On the strengthening behavior of ultrafine-grained nickel processed from nanopowders

Bulk ultrafine-grained nickel specimens having grain sizes in the range of 0.25–5 μm were processed by a spark plasma sintering method. The resulting microstructures were characterized by electron backscattering diffraction, transmission electron microscopy and X-ray diffraction analysis. Compression tests were carried out at room temperature and at a strain rate of 1.6 × 10−4 s−1. It was found that the fine-grained microstructure and the presence of NiO phase were the main strengthening factors in the as-processed bulk materials. The contribution of the oxide phase to strengthening was even more pronounced for lower grain sizes. This contribution was calculated as the difference between the measured strength and the value obtained from a Hall–Petch plot of oxide-free samples, and this yielded a flow stress increment of about 635 MPa for the lowest grain size studied here. In addition, a transition from work-hardening to -softening occurred for materials having a mean grain size smaller than about 300 nm and having boundaries that could have been weakened by the presence of a high amount of NiO phase.