We reported that work softening takes place during room-temperature rolling of nanocrystalline Ni at an equivalent strain of around 0.30. The work softening corresponds to a strain-induced phase transformation from a face-centered cubic ͑fcc͒ to a body-centered cubic ͑bcc͒ lattice. The hardness decreases with increasing volume fraction of the bcc phase. When the deformed samples are annealed at 423 K, a hardening of the samples takes place. This hardening by annealing can be attributed to a variety of factors including the recovery transformation from the bcc to the fcc phase, grain boundary relaxation, and retardation of dislocation gliding by microtwins. © 2008 American Institute of Physics. ͓DOI: 10.1063/1.3062849͔The deformation mechanisms of nanocrystalline ͑nc͒ materials are very different from those of coarse grain materials. [1][2][3][4][5][6][7] It has been found that in face-centered cubic ͑fcc͒ nc Ni, mechanisms such as deformation twinning, formation of extended and full dislocations from the grain boundaries ͑GBs͒, GB sliding, and grain rotation can contribute to plasticity. 6,[8][9][10][11][12][13][14][15][16] Recently, we have reported that for nc Ni, a phase transformation from fcc ͑␥͒ to body-centered cubic ͑bcc, ␣͒ can provide another deformation mechanism to realize plastic deformation to relatively large plastic strain during room-temperature rolling, 17 i.e., a far-fromequilibrium microstructure such as nc Ni can accommodate plastic strain via a change in lattice structure upon mechanical loading. In this letter we report that such a strain-induced phase transformation from ␥ to ␣ leads to work softening during cold-rolling and that subsequent annealing at 423 K results in hardening.Fully dense, electrodeposited nc Ni sheets ͑99.8% purity͒ were procured from Goodfellow, Inc. The as-received sheets were 200 m thick with an average grain size of about 20 nm. Samples of the nc Ni of 10ϫ 10 mm 2 in size were rolled at room temperature to various von Mises equivalent strains VM calculated as VM = ͉2 / ͱ 3ln͑1+␦͉͒, where ␦ is the rolling reduction. The microstructure of the samples was examined using x-ray diffraction ͑XRD͒ ͑Riguta D/max-RC͒ and high-resolution transmission electron microscopy ͑JEM 2010F operated at 200 kV͒. Microhardness measurements were taken using a load of 0.196 N ͑HVS-1000͒. The reported hardness values are each averaged from three indents. The hardness provides a measure of the resistance to deformation by surface indentation or by abrasion and is a combined property of elasticity, plasticity, strength, and toughness. However, the hardness value is determined by the resistance both to initial and continuous plastic deformation. In general, a high hardness corresponds to a high tensile strength, although there is not an exact relationship between the two values. Considering the small dimensions of the deformed samples, in the experiment we use the sample hardness to investigate changes in the mechanical properties.The experimental result shows that once a deformation strain ...

No abstract