The Very High Cycle Fatigue Behaviour of Ti-6Al-4V Alloy

The high cycle and very high cycle fatigue properties of the titanium alloy Ti-6Al-4V with a duplex microstructure were investigated at room temperature. High cycle fatigue tests were performed in the range from 10⁴ to 10⁷ cycles by rotating bending at the frequency of 30 Hz. The very high cycle fatigue tests were carried out in the range from 10⁷ to 10¹⁰ cycles in tension-compression on an ultrasonic fatigue testing machine at the frequency of 20 kHz. The stress amplitude was found to decrease with increasing number of cycles in the whole range from 10⁴ up to 10⁹ cycles and only at the highest number of cycles $(N_{F}=10^9)$ the alloy exhibits the fatigue limit of 460 MPa. The detail fractographic analysis was performed to characterize the fatigue failure mechanisms. Both subsurface and surface crack initiation were observed in very high cycle fatigue region. No inclusions, but only local chemical inhomogeneity in microstructure was observed at the locations of subsurface fatigue crack initiation in alpha-grains.