Higher-order boundary conditions for passivation mediated by plastic strain gradients

The theory of higher-order strain gradient plasticity is applied to investigate passivation effects, due to its superior capability in handling boundary conditions. In this study, a higher-order boundary condition accounting for the plastic strain gradient at the boundary layer is implemented based on the Fleck-Hutchinson-Willis theory. The role of the higher-order boundary condition is investigated under three non-proportional loading conditions: stretch-passivation, bending-passivation, and torsion-passivation. The higher-order boundary condition significantly reduces the strain hardening rate during plastic flow, in contrast to the conventional boundary condition where dislocations are fully blocked. The plastic strain gradient at the boundary layer controls the yield strength of the microscale metallic materials. The theoretical predictions align with the experimental results. This study provides valuable insight into the underlying mechanisms governing passivation effects in microscale metallic materials.