Plastic deformation and ductility of magnesium AZ31B-H24 alloy sheet from 22 to 450 °C

Mg alloy AZ31B-H24 sheet was investigated through microstructural characterization and mechanical testing. Tension tests were conducted across a broad range of strain rates for temperatures from 22 up to 450 °C, as were gas-pressure bulge tests for several forming pressures at 350 °C. Tensile data were used to calculate flow stresses, tensile elongations, activation energies for creep and the Lankford coefficient, or R-value. The AZ31B material exhibits strong plastic anisotropy (R=7) at room temperature because of its basal crystallographic texture. Plastic anisotropy decreases with increasing temperature but demonstrates no sensitivity to strain rate until recrystallization occurs. Upon recrystallization, the R-value becomes sensitive to strain rate and continues decreasing with increasing temperature until it reaches a minimum (R=1-3) near approximately 300 °C. Plastic anisotropy at these high temperatures is greatest at the fastest strain rate. This sensitivity to strain rate is attributed to a competition between dislocation-climb creep, which produces anisotropic flow and dominates at fast strain rates, and grain-boundary-sliding creep, which produces isotropic flow and dominates at slow strain rates. The mechanistic understanding developed for plastic flow in AZ31B was implemented in a material constitutive model for deformation at 350 °C. A new aspect of this model is the inclusion of dislocation pipe diffusion as a potential accommodation mechanism for dislocation-climb creep. This model was validated against independent gas-pressure bulge test data through the predictions of a finite-element-method simulation, and the model provided quite accurate predictions of the experimental data.