Influence of lamellar orientation on fatigue crack propagation behavior in titanium aluminide TiAl

Fatigue crack propagation (FCP) behavior of a titanium aluminide (TiAl) with a nearly fully lamellar microstructure has been studied on two different FCP directions relative to the lamellar orientation; i.e. parallel (type A specimen) and perpendicular (type B specimen) to the lamellar orientation, at ambient temperature in laboratory air. It was found that the FCP resistance of the former was considerably lower than that of the latter. Close examinations of crack morphology revealed significant differences between the two FCP directions. In type A specimens, several cracks along lamellae were seen on the surfaces and sections of the specimens, thus uncracked ligaments were formed in the wake of the crack tip. On the contrary, such ligaments were scarcely produced in type B specimens because only the main crack could propagate without remarkable deflections and branching. The FCP rates of type A specimens were decreased gradually with crack extension under constant stress intensity factor range, ΔK, tests, suggesting the role of crack bridging by uncracked ligaments. Finite element method (FEM) analysis indicated considerably reduced ΔK experienced at the crack tip, thus the difference in FCP resistance between two FCP directions based on the actual ΔK at the crack tip after allowing for crack bridging became much larger than that based on the nominal or applied ΔK.