High cycle fatigue of isothermally forged Ti-6.5Al-2.2Mo-2.2Zr-1.8Sn-0.7W-0.2Si with different microstructures

The high cycle fatigue properties of Ti-6.5Al-2.2Mo-2.2Zr-1.8Sn-0.7W-0.2Si with three typical microstructures (equiaxed, bimodal and full lamellar) prepared by carefully controlled isothermal forging and heat treatment procedures are investigated by comparison. Equiaxed and bimodal microstructures exhibit a higher HCF resistance than the full lamellar one and a mismatching of HCF strength with yield strength for equiaxed and bimodal microstructures is observed. Fractography analysis shows crack initiates primarily at the subsurface of ∼30 μm-300 μm for the three microstructures. Interior initiation occurs in 5 specimens of the 19 specimens that were measured. For equiaxed and bi-modal microstructures, the interior initiation samples have significant higher fatigue life than those initiated at (sub)surface under the same fatigue stress, while in lamellar structure, the initiation location shows no such effect on fatigue life. It is found that two factors, i. e. the effective slip length and the intrinsic strength, jointly decide the fatigue crack initiation in the present alloy. The dependence of crack initiation on microstructure is responsible for the microstructural sensitivity of HCF properties.