Phase transformations during high-pressure torsion (HPT) in titanium, cobalt and graphite

Abstract

Several elements were processed by high-pressure torsion and phase transformations were investigated. Titanium exhibited a transition from hcp to an ω phase at pressures higher than 4 GPa, while the transition was facilitated with increasing the shear strain. The stability of ω phase appeared to decrease with decreasing the grain size to the nanometer level (e.g., by processing at cryogenic temperatures and/or by consolidation of powders). Cobalt exhibited a transition from metastable fcc to hcp until the grain size reached the submicrometer level, but the fcc phase appeared to be more stable with decreasing the grain size to the nanometer level. Graphite exhibited a transition to diamond-like carbon (DLC), while the formation of DLC was facilitated with increasing the pressure, temperature and strain.