Effect of carbon content on the mechanical properties and microstructural evolution of Fe-22Mn-C steels

A Fe-22Mn-0.6C (wt%) sheet steel was decarburized and heat treated to obtain four Fe-22Mn-C alloys with carbon contents of 0, 0.2, 0.4 and 0.6 wt% with similar grain sizes, with the objective of determining the effect of varying the alloy stacking fault energy (SFE) on the microstructural evolution and mechanical properties of these alloys as a function of applied strain. Microstructural observations determined the deformation products to be mechanical twins for the 0.6C alloy and mechanical epsilon-martensite for the OC and 0.2C alloys. For the 0.4C alloy, both mechanical twins and epsilon-martensite were observed during deformation. As expected, the ultimate tensile strength and uniform elongation increased with increasing alloy carbon content. For the epsilon-martensite dominated OC and 0.2C alloys, the critical damage mechanism was determined to be void formation at matrix/inclusion and austenite/epsilon-martensite boundaries, leading to cleavage-like features on the fracture surface. For the twinning dominated 0.4C and 0.6C alloys, the critical damage event was void nucleation at grain and matrix/inclusion boundaries, leading to ductile tearing features on the fracture surfaces. In the case of the higher SFE (i.e. SFE >= 15 mJ/m(2)) OAC and 0.6C twinning dominated alloys, it was determined that the sum of the volume fractions of mechanical twins and epsilon-martensite saturated at higher strains, where the sum of the saturation volume fraction of these two deformation products was approximately 0.37 for all experimental alloys and was found to be consistent with similar systems in the high-Mn twinning induced plasticity (TWIP) steel literature. From this data, it was concluded that the combined twinning and epsilon-martensite formation kinetics determined the initiation of critical damage in the higher SFE, mechanical twinning dominated alloys. It was also found that the saturated volume fraction of epsilon-martensite had a sigmoidal decay relationship with increasing values of the SFE for the present alloys and for a variety of high-Mn steel systems taken from the literature. It was further determined that the volume fraction of epsilon-martensite in the low SFE (i.e. SFE <= 10 mJ/m(2)) alloys at fracture was independent of SFE, suggesting that the epsilon-martensite formation kinetics, as dictated by the alloy SFE, dominate the critical damage processes in these alloys. Finally, it was found that the stress for the activation of twinning and the related increase in work hardening rates for the higher SFE, twinning dominated alloys could be described by a simple linear relationship with the SFE, leading to the conclusion that the observed work hardening characteristics and properties of high-Mn TWIP alloys are largely determined by the alloy twinning kinetics. (C) 2014 Elsevier B.V. All rights reserved.