

Abstract

A sigmoidal relationship between strain rate and stress was observed in a superplastic Zn-22% Al eutectoid alloy with grain sizes in the range of 2.1–7.5 μm. The relationship was independent both of the testing technique employed (whether constant stress or constant strain rate) and of the mode of deformation selected (whether tensile or shear).In the superplastic region (strain rates of ~10−5-10−2 sec−1), the stress exponent was ~2.25, the exponent of the inverse grain size was ~2.3, and the activation energy was close to that for grain boundary diffusion. These results are in good agreement with the predictions of a model based on grain boundary sliding accommodated by the climb of dislocations into boundaries. At very low strain rates ≲10−5 sec−1), the stress exponent was ~4.1, the exponent of the inverse grain size was ~2.4, and the activation energy was close to that for volume self-diffusion. These results are not consistent with any of the existing deformation mechanisms, but suggest that the sigmoidal relationship may arise through the sequential operation of two different processes