High-temperature deformation in a superplastic 7475 Al alloy with a relatively large grain size

Abstract

Tensile tests were conducted on a superplastic 7475 Al alloy having a grain size of 14 μm, which is relatively larger than that of conventional micrograin superplastic alloys, at temperatures in the range of 723–789 K. The objective of the present investigation was to examine the possible rate-controlling mechanisms that govern superplastic deformation of the alloy at both intermediate and low stresses. The values of the stress exponent and the activation energy, as well as microstructural observation, indicate that deformation of the alloy at intermediate stresses is rate-controlled by lattice diffusion. At low stresses, the stress exponent and the activation energy are higher than those at intermediate stresses. The mechanical behaviour at low stresses may be attributed to the existence of a threshold stress which strongly depends on temperature. The origin of the threshold stress is discussed in terms of the various dislocation-particle interaction models that have been proposed for the creep of dispersion-strengthened alloys. The present analysis reveals that the mixed climb model can qualitatively explain the very strong temperature dependence of the threshold stress, but not quantitatively. Therefore an alternative explanation is suggested for the very strong temperature dependence of the threshold stress on the basis of the role played by interface diffusion in recreating the dislocation core segment that is smeared out following attachment of dislocation to the dispersoids.