Structure and microstructure evolution during martensitic transformation in wrought Fe-26Mn-0.14C austenitic steel: an effect of cooling rate

Structure and microstructure evolution under various cooling rates of a wrought austenitic steel, Fe-26Mn-0.14C (composition in mass %), were studied by the Rietveld method of X-ray diffraction pattern fitting, grain boundary characterization by electron back-scattered diffraction (EBSD) and optical microscopy. Cooling rate, density of stacking faults, and austenite grain size and grain boundaries influence the observed gamma(fcc) --> epsilon(hcp) transformation and lead to significant anisotropic X-ray line broadening. Depending on the cooling conditions, the grain boundaries are misoriented at both lower and higher angles. In the epsilon-martensites, the dominant planar fault is twins (similar to 10(-3)). The austenite grains were found to contain low to moderate density of stacking faults (similar to 10(-4)-10(-3)), which act as efficient nucleation sites of the epsilon-martensites. Both X-ray and EBSD analyses estimated negligible twins in the austenite. Approximate average dislocation densities have been estimated and correlated with the grain structure.