Thermodynamics and crystallography of the gamma→epsilon→alpha' transformation in feMnAlSic steels

FeMnAlSiC steels which exhibit two-stage transformation induced plasticity (TRIP) behavior characterized by the gamma→epsilon→alpha' dual stage martensitic transformation promise to take a leading role in the development of 3rd generation advanced high strength steels. The crystallographic orientation relationship of the gamma→alpha' and gamma→epsilon athermal martensitic transformations in these steels has been determined as the Kurdjumov-Sachs and the Shoji-Nishiyama, respectively. Six crystallographic variants of alpha-martensite consisting of three twin-related variant pairs were observed in epsilon-bands. A planar parallelism of {0001}epsilon || {110}alpha' and a directional relation of alpha' lying within 1° of epsilon existed for these variants. Two regular solution models have been developed to describe the thermodynamics for the gamma→epsilon, gamma→alpha', and subsequent epsilon→alpha' martensitic transformations which best described the behavior and microstructure of various FeMnAlSiC TRIP compositions when compared against other thermodynamic models from literature. The role of available nucleating defects of critical size, n*, has been linked to the intrinsic stacking fault energy (SFE) necessary to observe the athermal gamma→epsilon transformation and it is thus proposed that the amount of epsilon-martensite in the quenched microstructure is a function of material processing history as well as thermodynamic driving force. The developed thermodynamic model has been used to optimize alloy compositions that produce ideal two-stage TRIP behavior. Compositions with Al contents near 1.5 wt% adequately balance epsilon- and alpha-martensite start temperatures such that retained austenite is expected upon quenching to room temperature while also maintaining adequate transformation driving forces to ensure full two-stage TRIP behavior.