Elucidating the Effect of Bimodal Grain Size Distribution on Plasticity and Fracture Behavior of Polycrystalline Materials

Abstract

The 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 dilemma, 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 behavior of the polycrystal: the effect on plasticity is assessed by means of crystal plasticity finite element modeling [B. Flipon, C. Keller, L. Garcia de la Cruz, E. Hug and F. Barbe, Tensile properties of spark plasma sintered AISI 316L stainless steel with unimodal and bimodal grain size distributions, Mater. Sci. Eng. A 729 (2018) 248–256] while the effect on intergranular fracture behavior is studied by using boundary element modeling [I. Benedetti and V. Gulizzi, A grain-scale model for high-cycle fatigue degradation in polycrystalline materials, Int. J. Fract. 116 (2018) 90–105]. The analysis of the computational results, compared to the experimentally characterized 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.

An integral framework for computational thermo-elastic homogenization of polycrystalline materials

“…Computational tools based on integral equations and on the employment of the boundary element method (BEM) [15,16] for their solution have been already successfully developed for the analysis of polycrystalline materials, both in 2D [17] and 3D [18,19], and have been applied to either computational homogenization [20], multiscale materials modelling [21,22,23] or micro-cracking analysis [24,25,26], also considering piezo-electric polycrystals [27], high-cycle and low-cycle fatigue [28,29] and hydrogen assisted cracking [30]. Such formulations, often built on Voronoi tessellations [31], which provide a reasonable approximation of the microstructural morphology [32,33], are expressed uniquely in terms of displacements and tractions of points belonging to the boundary of the crystals in the aggregate, thus providing a reduction in the number of degrees of freedom needed for analysing a give microstructure.…”