Abnormal strain rate strengthening and strain hardening with constitutive modeling in body-centered cubic {332}<113> TWIP titanium alloy

Metastable body-centered cubic titanium alloys exhibiting {332}<113> twinning-induced plasticity (TWIP) have attracted extensive attention owing to their remarkable strain hardenability and unprecedented impact performance. However, the intrinsic correlation between the strain rate, {332}<113> twinning, and flow stress remains to be fully elucidated. In this study, the instantaneous strain rate sensitivity (ISRS) and strain rate sensitivity of strain hardening (SRSS) corresponding to strain rate strengthening and strain hardening, respectively, were systematically clarified in {332}<113> Ti-15Mo alloy exhibiting TWIP effect by conducting strain-rate-jump tests and constitutive modeling analyses. The positive ISRS with an increasing trend correlated well with the change in the dislocation activation volume, which was mainly controlled by local obstacles of interstitial O atoms, and was affected by geometrically necessary dislocations due to twin formation. The negative SRSS with a decreasing trend correlated well with the evolution of deformation microstructures, which was mainly due to the decrease in the twinning rate and an increase in the dislocation annihilation rate. The established constitutive model reasonably described not only the abnormal strain rate strengthening from the thermal activation of dislocation but also the abnormal strain hardening from concurrent back stress strengthening due to twin formation and dislocation strengthening.