Characteristics of the allotropic phase transformation in titanium processed by high-pressure torsion using different rotation speeds

An investigation was initiated to examine the effect of rotation speed on the allotropic α to ω-phase transformation in titanium. A grade 2 commercial purity titanium was processed by high-pressure torsion (HPT) at room temperature up to a maximum of 10 turns using a pressure of 5.0 GPa and different rotation speeds from 0.5 to 2 rpm. It is shown that the allotropic phase transformation occurs during HPT at different rotation speeds but the volume fraction of the ω-phase decreases when the rotation speed is increased. The results indicate that the hardness and strength are decreased by processing the specimens at higher rotation speeds. Additional annealing for 15 min at 423 and 473 K after HPT processing is also effective in changing the volume fractions of the ω-phase and producing an ω to α reverse phase transformation. The results are interpreted in terms of the temperature rise during HPT processing.