Role of dislocation density on the sample-size effect in nanoscale plastic yielding

The sample-size dependence of plastic yielding in nanoscale metal pillars is not a purely geometrical fact, explainable just in terms of shape and geometry of the pillar, but depends on the actual content of inner dislocations. The present study discusses the role of the initial density of bulk dislocations ρ on the power-law scaling that describes the strengthening effect in yield stress. This paper reports on in-situ mechanical tests on highly miniaturized focused-ion-beam (FIB)-machined Ni pillars and buttons (i.e., very short pillars) of diameters D ~ 200-300 nm specifically conducted to highlight the decrease in hardness and the disappearance of the power-law scaling that may accompany a controlled decrease in ρ. From an experimental standpoint, it is further demonstrated that nanoindentation tests on pillars make for a useful companion test to standard microcompression to assess actual hardness increase with reducing sample size.