LAUSR

Abstract Gradient-nanograined metals with high strength-ductility synergy would be used in applications of advanced coating materials. An in-depth understanding of their strengthening mechanisms plays a key role in the widespread use of gradient-nanograined structure for industrial area. Here, using molecular dynamics simulations, we report the plastic deformation response of gradient-nanograined metals, which depend on the three conditions: (1) the inverse Hall-Petch relationship, (2) the inverse and classic Hall-Petch relationship, (3) the classic Hall-Petch relationship. We also compare the difference of the plastic behavior between gradient-nanograined and random-nanograined body-centred cubic Fe films. The results show that the yield strength of gradient-nanograined Fe exceeds that of random-nanograined Fe, which is in good agreement of the previous experimental observation. It is owing to that the strong gradient stress and strain always occur during the deformation process of the gradient-nanograined structure. Interestingly, the deformation of middle region in nanograined film depends on the gradient stress, and the deformation of surface layer is controlled by the gradient strain. The theoretical model shows the gradient plastic strain relies on the distribution and size of the nanoscale grains, especially for below the critical grain size. Our results reveal the strengthening mechanisms of body-centred cubic Fe film at a wide size range of nanoscale grain, and have broader implications for designing the high strength of metallic thin films by high-throughput molecular dynamics simulations combined with experiments.