High-pressure torsion of pure cobalt: hcp-fcc phase transformations and twinning during severe plastic deformation

Cobalt with low stacking fault energy in the forms of bulk and powders was severely deformed using high-pressure torsion to investigate the influence of grain size on allotropic phase transformations. A phase transformation occurred from metastable fcc to hcp until the average grain size reached the submicrometer level. However, an hcp -> fcc phase transformation with nanotwin formation occurred together with the formation of a distorted hcp structure when the grain size was reduced well to the nanometer level (similar to 20nm). Mechanical and magnetic properties were also investigated. (C) 2013 AIP Publishing LLC.