Plastic deformation induced hexagonal-close-packed nickel nano-grains

Nickel has a stable face-centered-cube lattice structure with a high stacking fault energy. No phase transition has been identified in Ni at elevated temperatures and under high pressure up to 200 GPa. In this study, we discovered formation of hexagonal-close-packed Ni nano-grains induced by plastic deformation. In cryogenic deformation of Ni nano-grains below a critical size of 17 nm when both dislocation activities and grain boundary processes are suppressed, an FCC-to-HCP transformation is triggered forming a mixture of FCC and HCP nano-grains. The finding offers a novel approach to tailor lattice structures of metals that are stable in coarse-grained form. (C) 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.