Fracture behavior in tensile and fatigue tests of diffusion bonded TC4 titanium alloy at room and high temperatures

The synergistic effects of matrix microstructure and bonding interface characteristics on the tensile and fatigue properties of the diffusion bonded (DB) TC4 titanium alloy at room temperature and 400℃ were studied. A double-stage heat treatment with various cooling conditions (750-850℃) was performed on the DB joints due to the precision manufacturing requirements of the fan blade. Matrix microstructure evolution caused significant tensile strength and fatigue life degradation of the DB joint at room temperature. At 400 °C, all tensile specimens exhibited ductile fracture morphology, with tensile strength showing negligible dependence on interface or matrix microstructure evolution. However, the influence of matrix microstructure and DB interface on fatigue performance at 400℃ was strongly related to the load conditions. During fatigue testing at 400℃ and 315 MPa (50 % of the tensile strength), the petal-like α + β colonies with a volume fraction of 10.5 % inhibited the crack propagation of the DB joint. Conversely, the preferentially oriented β grains with the < 111 > slip direction aligned parallel to the fatigue loading direction, along with lath α + β colonies exhibiting a 21.5 % volume fraction, facilitated crack initiation and propagation. Notably, when fatigue cracks extended into the weakly bonded region at the DB interface, crack propagation was impeded, accompanied by a shrinkage of the plastic zone at the crack tip. Under higher fatigue loads (70 % of the tensile strength), crack propagation was accelerated at the micro-pores of the DB interface, irrespective of testing temperature (room temperature or 400 °C). The fracture surfaces exhibited minimal plastic strain, resulting in negligible differences in fatigue life between the two heat-treated DB joints with distinct α + β colony microstructures.