High-pressure torsion of titanium at cryogenic and room temperatures: Grain size effect on allotropic phase transformations

Titanium in the form of bulk and powder was processed by severe plastic deformation using high-pressure torsion (HPT) at cryogenic and room temperatures to investigate the influence of grain size on allotropic phase transformations. Almost a complete α (hexagonal close-packed, hcp) to ω (hexagonal) phase transformation occurred under a pressure of 6 GPa at room temperature until the grain size reached the submicrometer level, while the formation of β (body-centered cubic, bcc) phase was not detected. The ω-phase fraction and the ω → α transition temperature decreased with processing at cryogenic temperatures and/or with using powders, i.e. with decreasing the grain size to the nanometer scale during the deformation. First-principles calculations found the β phase to be dynamically unstable (neither stable nor metastable), while both α and ω phases are dynamically stable at 0 and 6 GPa. This explains why the β phase was not detected in this study using different methods such as X-ray diffraction analysis, high-resolution transmission electron microscopy, automated crystal orientation mapping and electrical resistivity measurements. Mechanical properties of the HPT-processed Ti were also examined.