The ultra-small strongest grain size in nanocrystalline Ni nanowires

Here we report the finding that the strongest grain size to achieve the maximum flow stress in nanocrystalline Ni nanowires was significantly smaller (d = 5 nm) than that in typical bulk metals (d > 10 nm). The discrepancy was due to the dominance of plasticity via surface slip in Ni nanowires at relatively large grain sizes, which was absent in bulk materials. An interesting grain size- and temperature-dependent “hardening”-to-“softening” transition in Ni nanowires was also revealed in this study.