Stacking fault energy and plastic deformation of fully austenitic high manganese steels: Effect of Al addition

Dependence of the dislocation glide mode and mechanical twinning on the stacking fault energy (SFE) in fully austenitic high manganese steels was investigated. Fully austenitic Fe–22Mn–xAl–0.6C (x = 0, 3, and 6) steels with the SFE in the range of 20–50 mJ/m2 were tensile tested at room temperature, and their deformed microstructures were examined at the different strain levels by optical microscopy and transmission electron microscopy. Deformation of all steels was dominated by planar glide before occurrence of mechanical twinning, and its tendency became more evident with increasing the SFE. No dislocation cell formation associated with wavy glide was observed in any steels up to failure. Dominance of planar glide regardless of the SFE is to be attributed to the glide plane softening phenomenon associated with short range ordering in the solid solution state of the present steels. Regarding mechanical twinning, the higher the SFE is, the higher the stress for mechanical twinning becomes. However, in the present steels, mechanical twinning was observed at the stresses lower than those predicted by the previous model in which the partial dislocation separation is considered to be a function of not only the SFE but also the applied stress. An analysis revealed that, of the various dislocation–defect interactions in the solid solution alloy, the Fisher interaction tied to short range ordering is qualitatively shown to lower the critical stress for mechanical twinning.