Influence of grain size on the strength size dependence exhibited by sub-micron scale nickel structures with complex cross-sectional geometries

Sub-micron scale columnar nickel structures with different cross-sectional geometries and grain sizes were successfully fabricated via electron beam lithography and electroplating methods. Uniaxial microcompression tests revealed the flow stresses of these structures increase with smaller grain sizes, in close agreement with the Hall-Petch relationship. Size dependent softening effects were only observed in nickel pillars with average grain sizes in the range of 9.4-13.2 nm. In contrast, pillars with grain size near ~22 nm do not exhibit any flow stress dependence on size, with the pillar strengths remaining relatively constant. In addition, for nickel pillars with the same microstructures, there is no observable dependence of flow stresses on the cross-sectional geometries.