Grain size effects on the mechanical properties of nanonickel examined by nanoindentation

Pulse-electrodeposited nanonickel was heat-treated to produce grain sizes from nanoscale to microscale. The hardness of individual grains was examined by nanoindentation. The results show that hardness not only depends on the grain size, but also on the ratio of grain size to the indent size. Beside of the conventional Hall-Petch relation different dependencies were found. The hardness scaled with the dislocation density in the range where the indent size was smaller than the grain size, when a critical load was reached in a single grain, evidence for dislocation emission in adjacent grains could be deduced from later pop-ins in the load-displacement curve.