In order to study the correlation between microstructure, strength and fracture toughness (K_IC) and optimize the strength-K_IC 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⋅m^0.5 at the strength level of ∼1460 MPa. By decreasing the tensile strength to ∼1260 MPa, the fracture toughness increases to ∼64 MPa⋅m^0.5. 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 K_IC gives a good fit in multiscale lamella structure. This suggests that continuum property (e.g. strength and ductility) governs K_IC in the same microstructure type. Comparing different microstructure type, multiscale lamella structure exhibited higher K_IC 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 K_IC. Meanwhile, coarse α lamella in multiscale lamella structure promotes the crack deflection. This crack shielding decreases the effective stress intensity (K_eff) 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 K_IC by both intrinsic toughening (a more homogeneous strain distribution) and extrinsic toughening (improved crack propagation resistance).

为了研究微观结构，强度和断裂韧性（K _IC）之间的关系并优化强度-K _IC组合，先进行BASCA（β退火慢冷加时效）和STA（α/β固溶处理加时效）热处理，然后进行热处理进行近β锻造以获得各种多尺度的片状结构和双峰结构。结果表明，多尺度薄片结构表现出良好的强度-韧性组合。在〜1460MPa的强度水平下，断裂韧性为〜52MPa·m ^0.5。通过将抗拉强度降低至〜1260 MPa，断裂韧性提高至〜64 MPa·m ^0.5。在薄板结构和双峰结构中，断裂韧性都随着屈服强度的降低而增加，这归因于较低强度的裂纹尖端处的塑性区更大。屈服强度和K _IC之间的线性关系非常适合多尺度薄片结构。这表明在相同的微观结构类型中，连续性（例如强度和延展性）决定着K _IC。比较不同的微观结构类型，多尺度薄片结构表现出更高的K _IC强度水平相似的双峰结构。这主要归因于变形期间的背应力效应。薄层结构表现出较低的α/β相间应力（背应力）和均匀的应变分布，从而阻碍了裂纹扩展并增加了K _IC。同时，多尺度薄片结构中的粗α薄片会促进裂纹偏转。这种裂纹屏蔽降低了裂纹尖端的有效应力强度（K _eff），并相应地提高了断裂韧性。我们的研究表明，通过BASCA处理形成多尺度薄片α相是克服高强度β钛合金强度和断裂韧性相互排斥的有效方法。多尺度薄片结构可改善K _IC 通过固有的增韧（更均匀的应变分布）和外部的增韧（提高的抗裂纹扩展性）。