Microscopic plasticity and damage in two-phase steels: on the competing role of crystallography and phase contrast

This paper unravels micromechanical aspects of multi-phase metals whose microstructure comprises grains of two or more phases. The local plastic response is determined by (i) the relative misorientation of the slip systems of individual grains, and (ii) the different mechanical properties of the phases. The relative importance of these two mechanisms is unclear: is the plastic response dominated by the grain's anisotropy, or is this effect overwhelmed by the mechanical incompatibility between the two phases? The answer impacts the modeling of such a material, but also gives insights in the resulting fracture mechanisms. Until now, this question has been addressed only for particular crystallographies and mechanical properties. In contrast, this paper presents a systematic study including a large set of distributions, crystallographies, and material parameters. It is found that the global and the plastic local response of the two extreme modeling choices (full crystal plasticity on one end and full isotropic plasticity on the other end) converge with increasing phase contrast. The effect of the crystallography is completely overwhelmed by the mechanical incompatibility when the yield stress of the hard phase is a factor of four higher compared to the soft phase. When this ratio is lower than two, its influence may not be neglected. However, even in this regime, fracture initiation is controlled by the local arrangement of phases. The latter is quantified in this paper through the average arrangement of phases around fracture initiation sites.