A mechanism-dependent material model for the effects of grain growth and anisotropy on plastic deformation of magnesium alloy AZ31 sheet at 450 °C

Abstract

Utilization of wrought magnesium sheet alloys for structural components in transportation industries has been severely limited by poor room-temperature formability, a result of the slip behaviors inherent to Mg’s hexagonal close-packed crystal structure. Today, the only production technology to clearly overcome this limitation uses hot forming to activate additional (non-basal) slip systems in Mg alloy sheet. The absence of an accurate material constitutive model that captures the complex mechanical response of Mg sheet alloys at elevated temperatures has been a persistent barrier to accurate forming simulations. This study addresses that issue using experimental measurements and mechanism-based modeling. The mechanisms of plastic deformation in a Mg AZ31 wrought alloy sheet at 450°C across strain rates from 10−4 to 10−1s−1 are identified as grain-boundary-sliding (GBS) creep and five-power dislocation-climb (DC) creep. GBS creep is subject to hardening from grain growth, and DC creep produces texture-dependent plastic anisotropy. Based on these mechanisms, a new material constitutive model for Mg AZ31 at 450°C is constructed to predict plastic response under general multiaxial loading. A unique aspect of this new model is that it accounts for hardening and plastic anisotropy by linking these effects to the two mechanisms controlling deformation. The model is validated against independent experimental data and provides accurate predictions for hot forming of a simple test shape. The new material model is the first for Mg AZ31 sheet that accurately predicts deformation at an elevated temperature under both uniaxial and biaxial stress states.