An experimental study on the stability of superplastic deformation of AZ31 Mg alloy

Optimum variable strain rate forming paths based on two different multiscale deformation-based stability criteria were developed. These criteria account for both geometrical necking (macroscopic feature) and microstructural evolution. In the first criterion, we defined the onset of instability using Hart's stability analysis, where a linear perturbation in a small region along the tensile specimen is introduced. In the second criterion, we introduced a modified one dimensional nonlinear long wavelength analysis, based on the one introduced by Hutchinson and Neale [5]. The two stability criteria are calibrated for the AZ31 magnesium alloy at 400°C, yielding two different variable strain rate forming paths. Uniaxial tensile tests are carried out following the derived forming paths to examine the validity of both criteria. Results show that the linear approach underestimates the amount of uniform deformation, and therefore prolongs the forming time to prevent failure compared to the nonlinear analysis.