Abstract
The effects of testing temperature, imposed strain rate, heating rate up to testing temperature and holding time before stretching on the total plastic strain have been investigated in warm-rolled 7475 aluminum alloy sheets. The value of strain rate sensitivity parameter, m, (=∂lnσ/∂lnε) and apparent activation energy for plastic flow were also measured. Mechanical characteristic and optical microscopic observations showed that in order to take large superplastic elongation the specimens should be heated quickly up to the optimum testing temperature and be stretched just before the advent of static recrystallization. The value of m was about 0.2 in the range of temperature from 583 to 650K, however, it increased with increasing temperature and reached about 0.55 above 770K. The apparent activation energy for plastic flow was 358kJ/mol in the temperature range of superplastic flow. Compensating by the change in grain size and elastic modulus at testing temperature, the activation energy reduced to 160kJ/mol. This suggests that the grain accommodation during superplastic flow is controlled by the bulk diffusion.
