Evolution of primary and secondary twins during tensile cyclic loading in magnesium alloy ZM21 by quasi in situ EBSD

Abstract

Large-scale microscopy at consecutive strains using quasi in situ EBSD was used to monitor the texture evolution in specific areas of extruded magnesium alloy ZM21 samples under cyclic tensile loading-unloading. Twin formation throughout the various cycles is highly dependent on slip and twin activity in the neighboring grains, so slip-trace analysis was conducted to monitor the activity of various slip systems. Slip traces were observed on three different glide planes: prismatic plane, first-order pyramidal plane, and second-order pyramidal plane. Formation of primary (including extension and contraction) twins and secondary twins was investigated in detail by overlapping the EBSD maps after pre-compression and after subsequent tension. Parent grains and twins were classified based on their orientations with respect to the ED (loading direction). The inverse pole figure map for the twins was plotted for the ED to calculate the position of the c-axis for twins. All extension twins that formed under reverse loading were located within 45°–90° of the ED axis, whereas the twins that formed during pre-compression were located within 45° of the ED axis. Extension twins are formed during tension loading perpendicular to the pre-existing twins from pre-compression (which have undergone a detwinning process under load reversal). Twin residues from compression interact with grain boundaries or new twins that have formed within the grains during tensile loading. Such interactions between twins and dislocations can result in damage-accumulation mechanisms which lead to crack initiation.