Twinning mechanism in TWIP steel: from single crystalline micro-pillar to bulk sample

Abstract

Twinning-induced plasticity(TWIP) steels have excellent combination of strength(1000 MPa) and ductility(60%) and are potential lightweight materials for automotive applications. The excellent mechanical properties of TWIP steels are due to the formation of intensive deformation twins during deformation. Understanding the twinning mechanisms in TWIP steels is essential for the successful application of TWIP steels in automotive industry. The first part of this work is to employ sub-micon and micron-sized single crystalline pillars to investigate the nucleation and growth mechanism of deformation twins. It is found that the nucleation and growth of deformation twins are due to emission and glide of successive partial dislocations. The twin thickness can range from nanometres to micrometres. A physical model is proposed to simulate the nucleation and growth of deformation twins and the model predictions agree well with experimental observations. The second part of this work investigates the effect of high strain rates on the deformation mechanism of bulk samples. By synchrotron X-ray diffraction experiments, the present work demonstrates that a higher strain rate leads to a lower dislocation density and a lower twinning probability. This unexpected suppression of dislocation and twin evolution have been attributed to the temperature increase due to dissipative heating at high strain rate deformation. A physically-based model has been proposed to simulate the evolution of dislocation and twin densities and to model the stress-strain relation. The modelling results agree well experimental findings.