Serration and recovery phenomena in twinning induced plasticity steel

Abstract

Growing economical and ecological requirements have driven the automotive industry towards lowering fuel consumption by reducing vehicle weight and in the meantime maintaining safety standards. This pushes the development of advanced light weight alloys. As a promising candidate for automotive applications, the twinning induced plasticity (TWIP) steels have received tremendous attention due to their superior mechanical properties, which combines high ultimate tensile strength and ductility simultaneously. However, their plasticity mechanism and recovery behavior are still not completely understood.

In the first part of this investigation, a modified physical-based model is developed based on the recent observations of strong influence of grain orientation and Bauschinger effect. The model describes well the serrated plastic flow of Fe-22Mn-0.6C TWIP steel under quasi-static tensile loading at ambient temperature. The discrete twinning is considered responsible for the serration phenomenon in Fe-22Mn-0.6C TWIP steel. The model also provides a new approach to estimate the volume fraction of deformation twins.

In the second part of this investigation, the recovery of pre-strained Fe-22Mn-0.6C TWIP steel is studied. A model describes the evolution of yield stress during annealing is proposed, which indicates that the recovery treatment does not remove deformation twins induced by cold rolling but decreases dislocation density in matrix and removes the dislocations piled up against twin and grain boundaries. The recovery activation energy rec Q is derived from experimental results. The value Qrec = 1.73 eV = 167 kJ/mol implies that the diffusion from dislocations cores governs the recovery of investigated TWIP steel.