Size effects on yield instabilities in nickel

Dislocation events are seen as excursions, or pop-in events, in the load-displacement trace of nanoindentation experiments. When indenting single crystal metals these events occur frequently during quasi-static and dynamic loading. A single crystal of Ni (111) has been indented quasi-statically using three different loading rates (10, 100, and 1000 μN/s) as well as with three different radii diamond indenter tips (1000 nm cone, 300 nm Berkovich, and 50 nm cube corner) to examine the occurrences of excursions. As expected, excursions at higher loads have larger displacements, and that initial loading follows Hertzian behavior up to the point of yield. Also, as the tip size is reduced the excursion loads are reduced. The excursion events depend mostly on the statistical distribution of surface sources and substructure dislocation arrangements.