Elemental partitioning-driven β-phase stability engineering in Ti-3Al-5Mo-4.5V alloy: Unlocking hierarchical deformation mechanisms for aerospace fastener cold formability

Fundamental compression mechanisms in β-rich titanium alloys remain inadequately explored, hindering aerospace fastener cold-heading solutions. This study reveals unconventional β-phase regulation and β-phase-dominated compressive behavior in Ti-3Al-5Mo-4.5V via tailored cooling rates. FC (furnace cooling) facilitates sufficient Mo/V diffusion, achieving a stable β-phase with high Mo_eq. (18.8) and hardness (5.7 GPa, +160 % vs α-phase). Conversely, water quenching/WQ and air cooling/AC treatments generate metastable β-phases (Mo_eq. = 7.49–7.84) with elemental trapping (reduced hardness) and ω-phase precipitation. The preserved equiaxed β grain in FC enables a novel deformation hierarchy: β-phase's superior hardness initiates a α-phase preferential yielding → β-phase coordinated deformation sequence. β-phase deformation induces stress relaxation manifested as discontinuous yielding through macroscopic stress drops, while its superior stability enables enhanced plastic deformation capacity and reduced deformation resistance. Comparatively, AC exhibit diminished stress drop amplitudes due to reduced phase hardness contrast, though deformation-induced α″ phase formation strengthens work-hardening effects, resulting in more pronounced double yielding. For WQ, the highly metastable β-phase undergoes plastic deformation that triggers profuse nanoscale α-phase precipitation, elevating compressive resistance and initiating shear-dominated fracture. These findings redefine β-phase stability engineering in duplex titanium alloys, solving cold-heading instability for aerospace fasteners through β-dominated deformation mechanisms.