Superplasticity mechanisms of a near-α titanium alloy: Competition and synergy of grain size and phase ratio

To satisfy the demands for lightweight design and high-temperature mechanical properties, Ti65 alloy has emerged as a promising structural material for critical aero-engine components. This study investigates the superplastic deformation behaviour, microstructure evolution and underlying mechanisms at 890–970°C and 0.0005–0.004 s⁻¹, focusing on the competitive and synergistic effects of grain size and phase ratio evolution on superplasticity. The results indicate that the Ti65 alloy exhibits exceptional superplasticity, achieving elongations exceeding 350% under all tested conditions and over 500% at 890–950°C or 0.0005–0.002 s⁻¹, reaching a peak elongation of 659.2% at 910°C and 0.001 s⁻¹. The strain rate sensitivity exponent (~0.48) and activation energy (~249.4 kJ/mol) analysed at 0.001 s⁻¹ confirm grain boundary sliding (GBS) as the dominant deformation mechanism. Microstructural observation reveals that, driven by dynamic strain, the β-phase fraction in the gauge region is significantly higher than that in the grip region, while the primary α grain size is substantially refined through continuous and discontinuous dynamic recrystallization (CDRX and DDRX). The enhanced α→β dynamic phase transformation at higher temperatures and lower strain rates increases the β-phase volume fraction from ~6.5% to ~57.8%, where a β-phase fraction not exceeding 50% helps reduce flow stress and promote GBS by providing more α/β phase interfaces with lower slip resistance. Conversely, as temperature increases and strain rate decreases, both the primary α and β-phase grains grow or even coarsen, leading to a reduction in grain/phase boundary density and subsequent deterioration of superplasticity. The contradictory evolution of grain size and phase volume fraction defines an optimal synergistic processing window, characterized by a β-phase fraction of approximately 10%-25% and an average primary α grain size below ~7.0μm, within which GBS efficiency and intragranular accommodation capability reach an optimal balance.