Microstructural Characterization and Crack Propagation Behavior of a Novel β-Solidifying TiAl Alloy

Novel β-solidifying TiAl alloys have great potential for engineering applications in the aerospace and automotive industries. The introduction of the β0 phase will inevitably affect crack propagation. However, the related mechanism is unclear. In this study, the crack propagation behavior of different β0-containing microstructures was systematically investigated by three-point bending tests. The results show that the coarse γ/α2 lamellar microstructure exhibits better fracture toughness than the fine-grain microstructure because large numbers of γ/α2 lamellar boundaries can effectively hinder crack propagation. The propagation direction depends largely on the orientation of the γ/α2 lamellae. When the angle between the crack propagation direction and the γ/α2 lamellar boundary is small, the crack tends to propagate along γ/α2 lamellae. When the angle is close to 90°, the crack generally propagates by the trans-lamellar mode. Moreover, the crack tends to traverse across the fine β0/γ duplex region due to the low resistance of fine grains in the crack propagation. The transgranular and intergranular modes are the main fracture mechanisms in the microstructure of the fine β0/γ grains. Some shear ligaments can also be identified in the lamellar microstructure and these can consume propagation energy. The enlarged image shows that the crack propagation direction can be changed by the β0 phase, owing to its high hardness. The crack tends to stop at the β0 phase region.