X-ray diffraction study on cooling-rate-induced γfcc → εhcp martensitic transformation in cast-homogenized Fe-26Mn-0.14C austenitic steel

The thermal stability of a cast homogenized austenitic steel with basic composition Fe-26Mn-0.14C has been studied using the Rietveld entire X-ray diffraction (XRD) pattern fitting technique. The decomposition of austenite was observed to be very sensitive to the cooling rate. A very high proportion (49 to 70 pet) of epsilon-martensites was found in the differently cooled specimens as well in the as-cast specimen. The density of various planar (stacking and twin) faults was also estimated in both epsilon-martensites and retained austenite. The significant variation in the extent of gamma(fcc) -> epsilon(hcp) martensitic transformation was interpreted in terms of athermal and isothermal martensites formed during cooling and grain size of austenite. The austenite grains were found to be comprised of a high to moderate density of stacking faults (similar to 10(-3) to 10(-2)) and negligible twin fault probability (similar to 10(-5)). On the other hand, in the epsilon-martensites, the dominant planar fault was twins (similar to 10(-3)). Approximate values of the dislocation density within the retained austenite were also evaluated using the crystallite size and root-mean-square (rms) strain values obtained from the size-strain-shape analyses.