Interface effects on the strength and ductility of bimodal nanostructured metals

Bimodal nanostructured (NS) metals possess both ultrahigh strength and good ductility. It is the nanograined (NG) matrix phase that leads to their ultrahigh strength and the coarse-grained (CG) inclusion phase that renders their good ductility. But the overall strength and ductility can also be significantly affected by the behavior of the interface regions. In this study, we employ a cohesive finite-element method to investigate the tensile fracture process of the bimodal NS Cu that includes the interface effects. We develop three types of cohesive elements in the bimodal NS Cu: (i) cohesive elements in the CG phase, (ii) those in the NG phase, and (iii) those at the CG–NG interface. Our objective is to uncover how the strength and ductility of the bimodal NS Cu can be affected by the interface property. In this process, we will also examine how the distribution and shape of the CG inclusions can contribute to the variation of the tensile fracture behavior of the bimodal NS Cu. By an extensive simulation, we find that, even at the small ratio of 1.6% of interface cohesive elements to all cohesive elements, a small change in the cohesive strength of interface elements could lead to a significant change in the overall strength and ductility. We also find that, when the cohesive strength of interface elements exceeds a certain level, the strength and ductility of the bimodal NS Cu will reach a saturation state.