Abstract:The fatigue crack growth rate of a novel Ti-6Al-4V-1Mo titanium alloy, which is developed for laser directed energy deposition technique, was investigated before and after cyclic heat treatment (CHT). Changes in microstructure, fracture surfaces, and crack growth paths were analyzed before and after CHT. Results indicate that in the stable crack growth region, the growth rates for the as-deposited and cyclic heat-treated specimens follow the relationships da/dN=1.8651×10-8(ΔK)3.2271 and da/dN=1.4112×10-8(ΔK)3.1125, respectively. Compared with that at the as-deposited state, the microstructure after CHT is transformed from a uniform basket-weave microstructure to a dual-phase microstructure consisting of near-spherical α and β-transformed matrix phases. The cyclic process also disrupts the continuity of the grain boundary α (αGB) at the primary β-phase grain boundary. The coarsening of primary α and the disruption of αGB continuity are the primary factors to release stress concentration and promote crack deflection, thereby decreasing the fatigue crack growth rate. Additionally, the increased occurrence of crack branching, secondary cracking, and crack bridging in cyclic heat-treated specimens further reduces the crack driving force and slows the fatigue crack growth rate.