Influence of grain size on high temperature fracture in a Mg AZ31 alloy

Tensile experiments at 673 K and grain sizes from ∼8 to 17 μm revealed large ductility at a low strain rate and a reduced ductility at a high strain rate, corresponding to a change from a high to a low value for the strain rate sensitivity. High strain rate deformation led to fracture by flow localization, whereas low strain rate deformation involved fracture by cavity nucleation and growth. Analysis revealed that grain boundary migration can assist significantly in reducing the stress concentrations caused by grain boundary sliding, thereby retarding cavity nucleation. Calculations demonstrate that the interlinkage of voids parallel and perpendicular to the tensile axis occurs significantly, so that it is not always possible to use the cavity shapes to distinguish between diffusion and plasticity controlled growth. Cavitation damage evolves slowly in materials with a coarser grain size because of reduced nucleation related to a reduction in the strain rate sensitivity and associated grain boundary sliding.

Research highlights▶ At higher strain rate, samples of AZ31 alloy failed by external flow localization. ▶ At lower strain rate, it failed by nucleation, growth and interlinkage of voids. ▶ Cavity growth occurs initially by diffusion processes. ▶ Cavities with size larger than ∼7 μm grow by a plasticity process. ▶ Cavity shapes do not necessarily reflect the dominant growth mechanisms.