Diffusive and displacive phase transitions in Ti–Fe and Ti–Co alloys under high pressure torsion

Abstract

The high pressure torsion (HPT) of Ti–4 wt. % Fe and Ti–4 wt. % Co alloys has been studied. Before HPT, the samples were annealed above and below the temperature of eutectoid transformation. After quenching, the first samples contained a mixture of α and β phases. The latter samples contained a mixture of α-Ti saturated with Fe or Co and respective intermetallic phases TiFe. During HPT the high-pressure ω-phase formed in all samples. The amount of ω-phase was higher when the initial samples contained the α+β mixture then in the case of α+TiFe or α+Ti2Co mixture. Most probably, the martensitic β→ω transformation occurs easier than the α→ω one. The amount of ω-phase was higher in Ti–Fe alloy than in the Ti–Co alloy for both initial states. It seems that cobalt additions disadvantageously change the lattice parameters in Ti for martensitic β→ω and α→ω transformations. The phase transformations with mass transfer (diffusive ones) also take place by HPT of Ti–Fe and Ti–Co alloys. Both α-based solid solutions partially decomposed after HPT. After HPT, the lattice parameters of α-phase containing cobalt or iron changed towards those of pure α-Ti. It has been observed for the first time that the composition of TiFe phase also changes after HPT.