The compression deformation behavior of electrodeposited nanocrystalline copper pillars with average grain sizes (d) of 360, 100, and 34 nm has been investigated as a function of specimen size (D). The yield stress for nanocrystalline pillars with d = 360 and 100 nm does not depend on specimen size, exhibiting essentially the bulk yield stress until the specimen size is reduced down to the critical values ((D/d)∗ = 35 and 85), below which the yield stress decreases with the decrease in specimen size. In contrast, the yield stress for nanocrystalline pillars with d = 34 nm does not depend much on specimen size, exhibiting the bulk yield stress value for all specimen sizes investigated. The dominant deformation mechanism changes from dislocation glide for pillars with d = 360 and 100 nm to grain boundary diffusional creep for pillars with d = 34 nm. Grain-size induced softening occurs for pillars with d = 34 nm being consistent with the occurrence of change in deformation mechanisms, whereas the bulk yield stress for pillars with d = 360 and 100 nm increases with the decrease in grain size according to the classical Hall–Petch relationship. The critical (D/d)∗ values determined for nanocrystalline Cu pillars with d = 360 and 100 nm increases with the decrease in grain size so as to conform to the same power law scaling obtained for coarse-grained Cu polycrystals. This is the first indication that the specimen size-induced softening extends from micrometer to nanometer scales as far as the dominant deformation mechanism is dislocation glide. The considerably large critical (D/d)∗ values determined for nanocrystalline Cu pillars with d = 360 and 100 nm are discussed in terms of strain continuity among neighboring grains and the generation of geometrically necessary dislocations to maintain strain continuity at the grain boundaries.
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