Influence of temperature and grain size on threshold stress for superplastic flow in a fine-grained magnesium alloy

The parametric dependencies of threshold stress for superplastic flow were characterized over a wide range of temperatures using a fine-grained magnesium alloy with various grain sizes. The threshold stress was proportional to the reciprocal of the square root of the grain size at 473 K, whereas it was independent of the grain size at 598 K. The threshold stress decreased with increasing deformation temperature more quickly than the shear modulus did. It was suggested that the temperature dependence of the threshold stress is associated with the interaction between the solute atoms and dislocations. The origin of the threshold stress probably resulted from the dislocations breaking away from the solute atmospheres formed in the grain boundaries and in the lattice at low temperatures and at high temperatures, respectively.