Microstructure evolution during the progressive transformation induced plasticity effect in a Fe-0.1C-5Mn medium manganese steel

This study investigated the microstructure evolution of a cold-rolled (CR) and intercritical (IA) annealed medium-Mn steel (Fe-0.10C-5Mn) during uniaxial tensile test. In-situ observations under scanning electron microscope (SEM), transmission electron microscope (TEM) and X-ray diffraction (XRD) analysis were conducted to characterize the progressive TRIP (Transformation-Induced Plasticity) process and the associated fracture initiation mechanisms. These findings were discussed with local strain measurements by digital image correlation (DIC). The results indicated that the Lüders band formation in the steel was limited to a strain of 1.5%, mainly due to the the transformation of relatively large-sized blocky retained austenite (RA) into α’-martensite, retarding the yielding. The small-sized RA exhibited higher stability, progressively transforming into martensite and endowed stably extended Portevin-Le Chatelier (PLC) effect. The volume fraction of RA displaying a gradual decrease from 26.8% to 8.2% before fracture. In the late deformation stage, fracture initiation primarily occurred at the, austenite/martensite and ferrite/martensite interfaces as well as the rupture of ferrite phase.