Modelling of the intergranular fracture of TWIP steels working at high temperature by using CZM–CPFE method

Abstract

Grain boundaries (GBs) of metallic materials will become the weakest and most vulnerable place when the materials deform at high temperature. This could directly affect deformation flow, plastic strengthening, the initiation and propagation of microcracks, and finally the fracture failure of materials. In this research, to reveal the high temperature deformation mechanisms of twinning–induced plasticity (TWIP) steels, the crystal plasticity finite element method (CPFEM) was employed to model the mechanical response of grain interiors. The cohesive zone model (CZM) of the bi–linear traction separation law (TSL) was developed to describe the damage and failure of GBs. By combining CZM and CPFEM, an analysis method was proposed to investigate the effect of the microstructure morphology, orientation and size of grains, and temperature. Furthermore, the damage initiation, accumulation and fracture at GBs were represented by the representative volume element model based on the CZM–CPFE method to explore the influences of GB angle, grain size and local microvoids (including the location and size) on the GB cracking. The stress damage cloud maps, the distribution of quadratic stress damage initiation criterion (QUADSCRT) and the scalar stiffness degradation (SDEG) describing the damage and fracture along the GBs were used to articulate the relationships of GB angle, size effect, local microvoids and the expected failure strain. The results show that low angle grain boundaries (LAGBs), the small average grain size (<21.2 μm), microvoids at the horizontal GBs and microvoids with a diameter below 1.5 μm can delay the initiation of microcracks and fracture failure along the GBs. The predicted damage and failure of GBs and the intergranular cracking characteristics in plastic deformation of TWIP steels at high temperature were corroborated by microscopic in–situ SEM experiments, in which the deformation characteristics inside the grains, and microcrack initiation and propagation near GBs were observed at 500 and 750°C. This research enhances the understanding of intergranular fracture, mechanical response and microstructure evolution of TWIP steels worked at different temperatures, and also provides a new way and strategy for studying the deformation behaviours of TWIP steels considering GB properties.