The Phase-Dependent Behavior of High-Temperature Deformation in Ti65 Titanium Alloy: Flow Softening, Recrystallization Mechanisms, and Slip System Evolution

This study investigates the flow behavior, dynamic recrystallization mechanisms, and slip system evolution of the Ti65 high-temperature titanium alloy in both the α+β biphasic region and the β single-phase region through isothermal compression tests. The results show that the flow stress in the α+β biphasic region is significantly higher than that in the β single-phase region, and the stress–strain curves exhibit pronounced softening behavior. The deformation activation energy in the α+β biphasic region (600–638 kJ·mol -1 ) is considerably higher than that in the β single-phase region (151–240 kJ·mol -1 ). With increasing temperature, the grain size increases markedly. In the α+β biphasic region, continuous dynamic recrystallization (CDRX) dominates, with recrystallized nuclei forming via subgrain rotation and coalescence. In contrast, the β single-phase region is mainly governed by discontinuous dynamic recrystallization (DDRX) and dynamic recovery (DRV), where nucleation occurs at grain boundaries through a bow-out mechanism. Accordingly, the dominant recrystallization mechanism gradually transitions from CDRX to DDRX with increasing temperature. Geometrically necessary dislocation (GND) density analysis reveals that low-temperature and high-strain-rate conditions lead to higher GND density and thus greater stored energy. At 900 °C, the stored energy reaches 42.3 kJ, which promotes recrystallization. Schmid factor analysis indicates that plastic deformation in the α phase is primarily accommodated by the prismatic and <c+a> pyramidal slip systems. As temperature increases, the α→β phase transformation is promoted, activating multiple slip systems in the β phase. The enhanced cooperative deformation capability of the β phase, together with the transition in recrystallization mechanisms, effectively suppresses macroscopic flow softening. • It is quantitatively shown for the first time that the deformation activation energy of Ti65 alloy in the α+β phase region(600–638 kJ·mol -1 ) is significantly higher than in the β single-phase region(151–240 kJ·mol -1 ). • It is clarified that continuous dynamic recrystallization (CDRX) dominates in the α+β phase region, while discontinuous dynamic recrystallization (DDRX) and dynamic recovery dominate in the β phase region. • At low temperatures and high strain rates, the high dislocation density in the α+β region significantly increases stored energy, driving the CDRX mechanism. • Schmid factor analysis shows that deformation in the α+β region is dominated by HCP and <c+a> slip systems, while the β phase region activates more BCC slip systems. • A unified mechanism framework is established, elucidating the relationship between dislocation, stored energy, recrystallization, slip systems, and rheological behavior.