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 (∼20 nm). Mechanical and magnetic properties were also investigated.
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