How magnesium accommodates local deformation incompatibility: A high-resolution digital image correlation study

The plastic deformation of single crystal magnesium is strongly anisotropic. This gives rise to deformation incompatibilities between grains during polycrystalline deformation, which are thought to limit ductility and formability. Wrought polycrystalline magnesium alloys are far from brittle, especially in uniaxial tension, implying that these incompatibilities can be accommodated to some extent, although it is not clear how. We have used high-resolution digital image correlation (HRDIC), supported by electron backscatter diffraction (EBSD), to study quantitatively and at the microstructural scale the accommodation of deformation incompatibility in an AZ31 magnesium alloy. Using a new gold remodelling procedure that improves the spatial resolution to 44 nm, we quantified the deformation heterogeneity after a small stretch in uniaxial tension. Our results confirm that polycrystalline deformation is very heterogeneous, with local axial true strains at grain boundaries 32 times higher than the applied average strain of 0.027, and 18 times higher at slip bands within grains. The local and macroscopic deformation gradients are very different in character as well as magnitude. The resultant deformation incompatibility is accommodated primarily by gradients in basal slip and the activation of difficult slip in “hard” grains, giving rise to grain breakup, with a smaller contribution by enhanced grain boundary shear and twinning. These results support the idea that a homogeneous distribution of “hard” and “soft” grains can prevent the development of strain localization and, therefore, that controlling texture and microtexture is a powerful way of enhancing the formability of magnesium alloys without reducing their single crystal plastic anisotropy.