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

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) over bar2} contraction twins, {11 (2) over bar1} extension twins, and {10 (1) over bar2} extension twins. A special type of slipped {11 (2) over bar1} 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 degrees 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) over bar2} twins, {11 (2) over bar1} twins, {10 (1) over bar2} twins and slipped {11 (2) over bar1} 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 similar to 41 to similar to 1 mu m during subsequent hot-deformation. (C) 2018 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.