Evidence for a transition in deformation mechanism in nanocrystalline pure titanium processed by high-pressure torsion

Nanocrystalline titanium with an average grain size of about 60–70 nm was prepared by high-pressure torsion. The results of hardness and structural evolutions indicate that a strain-induced hardening–softening–hardening–softening behaviour occurs. For coarse-grained titanium, ?a?-type dislocation multiplication, twinning and a high pressure-induced ?-to-? phase transformation play major roles to accommodate deformation, leading to a significant strain hardening. As deformation proceeds, dynamic recrystallisation leads to a decrease in dislocation density, especially for ?a?-type dislocations, leading to a slight strain softening. The ?c?-component dislocation multiplication dominates the deformation when the grain size decreases to 100 nm and ?c?-component dislocation multiplication, grain refinement and the ?-to-? phase transformation contribute to the second strain hardening. The following strain softening is attributed to dynamic recovery