A bimodal bulk ultra-fine-grained nickel: Experimental and micromechanical investigations

Bulk nanocrystalline and ultrafine-grained metals are materials having grain size in the submicron range and have motivate considerable attention due to their interesting physical and mechanical properties. An important issue in the field of submicron grain-sized materials is how to achieve both high strength and high ductility? It has been suggested that, one strategy for enhancing the ductility of high-strength nanocrystalline materials is to develop a bimodal grain-size distribution, in which the fine grains provide strength, and the coarser grains enable strain hardening. In this paper, we report on the micromechanical behaviour of bulk nickel samples having bimodal microstructures. The samples were processed by hot isostatic pressing of blends of nano and micrometer-sized powder particles. The resulting microstructure is a bimodal randomly distributed grains considered here as a mixture of two unimodal log-normal distributions. An efficient modelling approach (i.e. a generalized self-consistent approach) previously developed by Jiang and Weng, 2004a and Jiang and Weng, 2004b is then applied to such experimental data to investigate, among others, local plastic strains and internal stresses fields as well as local magnitudes deviations.