Fatigue crack propagation behavior and damage mechanism of Ti-Mo-Cr-V-Nb-Al alloy in the near-threshold region

To meet the requirements of damage tolerance design for high-strength and high-toughness metastable β titanium alloys, it is of vital significance to regulate and obtain an appropriate microstructure for enhancing the fatigue resistance of such titanium alloys. This work mainly conducts a comparative study on the fatigue crack growth threshold value ΔK_th of a novel high-strength and high-toughness Ti-Mo-Cr-V-Nb-Al titanium alloy with basketweave and bi-modal microstructures, exploring the influence of its microstructure on fatigue crack initiation and growth behavior and the corresponding damage mechanism. It is discovered that equiaxed α_P phases are more prone to causing crack deflection compared with coarse lamellar α_P phases. Furthermore, the crack resistance of this alloy mainly originates from the crack deflection induced by α_P phases and the crack tip blunting caused by α_S phases. Based on the analysis of crack growth paths and slip traces, it is considered that basal slip provides a favorable path for crack growth, while second-order pyramidal slip exhibits greater resistance to crack growth. Additionally, microplastic deformation occurs in the α phase at the crack tip, resulting in lattice rotation. The residual dislocations in the α phase at the crack tip indicate the existence of slip transfer. Meanwhile, the phase boundaries serve as both sources and barriers for dislocations, which may lead to crack propagation along the phase boundaries.