Transformations of α` martensite in Ti–Fe alloys under high pressure torsion

Titanium and titanium alloys are attractive materials for various applications due to their high strength-to-density ratio, excellent corrosion resistance, and good biocompatibility. To improve capability of the titanium based alloys, severe plastic deformation revealed to be a powerful tool for tailoring their structure and properties.In the present work, we observed for the first time the phase transitions in α`-Ti martensite driven by the high pressure torsion (HPT) as well as α` to ω, high-pressure phase, transformations in Ti–Fe alloys. The solubility of Fe in β-Ti is high (up to 25 wt. %), but in α-Ti it does not exceed 0.05 wt. %. The as-cast alloys were initially transformed into the α`-Ti martensite after annealing in the β-(Ti,Fe) solid solution region and subsequent quenching. Pure titanium and four Ti–Fe alloys with 0.5, 1, 2.2 and 4 wt.% Fe were subjected to HPT at 7 GPa at room temperature. During HPT, α`-Ti martensite transformed into the mixture of ω- and α-phases. However, at first HPT decreases the iron content in the α`-Ti martensite, “purifies” it and drives the α`→α transformation. X-ray diffraction measurements clearly indicate increased lattice parameters up to the level of pure α-Ti subjected to HPT. Our results point towards the following consequence of phase transformations: α` → α and α → ω in Ti–Fe alloys at HPT. The reason of accelerated mass transfer is not only the shear-driven atom movement but also the extremely high concentration of non-equlibrium vacancies created during HPT at room temperature.