Dynamic recrystallization of titanium: Effect of pre-activated twinning at cryogenic temperature

Abstract A pre-cold-deformation process was applied for commercially pure titanium at cryogenic temperature to activate high-density deformation twins, and subsequent hot-deformation was used to induce dynamic recrystallization (DRX). Three major types of twins were effectively activated during cryogenic deformation, including { 11 2 ¯ 2 } contraction twins, { 11 2 ¯ 1 } extension twins, and { 10 1 ¯ 2 } extension twins. A special type of slipped { 11 2 ¯ 1 } “twins” was also activated by a sequential effect of twinning and slip. Selection of twinning variants followed Schmid's law well, where only the twinning systems with a Schmid factor of m ≥ 0.4 could be activated. The pre-activated twinning led to a remarkably stable flow stress at 500 °C up to a true strain of 1.0, due to the attainment of dynamic equilibrium between strain hardening and high-temperature softening. Two DRX stages occurred: (1) twin-active DRX stage, and (2) discontinuous dynamic recrystallization (DDRX) stage. The DRX mechanisms identified were twinning-induced DRX (or TDRX) and DDRX. While the low-temperature slip alone had little influence on DRX, the pre-activated { 11 2 ¯ 2 } twins, { 11 2 ¯ 1 } twins, { 10 1 ¯ 2 } twins and slipped { 11 2 ¯ 1 } “twins” contributed effectively to DRX in the form of spheroidization of twin lamellae due to twin-dislocation interactions, leading to a substantial grain refinement from ∼41 to ∼1 μm during subsequent hot-deformation.