Study of fatigue properties of forged BT25 titanium alloy based on fractographic and quantitative microstructural analysis

The high cycle fatigue (HCF) properties of forged BT25 titanium alloy was examined by fractographic and quantitative microstructural analysis. Two different height reductions (50 and 80%) were applied to the isothermal compression of BT25 alloy with initial lamellar microstructure. Fatigue limits obtained through up and down method were 440 and 450 MPa respectively. Microstructural analysis showed that the globularisation volume fraction of the two microstructures (MA represents the microstructure of the alloy with the height reduction of 50%, while MB is used for 80%) were 62.72 and 84.13% respectively. There was more lamellar alpha in which Feret ratio was greater than 3.5 remained in MA, 16.96%. Hence, a larger globularisation volume fraction is more beneficial for resistance to fatigue failure. The fracture surfaces of the failed testing specimens were examined and the fatigue crack initiation mechanisms were discussed. Cracks initiated in grains by means of constituting extrusions through combined glide and dislocation annihilation in slip bands, while those initiating along the alpha/beta phase interface arose from discordant deformation of alpha and beta phases with different Young's modulus.