Variation of true strain-rate sensitivity exponent as a function of plastic strain in the PM processed superplastic 7475Al+0.7Zr alloy

The variation of strain rate–stress relationship for the high-strain-rate superplastic PM7475Al+0.7Zr alloy has been studied as a function of plastic strain. The increase of true strain-rate sensitivity exponent, mt, from ∼0.3 to >0.5 was observed to occur with plastic strain when tested at an optimum strain rate of 10−1 s−1 for superplasticity. In contrast, when tested at a lower strain rate of 10−4 s−1 where only limited tensile elongation was obtained in the elongation-to-failure test, low mt values less than 0.5 were revealed throughout the test. The true activation energy for plastic flow was estimated for the dynamically recrystallized microstructure and found to be similar to that for lattice diffusion in pure aluminum and that for the same alloy with the initial microstructure with a high portion of low angle boundaries. Threshold stresses and its related activation energy for the recrystallized alloy were higher and lower, respectively, by comparison with those for the alloy with the initial microstructure. Tensile elongation behavior as a function of strain rate could be explained in terms of true m value varying with strain.