Science of deformation mechanisms in twinning-induced plasticity steels

Abstract

The demands to save energy and reduce the CO2 emission have been pushing the automobile industry to reduce the vehicle weight while maintaining a high safety standard. Since 1980’s, a tremendous amount of research efforts have been made to study the twining-induced plasticity (TWIP) steels, which are considered as promising candidates for structural application in vehicle. It has been demonstrated that the TWIP steels generally exhibit an unusual work-hardening rate, which leads to excellent combination of high strength and exceptional ductility. Such excellent mechanical properties and unusual work-hardening behaviour of TWIP steels are to a significant extent controlled by the deformation mechanisms, which include the dislocation and twinning kinetics and the associated dislocation-twin interaction, dislocation-dislocation interaction and dislocation-solid atom/precipitate interaction. It is of fundamental importance to understand these deformation mechanisms in order to truly appreciate the origin of the exceptional strain hardening ability of TWIP steels. In the first part, systematic experimental and modelling works have been carried out to study the deformation mechanisms of TWIP steels over a wide range of strain rates (〖10〗^(-4) to 〖10〗^3 (s^(-1)) and temperatures (room temperature to 973 K). It was found that the present TWIP steel exhibits a stable instantaneous strain rate sensitivity and a negative strain rate sensitivity of work-hardening rate, which is opposite to conventional face-centred cubic metals such as aluminium and copper. Based on the experiments, a constitutive model, which is strain rate- and temperature-dependent, was developed to reveal the underlying physics of the unusual strain rate sensitivity. Besides, it was found that there is a transition among different plasticity mechanisms, including deformation twinning, dynamic strain aging and dislocation slip, with temperature, leading to the change of the work-hardening rate and the resultant mechanical properties. In addition, systematic experiments have been conducted to quantify the respective hardening contributions of twins and dislocations. In contrast to the literature, it was found that twins contribute to a limited amount of strength. Instead, dislocations, via forest hardening, can account for up to ~90% of the flow stress increment after yielding, i.e. dislocation evolution dominates the work-hardening rate of TWIP steels. In the second part, the twinning mechanism in TWIP steels has been studied by systematic experimental and modelling works on micrometre- and nanometre-sized single crystal pillars. Emissions of partial dislocations from free surface (surface sources) and the pre-existing perfect dislocations inside the pillars (inner sources) are identified as essential processes for the formation of deformation twins. Based on experiments, a physically-based model, which integrates source introduction methods and source activation criterions for partial dislocation emission, is developed to predict the twin evolution. Besides, the critical twinning stress increases considerably at decreasing pillar diameter, demonstrating a substantial size effect with a power law exponent of 0.43. As revealed by the model, the increase of twinning stress in smaller pillars should be attributed to the reduction in the number of the pre-existing dislocations inside the pillar for twin nucleation and in the size of the radiation-induced surface dislocations for twin growth.