Micromechanical aspects of the austenite reversion in a high-Mn steel evaluated by in-situ synchrotron X-ray diffraction

High-Mn steels are very attractive for automotive applications, since they combine high strength and ductility. The transformation-induced plasticity (TRIP) is one of the complex strain hardening mechanisms presented by these steels. With regard to the TRIP effect, metastable austenite (gamma) transforms into epsilon- and/or alpha'-martensite. Reversion annealings of Mn-based steels transforms the alpha'-martensite back to austenite, leading to remarkable bulk-scale grain refinement. In this work the austenite reversion in a 17.6 wt.%Mn steel was tracked by means of in-situ X-ray diffraction (XRD). This experiment was carried out at the X-Ray Diffraction and Thermo-Mechanical Simulation (XTMS) workstation placed at the LNNano. The material was deformed to an engineering strain of 0.2 under tensile testing and further annealed up to 800°C with a heating rate of 3°C/min. During in-situ experiments, images of the diffraction cones were collected under a rate of 6 pictures per minute. The 2D images were converted into usual diffractograms which were fitted with a pseudo-Voigt function to obtain the integrated area, peak position, and the full width at half maximum (FWHM) of the peaks. The phase quantification was performed using the direct comparison method. The modified Williamson-Hall approach was used to separate the contributions from the crystallite size (D) and lattice strain (beta) to the peak broadening. The starting and finishing temperatures for the epsilon -gamma reaction was 245 and 450°C, respectively. The corresponding temperatures for the alpha'- gamma transformation were 450 and 700°C, respectively. From the beta behavior, it was observed that tempering occurs in the alpha'-martensite in the early stages of the gamma reversion. The crystallite size in turn increases considerably in the end of this reaction. In addition, the changes in the molar volume of the steel was assessed from the lattice parameters of the phases. (author)