Fracture toughness anisotropy of Ti17 billet processed by the β forging

Highlights: • The delamination induced toughening is firstly found in the β proceed Ti17 alloy with severe deformation. • A way to obtain a good combination of strength and fracture toughness is found. • The crack propagation mechanism is illustrated. • The intrinsic and extrinsic toughness are discussed based on a fracture toughness prediction model. The fracture behavior of Ti17 billet processed by the β forging was examined. The results demonstrate that the specimen orientation has a pronounce effect on the fracture toughness value as well as on the crack propagation path. The fracture anisotropy can be attributed to the elongated and aligned β grain. The crack preferentially nucleates and propagates along the precipitate-free zone nearby the grain boundary. It causes either crack deflection or delamination, enhancing the fracture resistance as compared to one weak specimen orientation. The exceptionally good combination of strength and fracture toughness can be achieved by activating delamination toughening. The delamination transforms the fracture under plane-strain condition into a series of fracture processes in plane-stress condition through the thickness, and the fracture toughness is enhanced. Furthermore, a fracture toughness prediction model considering both the intrinsic and extrinsic contributions is constructed. The results show that the extrinsic toughness varies greatly than the intrinsic toughness in different orientations, indicating that the extrinsic toughness plays a significant role in the fracture toughness anisotropy of the Ti17 billet.