Elucidating the effect of bimodal grain size distribution on plasticity and fracture behavior of polycrystalline materials

International audienceThe refinement of grains in a polycrystalline material leads to an increase in strength but as a counterpart to a decrease in elongation to fracture. Different routes are proposed in the literature to try to overpass this strength-ductility dilemna, based on the combination of grains with highly contrasted sizes. In the simplest concept, coarse grains are used to provide relaxation locations for the highly stressed fine grains. In this work, a model bimodal polycrystalline system with a single coarse grain embedded in a matrix of fine grains is considered. Numerical full-field micro-mechanical analyses are performed to characterize the impact of this coarse grain on the stress-strain constitutive behaviour of the polycrystal: the effect on plasticity is assessed by means of crystal plasticity finite element modelling [1] while the effect on intergranular fracture behaviour is studied by using boundary element modelling [2]. The analysis of the computational results, compared to the experimentally characterised tensile properties of a bimodal 316L stainless steel, suggests that the elasto-plastic interactions taking place prior to micro-cracking may play an important role in the mechanics of fracture of this steel