Twinning-induced negative strain rate sensitivity in wrought Mg alloy AZ31

Highlights: → The SRS of AZ31 is affected by texture and active deformation modes. → AZ31 plate exhibits the negative SRS when twinning dominates deformation. → An increased twinning activity at higher strain rate can lead to negative SRS. - Abstract: This paper reports the effects of the texture and strain on the strain rate sensitivity of wrought magnesium alloy AZ31. Specimens were prepared from strongly textured AZ31 plate such that the uniaxial loading direction was either parallel or perpendicular to the basal planes of the majority of grains. The strain rate sensitivity of the specimens was determined from strain rate jump tests in tension and in compression, and the microstructure evolution during the tests was tracked by electron backscatter diffraction microscopy of ex situ samples. Specimens oriented unfavorably for {011-bar 2} tension twinning show a decreasing strain rate sensitivity with increasing strain. In contrast specimens oriented favorably for tension twinning exhibited negative strain rate sensitivity at low strains followed at higher strains by positive strain rate sensitivity. The flow stress and consequently the strain rate sensitivity was modeled using an analytical constitutive equation. This model was able to account for not only the negative strain rate sensitivity of the samples undergoing tension twinning at low strains but also the observed trend of strain rate sensitivity with strain for all samples. From this we deduce that an increase in twinning activity with strain rate is responsible for the strain rate sensitivity.