Shear induced ω-phase in titanium

The metastable omega-phase of titanium, which has a nanocrystalline structure (with a crystallite size of 50 nm) was formed by severe plastic deformation (SPD), namely, torsion under high quasi-hydrostatic pressure (6 GPa). It is shown that after a short incubation period (300 seconds), more than 90% of alpha-titanium is transformed into omega-phase, while the static load applied for the same time does not lead to phase transformation. TEM shows that the omega-phase is formed near the grain boundaries (GBs) of the the initial phase, which allows to hypothesize that the alpha ->omega martensitic transformation occurs due to the increase of internal stress near the grain boundaries, greatly exceeding the external load and is the channel of dissipation of plastic energy.