SUPERPLASTICITY IN AN AL-ZN-MG-CU ALLOY

The deformation behaviour of a fine-grained Al-Zn-Mg-Cu alloy was investigated in a broad strain rate interval at temperatures between 672 K and 789 K. Superplastic characteristics were found at all temperatures used at strain rates between 10(-4) s-1 and 10(-3) s-1. The non-superplastic behaviour at low strain rates (in what is known as region I) was explained taking into account the existence of a threshold stress. Using a suitable value of the threshold stress, region I seems to be a continuation of the superplastic region II with nearly the same values of the strain rate sensitivity parameter m and activation energy. Possible sources of the threshold stress are discussed.