An effective stacking fault energy viewpoint on the formation of extended defects and their contribution to strain hardening in a Fe-Mn-Si-Al twinning-induced plasticity steel

A deformation-dependent stacking fault energy (SEE) viewpoint is invoked to interpret the low strain rate tensile deformation of a Fe-27Mn-2.5Si-3.5Al austenitic steel at room temperature by using X-ray diffraction analyses and transmission electron microscopy (TEM) observations. The effective SFE of austenite increased with strain from similar to 18 to 40 mJ m(-2), and it was reasoned that this was the result of the increase in strain energy of the stacking faults (SFs) per unit area due to its dependence on the dislocation character and density. Twinning was observed at 2% strain and confirmed to occur by a glide mechanism of a/6 < 121 > Shockley partial dislocations, leading to the formation of overlapping intrinsic-extrinsic SF pairs representing a three-layer twin embryo, revealing periodic dislocation contrast in TEM. The early onset of twinning is attributed to the unusually low critical twinning stress of the steel, similar to 200 MPa. In spite of twinning, the stacking fault probability (P-sf) of twinned austenite was remarkably low (similar to 10(-4)) at low strains, but increased moderately (to similar to 10(-3)) up to failure strain. At the emergence of twinning, the corresponding perfect dislocation density was low (similar to 10(14) m(-2)) but was well above the critical dislocation density required for twinning occurrence. Dislocation character analysis indicated that increasing deformation gradually changed the dislocation character from edge to screw type. The microstructural parameters of the steel estimated in direct or indirect relation to its SFE could explain its flow stress and strain hardening behavior. (C) 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.