Formation of the $\omega$ phase in the titanium–iron system under shear deformation

The effect of the phase composition on the $\alpha/\beta$-Ti(Fe)$\to\omega$-Ti(Fe) transformation in the Ti-4 wt % Fe alloy under shear strain with high-pressure torsion (HPT) has been studied. For shear deformation by means of HPT, two initial states of the alloy were used, which significantly differed in the morphology of the phases and the concentration of iron atoms in the $\beta$ phase. During HPT, a stationary state occurred in both sample series, which is characterized by the presence of a single $\omega$ phase containing 4 wt % Fe and by a grain size of about 200 nm. Thus, the HPT state is equifinal and independent of the initial phase composition of the samples. It was found that under the influence of HPT in Ti-4 wt % Fe alloys not only martensitic (shear) transformation into the $\omega$ phase occurs, but also a significant mass transfer of atoms of the alloying element. An analysis of the change in the torsion torque directly in the HPT process made it possible to estimate the rate of deformation-induced mass transfer. It is 18–19 orders of magnitude higher than the rate of conventional thermal diffusion at the processing temperature $T_{\text{HPT}} = 30^\circ$ C, while it is close to the diffusivity values at 700–800$^\circ$ C. This is because HPT increases the concentration of lattice defects, which in turn is equivalent to an increase in temperature. A similar combination of accelerated mass transfer during HPT and martensitic (shear) transformation was previously observed in copper-based shape memory alloys, but for the first time studied for the formation of $\omega$-phase in titanium alloys.