Deformation behaviour of TWIP steels: Constitutive modelling informed by local and integral experimental methods used in concert

This article addresses the influence of stacking fault formation on the deformation and strain hardening behaviour of steels showing the twinning-induced plasticity effect. The role of the twin nucleation process was demonstrated on the results of mechanical testing, microstructure analyses, and constitutive modelling that were performed exemplarily for a high-alloy austenitic steel X2CrMnNi16-7-10. The microstructural evolution of the steel was studied in situ by acoustic emission measurements and post mortem by X-ray diffraction, and by scanning and transmission electron microscopy. The acoustic emission data provided information about the movement of partial and perfect dislocations and about the onset and extent of the deformation-induced twinning. The X-ray diffraction quantified lattice strains caused by the microstructure defects, and revealed averaged dislocation densities and stacking fault probabilities. The scanning electron microscopy delivered grain sizes and dislocation densities and unravelled the relationship between local grain orientation and the governing deformation mechanism. Transmission electron microscopy confirmed the validity of the microstructure model that was used for evaluation of the X-ray diffraction data, and visualized the interplay between individual deformation mechanisms. For the description of the mechanical response of the TWIP steel, a constitutive model was formulated, and supplied with the entirety of the experimental data. This approach helped to reduce the number of the free parameters of the constitutive model. Even with less free parameters, the model was able to describe the strain hardening of the steel reliably. It was found that, in contrast to dislocation motion, deformation twinning contributes to strain only marginally. The major impact of twinning on the enhanced plasticity arises from the reduction of the dislocation mean free path and the associated dislocation-related strain hardening. Based on the experimental data and model predictions, general guidelines for austenitic steel design were established.