Thermal stability of an ultrafine-grained dual phase TWIP steel

This work investigates the thermal stability of an ultrafine-grained dual phase twinning-induced plasticity (TWIP) steel produced by friction stir processing (FSP). In exploring this, a set of annealing treatments were conducted on the friction stir processed steel in the temperature range of 500-1000 °C for 1 h. The results indicate that the microstructure of the ultrafine-grained steel possesses high thermal stability up to 700 °C, where the grain size remained in submicron level. At higher temperatures, however, the grains experienced a significant growth. Moreover, the apparent activation energy for grain growth was calculated through isothermal annealing at 1000 °C for different durations. The micro-texture analysis shows that the annealing has been accompanied by a substantial shear-type texture weakening. Concurrently, shear punch mechanical testing and microhardness measurements were employed to study the mechanical behavior of the annealed materials. The results were in line with the microstructural evolution i.e., a high stability of strength was recorded by 700 °C, while material׳s strength significantly dropped at higher temperatures. The evolution of microstructure and mechanical properties were elaborately discussed considering the nature of FSP induced microstructure, restoration processes taking place during annealing and the variations of ferrite volume fraction with annealing temperature.