High-cycle fatigue crack initiation and propagation in laser melting deposited TC18 titanium alloy

This article examines fatigue crack nucleation and propagation in laser deposited TC18 titanium alloy. The Widmanstätten structure was obtained by double-annealing treatment. High-cycle fatigue (HCF) tests were conducted at room temperature with the stress ratio of 0.1 and the notch concentration factor K_t = 1. Fatigue cracks initiated preferentially at micropores, which had great effect on the HCF properties. The effect decreased with the decrease of pore size and the increase of distance from the pore location to the specimen surface. The crack initiation region was characterized by the cleavage facets of α lamella and the tearing of β matrix. The soft α precipitated-free zone formed along grain boundaries accelerated the crack propagation. Subsurface observation indicated that the crack preferred to propagate along the grain boundary α or border of α lamella or vertical to α lamella.