Fracture toughness of high-strength bimodal Ti-5553 titanium alloy with pancake-shape prior β grain

Grain-boundary delamination toughening (GBDT) is an ingenious toughening mechanism for high-strength metallic materials, such as the advanced high-strength steels. In the present work we attempt to improve the fracture toughness of high-strength Ti–5Al–5V–5Mo–3Cr (Ti-5553) titanium alloy by GBDT. Based on the microstructure requirement of GBDT, two kinds of bimodal Ti-5553 alloys with pancake-shape prior β grain, namely GBDT-1 bimodal alloy and GBDT-2 bimodal alloy, were fabricated by sub-transus hot-rolling, solution treatment and single/dual ageing treatment. GBDT-1 alloy processes the fine grain boundary α precipitation and fine intragranular α precipitation. Yield strength (σ_y), tensile elongation (δ) and fracture toughness (K_IC) are 1400 MPa, 9 % and 25.2 MPa m^0.5. In contrast, GBDT-2 alloy processes the coarse grain boundary α precipitation, coarse intragranular α precipitation and ultra-fine intragranular α precipitation. σ_y, δ and K_IC are 1270 MPa, 16 % and 30.3 MPa m^0.5. Fracture mechanism for the compact tension specimen of GBDT alloys includes the blunting of I-type crack and the propagation of I-type crack, and the blunting load of the load-displacement curve is used to calculate K_IC. Although we aim to improve K_IC of high-strength bimodal Ti-5553 alloy by GBDT-mechanism, the mechanism does not work during the fracture of the compact tension specimen. It is possible that the aspect ratio of the pancake-shape prior β grain might be too low to trigger GBDT. The findings of present work suggests that the conflict between strength and toughness of high-strength Ti-5553 alloy is mainly caused by the preferential plastic deformation of β phase, which occurs at relative low load, during the blunting of the I-type crack. In future we will investigate the role of microstructure parameters for prior β grain in the activation of GBDT-mechanism for high-strength bimodal Ti-5553 alloy.