Manipulating the orthorhombicity of α" martensite: A novel strategy to tailor yield ratio and break strength–ductility trade-off in titanium alloys

In addressing the critical challenge of tailoring yield ratio and breaking the strength–ductility trade-off in titanium alloys, we have proposed a novel design strategy by forming a dual-phase microstructure, which consists of primary α phase (α_p) and α" martensite in a type of titanium (Ti) alloy with an aluminum equivalency ([Al]_eq) of approximately 6 and a molybdenum equivalency ([Mo]_eq) in the range of 7–8 (i.e., Ti-6[Al]_eq-7∼8[Mo]_eq). Through precise manipulation of the orthorhombicity of α" martensite via solution treatment and low-temperature ageing, the yield strength of the Ti6Al4V5.5Cu (wt.%) model alloy can be progressively enhanced from 465 MPa to 1050 MPa. Meanwhile, the great elongation (ε_p, ∼17 %) and high ultimate tensile strength (UTS, ∼1200 MPa) are preserved, thus achieving a desirable combination of yield strength and yield ratio over a wide range and overcoming the yield strength–ductility trade-off simultaneously. The adjustable yield strength and yield ratio arise from the modulation of the orthorhombicity of α" martensite, and the great ductility and work-hardening capacity are attributed to the strain-induced continuous transformation of α" martensite from an orthorhombic structure to a near-hexagonal structure during tensile deformation. The diverse combinations of tensile properties achieved through this α" martensite orthorhombicity engineering provide new insights for designing versatile Ti alloys, enabling them to meet the demands of various processing and application scenarios.