Cyclic Strain Heterogeneity and Damage Formation in Rolled Magnesium Via In Situ Microscopic Image Correlation

Inherently tied to the complexity of their deformation mechanisms, Magnesium alloys show a high propensity for strain localization at multiple length-scales. Understanding this aspect of Magnesium deformation is essential for enabling new-generation models that seek fidelity at the microstructural level. We present a comprehensive investigation of strain distributions over a cyclic load path in rolled Magnesium AZ31. Over the asymmetric stress-strain curve, the spatial structures that correspond to twin-, detwin- and plasticity-dominated deformation regimes are targeted, with an emphasis on their cyclic interactions. A robust in-situ implementation of digital image correlation with area-scanning optical microscopy is performed that entails a full bridging of grain and sample scales. This proves essential to uncover the pronounced long-range coordination of the strain patterns in this material. Over the compression-tension-compression cycle, two such patterns have dominant presence: (i) tensile-twin-driven (TTD) bands that are activated in compression and (ii) preferential plasticity in micro-texture bands, heavily realized in tension. Strain heterogeneity levels show a distinct asymmetry at the mid- and end points of the cycle. Both dominant patterns come to co-exist in the latter as a second-wave of TTD bands superpose over remnant strains in the micro-texture bands. The compactness of second-wave TTD bands is significantly reduced compared to the first wave. The strain distributions over the intergranular localization network that make up the TTD bands are characteristic and can be targeted by advanced models. The long-range inherited micro-texture elements have a strong impact on the meso-scale strain heterogeneity and they warrant careful consideration.