Temperature and grain size dependence of superplasticity in a Zn-0.3 wt.% Al alloy

Abstract

The superplastic deformation behavior of quasi-single phase Zn-0.3 wt.%Al was investigated. A series of load relaxation and tensile tests was conducted at various temperatures ranging from RT (20 degreesC) to 200 degreesC. The recently proposed internal variable theory of structural superplasticity was applied. The flow curves obtained from load relaxation tests were shown to consist of contributions from interface sliding (IS) and accommodating plastic deformation. In the case of quasi-single phase Zn-0.3 wt.% Al alloy with an average grain size of 1 mum, the IS behavior could be described as a viscous flow process characterized by a power index of M-g=0.5. A large elongation of about 1400 % was obtained at room temperature and the strain rate sensitivity parameter was about 0.4. Although relatively large-grained (10 mum) single phase alloy showed a high value of strain rate sensitivity comparable to that of fine-grained alloy at very low strain rate range, IS was not expected from the analysis based on the internal variable theory of structural superplasticity at room temperature. As the temperature increased above 100 degreesC, however, the contribution from IS was observed at a very low strain rate range. A high elongation of similar to400 % was obtained in a specimen of 10-mum-grain-size at 200 degreesC under a strain rate of 2x10(-4)/sec.