Abstract
Fatigue crack propagation (FCP) behaviour of titanium aluminide TiAl with a nearly fully lamellar microstructure has been studied in ambient temperature 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. It was found that the FCP resistance of the former was considerably lower than that of the latter. Close examination on crack morphology revealed significant differences between two FCP directions. In Type A specimens several cracks along lamellar were seen on both surfaces and sections of the specimen, thus uncracked ligaments were formed in the wake of the crack tip. On the other hand, such ligaments were scarcely produced in Type B specimens because only main crack could propagate without remarkable deflection and branching. The FCP rate of Type A specimens decreased with crack extension under constant ΔK tests, suggesting the role of crack bridging by uncracked ligaments. 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 after allowing for crack bridging became much larger than that based on the nominal or applied ΔK.
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