The α↔ω phase transformations and thermal stability of Ti Co alloy treated by high pressure torsion

Omega Phase Formation in Ti–3wt.%Nb Alloy Induced by High-Pressure Torsion

“…For example, about 80% of the α-phase was transformed into the ω-phase in the sample pre-annealed at 450 °C, while in the sample pre-annealed at 600 °C, it was only 71%. Similar results were also obtained in a Ti–4wt.% Co alloy subjected to HPT under the same conditions [ 40 ], where it was observed that the α → ω phase transition during HPT was controlled by the cobalt concentration in the initial α-phase and microstructure peculiarities. The higher amount of ω-phase after HPT was observed at lower Co concentration and with smaller grains in the α- phase [ 40 ].…”

“…Similar results were also obtained in a Ti-4wt.% Co alloy subjected to HPT under the same conditions [40], where it was observed that the α → ω phase transition during HPT was controlled by the cobalt concentration in the initial α-phase and microstructure peculiarities. The higher amount of ω-phase after HPT was observed at lower Co concentration and with smaller grains in the αphase [40]. It should be noted that HAADF observation of the thin foil prepared from the deformed samples using the conventional twin-jet polishing technique was also performed.…”

“…This shift of the α-phase peaks can be associated with the appearance of the new α-phase with lower cobalt content, as well as with the increase of the lattice parameters due to thermal expansion. The same situation was observed in the case of a Ti-4wt.% Co alloy subjected to the HPT process [40].…”

“…The ω-phase is the metastable one, therefore the ω → α transition is exothermic. The exothermic nature of the reverse ω → α transformation was also observed in Ti–Fe- and Ti–Co-based alloys subjected to HPT deformation [ 29 , 40 ]. It can be seen in Figure 10 a that the amount of the ω-phase can be estimated by calculating the areas under the respective exothermic peaks.…”

“…Therefore, alloying with 3wt.% of Nb resulted in a twofold increase of the ω-phase volume fraction and an increase its thermal stability up to 250 • C. HPT of the Ti-4wt.% Co alloy led to the formation of the ω-phase in the amount of 65-80%, depending on the temperature of the preliminary annealing. Thermal stability of the cobalt-doped ω-phase was reached at around 450 • C [40]. In Ti-Fe alloys with iron content of 1, 2, 4 and 7 wt.%, HPT led to the formation of the ω-phase in amounts of 50, 80, 94 and 72%, respectively [29].…”