Effect of grain size and temperature on mechanical properties of nanocrystalline nickel

Molecular dynamics simulations were employed to perform the nanoindentation analysis of nanocrystalline nickel with polycrystalline crystal structure at different grain sizes and temperatures under an indenter of spherical shape. The results indicate that during the nanoindentation process, dislocations are mainly present near grain boundaries. Atomic rearrangements occur around the indentation area, which leads to the formation of an amorphous region along the grain boundaries. The indentation region contains dislocations and amorphous structures. As the grain size decreases, the indentation stress decreases. However, for grain sizes below 13.7 nm, a reverse Hall-Petch relationship is observed between grain size and hardness values. The elastic recovery rate in the depth direction increases with increasing grain size, and is greater than that in the width direction, so the influence of the grain size on the elastic recovery rate in the width direction is very small. At higher simulation temperatures, the load-displacement curve during nanoindentation exhibits significant fluctuations. The higher the simulated temperature, the greater the fluctuation of the load-displacement curve. The hardness values reaches the maximum value at 100 K, and then decrease with the increase of temperatures. When the indentation depth remains constant, the number of atoms experiencing higher shear strains increases with increasing temperature.