In situ study of austenite driven work-hardening behaviour in a TRIP-assisted dual phase steel

In this study we have subjected a high-strength, dual phase steel containing retained austenite to heat-treatment designed to suppress second phase banding resulting from chemical segregation and thermomechanical treatment. Loading experiments have been carried out in situ in synchrotron and ex situ to determine the effect of loading on the progression of the mechanically induced γ→α/ α’ transformation and the resultant effects on load partitioning and work-hardening of the phases present. Detailed powder diffraction analysis has been performed to characterise the evolution of the phase fraction and load partitioning resulting from the austenite-to-martensite transformation in order to understand the work hardening behaviour of the steel. Our results show that the mechanically induced transformation does not begin until the onset of matrix yielding and is coupled to an increase in work-hardening rate. We show that the mechanically induced transformation does not begin until the onset of stage II hardening and progresses linearly with strain and the presence of fresh martensite in the microstructure does not significantly affect the austenite stability. Yielding of martensite does not occur until all the completion of mechanically induced transformation. Plastic deformation of martensite is quickly followed by local plastic instability leading to necking and ultimate failure.