The behavior of crack initiation and early growth in high-cycle and very-high-cycle fatigue regimes for a titanium alloy

The behavior of crack initiation and early growth in high-cycle fatigue (HCF) and very-high-cycle fatigue (VHCF) regimes for a TC4 titanium alloy with equiaxed microstructure was investigated. Fatigue tests were conducted via ultrasonic axial cycling (20 kHz) superimposed by an amount of tensile load. The effect of tensile mean stress on fatigue endurance was plotted with a Haigh diagram formulated by Goodman, Gerber and the present formula. Fractography was observed by scanning electron microscopy, and three failure types of crack initiation were classified: surface-without-RA (rough area), surface-with-RA and interior-with-RA. Profile samples from the crack initiation region were prepared and examined by transmission electron microscopy with selected area electron diffraction. The observations show that a nanograin layer prevails underneath the fracture surface in the RA region only for the VHCF case with a stress ratio R = -1. The nanograin formation mechanism was explained by the numerous cyclic pressing (NCP) model.