Modelling of Interfacial Problems at Various Length Scales in Polycrystalline Materials

Abstract

The goal of this research project was to develop a modelling technique making it possible to simulate grain boundaries and inclusion-matrix material interfaces using numerical techniques. This is particularly of interest when investigating the effects of grain boundary sliding and decohesion of the material interfaces under hot deformation conditions. This can then be used to predict damage nucleation and growth at the grain boundaries applicable to both creep type damage and plasticity induced damage. A novel scheme was developed based on the Controlled Voronoi Poisson’s Tessellation technique (CVPT) to generate statistically equivalent microstructures based on the physical parameters of the microstructure in free cutting steel under hot forming conditions. The generated microstructure was then used to study the effect of different parameters on the global response of the material. Furthermore, this model was utilised to calibrate the crystal plasticity material model using the experimental data available for high strain rate deformation of free cutting steel at elevated temperatures. A micro-scale Representative Volume Element (RVE) was developed in which the grain boundaries and material interfaces have been represented by cohesive elements. The RVE consisting of an MnS inclusion surrounded by four austenitic grains was used to study the effect of inclusion orientation, grain orientations and the relative strength of grain boundaries and the matrix/inclusion interfaces on overall failure of the RVE. Furthermore, in the endeavour of finding the right modelling technique an extension to the conventional finite element method called XFEM proved to be capable of modelling strong and weak discontinuities independent of the FE mesh. The crystal plasticity material model was implemented in the open-source FE package OOFEM and was used to simulate interface decohesion and grain boundary motion using the XFEM technique.