The strain rate and temperature dependences of the low stress of the superplastic Al-Zn alloy were studied by conducting tensile tests between -80°C and 30°C. Microstructures before and after deformation were also examined. The principal results obtained are as follows:
(1) The linear relationships of logσ_y-logε., logσ_O2-logε, logσ_m-logε and logσ_6.0-logε were valid not only at high temperatures but at low temperatures.
(2) The strain rate sensitivity exponents, m, determined from the logσ-logε curves, were 0.23, 0.17 and 0.12 at, 30°C, 0°C and -20°C, respectively. However, the m-values, obtained by the Backofen method, depended on the strain rate. It was suggested that this was due to structural changes during deformation.
(3) The strain rate showed the Arrhenius type temperature dependence when deformation was beyond the maximum stress in tensile tests. The activation energy was independent of the stress and equal to 0.58eV, which was smaller than that of the bulk and grain boundary self-diffusion of Al and Zn.
(4) When deformation was carried out between -20°C and 30°C, many voids and cavities developed at grain boundaries, transverse to the tensile axis, and grew as the strain increased.
(5) Grain-and phase-boundary sliding was predominant at 30°C. It was still observed at 0°C. Any evidence of crystallographic slip inside grains were not detected at these temperatures.