Effects of sample orientation and processing temperature on dynamic recrystallization and mechanical behaviours of a Mg alloy under high strain-rate deformation

Abstract

Mg alloys have been extensively investigated because they are promising engineering materials with a low density and high specific strengths. However, poor formability at low temperature has been a major obstacle limiting their widespread industrial applications. Recent investigations indicated that plastic deformation of Mg alloys at relatively high strain rates at high temperatures promotes dynamic recrystallization (DRX) and this substantially improves both their formability and mechanical properties. However, there are still some outstanding issues on DRX during high strain-rate deformation, which include (1) it is not clear how crystallographic orientation and initial microstructure affect DRX and (2) how strain rate influences the mechanical responses at different temperatures is largely unknown. A commonly used Mg alloy, AZ31 (Mg–3Al–1Zn, in wt. %), was used to explore the abovementioned two outstanding issues in this study. The microstructural evolution and mechanical properties of samples with three different grain orientations (the c-axis of the samples being generally 0°, 45° and 90° away from the loading direction) were investigated. The following major conclusions were reached: (1) sample orientation and the original twins affect significantly the DRX behaviour deformed under high strain rate at high temperature. To promote DRX, it is crucial to have significant local dislocation accumulation. This can be achieved by dislocation activities starting from the early stages of deformation and effective barriers that block the dislocation motion. Early activation of dislocation slip is achieved by appropriate sample orientation. Both deformation twins and pre-existing twins can act as barriers for dislocation activities. The effect of pre-existing twins becomes significant if there was no deformation twinning; (2) at high temperature, increasing strain rate dramatically refines grain sizes in all samples, leading to remarkable increase of the flow stresses. The strain rate effect on microstructural evolution varies remarkably with sample orientation; and (3) at high temperature, samples present different basal texture after the deformations at different strain rates. It is because the basal texture of non-DRX areas is getting significantly stronger with the increase of strain rate in some samples. Meanwhile, the ratio of non-DRX areas is considerably higher in these samples than the other sample.