Phase transformations in a high-Mn steel: strain hardening mechanisms, austenite reversion, and athermal martensitic transformation

In this work, phase transformations in a high-Mn steel containing 17.6 wt.% of Mn and belonging to the Fe-Mn-Al-Si-C-Ni system were investigated for a variety of states, including cold rolled and annealed ones. The strain hardening mechanisms of austenite, ε-, and α\'-martensite were tracked during cold rolling. The complex superposition of several displacive reactions were revealed for each phase with the aid of the combinatorial use of XRD measurements (coupled with the software MAUD), ECCI-SEM, and EBSD. Dilatometry measurements revealed that the austenite reversion splits into two stages during continuous annealing. Such phenomenon is due to strong elemental partitioning between the growing austenite and the α\'-matrix, as simulated using the software DICTRA and confirmed via near-atomic resolution APT. Results provided new insights that the successful austenite nucleation is preceded by long-range elemental partitioning. Besides, the growth of austenite is also given by strong elemental partitioning and solute redistribution within both austenite and α\'-martensite. Magnetic properties were also investigated for several microstructures of the present steel, modified by means of straining and/or annealing. The formation of nano reversed γ-grains in the early stages of the austenite reversion is sufficient to induce strong magnetic shape anisotropy. Using in-situ magnetic measurements and thermodynamic modelling, the Curie temperature of the steel was evaluated, as well as the stability of austenite during controlled conditions of cooling. The influence of local changes in chemical composition on the magnetic properties was deeply investigated by means of magnetic measurements, thermodynamic simulations (Thermo-Calc), high-resolution microscopy, including STEM, and APT. The findings revealed that short- and long-range chemical fluctuations strongly affect the saturation magnetization of the steel and brought new insights on the use of magnetic probing as tool for quantification of phases in Mn-based steels. (AU)