Grain size gradient and length scale effect on mechanical behaviors of surface nanocrystalline metals

Substantial research has shown the remarkable mechanical properties such as high stress and excellent strain hardening of nanocrystalline (NC) materials with a grain size gradient on their surface region. Here, a constitutive model of the NC gradient metals is established considering the dislocation interaction among adjacent phases with different grain sizes by employing a rate-dependent stress gradient plasticity model. A material length scale which captures the size-dependent effect is introduced as the single fitting parameter under the physical meanings of the distance between two dislocation obstacles. The simulation results for the gradient NC coppers considering the nano-grain growth mechanism as well as the coarse grain (CG) coppers are in good agreement with the experimental data. The overall mechanical behaviors of the NC gradient metals can be optimized considering the effects of the NC layer thickness as well as the NC grain size distribution. Moreover, the strength-ductility tradeoff can be evaded for the NC gradient metals based on the failure analysis. Additionally, the introduced material length scale has a salient effect on the yield stress as well as the failure behavior. This proposed model can be served in designing functional gradient NC metals considering the stress gradient caused by the dislocation interaction in the NC phases with gradient grain size distribution.