Evolution of the structure of an $\alpha$-titanium single crystal during high-pressure torsion

The microstructure of an iodide titanium single crystal subjected to severe plastic deformation in Bridgman anvils at a pressure of 8 GPa and room temperature or the liquid-nitrogen temperature is studied by electron microscopy and X-ray diffraction. The initial $\alpha$-titanium single crystal undergoes the baric $\alpha\to\omega$ transformation during compression and deformation under pressure. A decrease in the deformation temperature to 80 K is found to activate twinning. Upon deformation up to a strain $e$ = 6, the strain hardening at room temperature exceeds the low-temperature strain hardening. At higher strains, the room-temperature strain hardening begins to decrease as compared to the low-temperature strain hardening due to the development of dynamic and postdynamic recrystallization. A decrease in the deformation temperature to 80 K suppresses recrystallization; correspondingly, titanium deformed in liquid nitrogen exhibits a larger microhardness increment in the strain range 6 $< e <$ 10.