Relationship between hardness and dislocation processes in a nanocrystalline metal at the atomic scale

By combining atomic force microscopy (AFM) and large-scale molecular dynamics (MD) simulations, we examine at comparable scales the atomistic processes governing nanohardness in electrodeposited nanocrystalline Ni with a mean grain diameter of 18.6 nm under confined contact deformation. Notably, this mean grain diameter represents the strongest size for Ni and other nanocrystalline materials where both crystal slip and grain-boundary deformation processes are intertwined to accommodate plastic flow. Accurate hardness measurements were obtained from shallow nanoindentations, less than 10 nm in depth, using an AFM diamond tip. We show evidence that the controlling yielding mechanism in the peak of hardness as a function of penetration depth corresponds to the emission of partial dislocations from grain boundaries. However, MD simulations also reveal for this grain size that the crystalline interfaces must undergo significant sliding at small penetration depths in order to initiate crystal slip. The strong interplay between intergranular and intragranular deformation processes found in this model nanocrystalline metal is discussed and shown to considerably reduce the local dependence of nanohardness on the initial microstructure at this scale, unlike past observations of nanoindentation in Ni electrodeposits with larger grain sizes. These new findings therefore constitute an important step forward to understanding the contribution of nanoscale grain-boundary networks on permanent deformation and hardness relevant for nanoscale materials and structures.