The interdependence of microstructure, strength and fracture toughness in a novel β titanium alloy Ti–5Al–4Zr–8Mo–7V

In order to study the correlation between microstructure, strength and fracture toughness (KIC) and optimize the strength-KIC combination, BASCA (β annealing slow cooling plus aging) and STA (α/β solution treatment plus aging) heat treatment followed by near β forging is conducted to obtain a variety of multiscale lamella structure and Bi-modal structure. The results show that multiscale lamella structure exhibits good strength-toughness combination. The fracture toughness is ~52 MPa⋅m0.5 at the strength level of ~1460 MPa. By decreasing the tensile strength to ~1260 MPa, the fracture toughness increases to ~64 MPa⋅m0.5. In both lamella structure and Bi-modal structure, fracture toughness increases with decreasing yield strength which is attributed to a lager plastic zone at crack tip of lower strength. A linear relationship between yield strength and KIC gives a good fit in multiscale lamella structure. This suggests that continuum property (e.g. strength and ductility) governs KIC in the same microstructure type. Comparing different microstructure type, multiscale lamella structure exhibited higher KIC than Bi-modal structure at similar strength level. This is mainly attributed to the effect of back stress during deformation. A lower α/β interphase stress (back stress) and homogeneous strain distribution are exhibited in lamella structure which retards the crack propagation and increases the KIC. Meanwhile, coarse α lamella in multiscale lamella structure promotes the crack deflection. This crack shielding decreases the effective stress intensity (Keff) at crack tip and increase fracture toughness accordingly. Our study indicates that forming multiscale lamella α phase by BASCA treatment is an effective approach to overcome the mutually exclusive properties of strength and fracture toughness in high strength β titanium alloys. Multiscale lamella structure improves KIC by both intrinsic toughening (a more homogeneous strain distribution) and extrinsic toughening (improved crack propagation resistance).