Instability of Omega phase induces synchronously improved strength and plasticity of metastable β-titanium alloys

Abstract

Metastable β-Titanium alloys combine excellent work-hardenability and good plasticity due to phase transformation induced plasticity (TRIP) and/or twinning induced plasticity (TWIP) effects, but exhibit poor yield strength. While isothermal ω-phase (ω_iso) precipitation effectively strengthens these alloys, it typically compromises their plasticity. This study develops a strategy to harness ω-phase precipitation for simultaneously enhancing tensile strength and maintaining plasticity in metastable β titanium alloys. Experimental results reveal that ω-phase particles significantly improve yield strength while initiating localized necking through dislocation channel formation. This necking facilitates the rapid multiplication of dislocations in the strain-localization area, leading to forest dislocation hardening. Additionally, there is extra work hardening due to the interaction of dislocations with stress-induced twins and α'' martensites. This dual work-hardening mechanism restrains and stabilizes premature necking. Continuous interactions between ω-phase particles and lattice defects (dislocations, twins, α" martensites), coupled with subsequent hardening from dislocation-defect interplay, induce two distinct necking events prior to final fracture. Each necking stage triggers corresponding hardening responses that progressively regulate deformation behavior. Consequently, an exceptional strength-ductility synergy is achieved, resulting in a 680 MPa yield strength and 51.77 % elongation after 150 °C aging. This work provides an instability-control strategy that coordinates forest hardening with dislocation-twin/α" martensites interaction hardening, thereby synchronously improving strength and plasticity (SISP) of metastable β titanium alloys.