Microstructural characterization of Fe–Mn–C martensites athermally transformed at low temperature by Rietveld method

The present study concerns X-ray characterization of the microstructures of Fe–Mn–C alloys with compositions 5.6, 5.8 and 6.0 Mn and 1.0 C (mass%), athermally transformed at low temperatures, following γ→α transformation reaction. The methodology applied is Rietveldʹs whole X-ray profile fitting technique adopting the recently developed software, Materials Analysis Using Diffraction (MAUD) which incorporates Popa model for crystallite (domain) size and microstrain (root mean square, r.m.s strain) and the preferred orientation of the crystallites. The analysis also considers the lattice-defect related features of the evoluted microstructures, viz. stacking, twin, compound fault probabilities and dislocation density values. The study revealed the highest degree of transformations with 36, 40 and 47% volume fractions of martensites for 5.6, 5.8 and 6.0 mass% of Mn at temperatures 190, 185 and 170 K, respectively. The respective microhardness values are 441, 398 and 385 kg mm−2. With increasing Mn concentration the crystallite size values of the respective martensites decrease and r.m.s strains increase, while the dislocation density and stacking fault density values increase with respect to those of the respective austenites. Depth profile study reveals that the martensite percentage decreases with decreasing microhardness values on removal of thin layers from the surface.