Size, geometry and nonuniformity effects of surface-nanocrystalline aluminum in nanoindentation test

In the present research, the mechanical behavior of the surface-nanocrystalline aluminum (SNCA) is investigated through nanoindentation experiment and theoretical modeling. Firstly, through microscopical observation and measurement for the SNCA material, a microstructure cell model is developed. Secondly, based on the microstructure cell model and the strain gradient plasticity theory, and based on introducing a parameter accounting for the grain size nonuniformity effect, the discrete features of the hardness–depth relations of the SNCA material are described. The “U-type” feature of the hardness–depth experimental curves is modeled and simulated. Thirdly, in the SNCA material the mechanical property of the grain boundary, i.e., the strength of plastic zone penetrating the grain boundary is characterized by introducing a criterion parameter, the critical effective plastic strain. The “waterfall-type” feature of the hardness–depth curves is modeled and simulated. It is worth pointing out that, in the present study, the length scale parameter in the strain gradient plasticity theory is taken as a universal material parameter instead of a simulation parameter, and it is determined through applying the strain gradient plasticity theory to the modeling of the corresponding single crystal aluminum.