The significance of crystal structure on grain refinement during severe plastic deformation.

Abstract

This study focused on the significance of crystal structure on the grain refinement occurring during severe plastic deformation without considering the activation of softening mechanisms which inherently hinder grain refinement. Pure metals of Fe, Co and Ni, with relatively close melting points and initial grain sizes, were selected as models for BCC, HCP and FCC materials, respectively, for processing by high-pressure torsion under similar condition at room temperature. Severe plastic deformation of the BCC, HCP and FCC materials led to grain refinement with average grain sizes of ~90, ~60 and ~120 nm, respectively, with the FCC material showing the lowest hardness and microstrain among the studied materials. The formation of shear bands in the BCC and HCP materials were effective in microstructure fragmentation during straining which facilitated a finer grain size compared to the FCC material without shear bands. A significant grain refinement with the highest dislocation density was obtained due to an insufficient number of slip systems in the HCP material which encouraged deformation-induced twinning and a transformation for strain accommodation. The results demonstrate the high potential for achieving exceptional grain refinement in materials with an HCP crystal structure by comparison with BCC and then FCC materials.