Influence of the volume content of α plus β colonies on the very high cycle fatigue behavior of a titanium alloy

A turbine engine blade alloy (Ti-8Al-1Mo-1V) under different heat treatment conditions was employed to conduct ultrasonic fatigue tests to investigate the influence of the colony content of bimodal titanium alloys on the high cycle fatigue (HCF) and very high cycle fatigue (VHCF) behavior. As a result, the increase in the content of colonies or the decrease in the content of primary alpha grains (alpha(p)) can be achieved by a double-annealed treatment below the alpha + beta to beta transformation temperature. A higher colony content will result in a higher HCF and VHCF properties. The crack initiation life has a lower sensitivity to the microstructure change in the alpha(p)/colony content than the fatigue crack growth life. With the fatigue life increasing from the HCF regime to the VHCF regime, the impact of the microstructure evolution on fatigue resistances gradually decreases. Irreversibly, slips mainly along the maximum shearing stress direction make the plastic strain accumulate within some favorably oriented alpha(p) grains and further cause the nucleation of fatigue micro-cracks. The formation of the fine granular area is attributed to the comprehensive effects of the vacuum-enhanced plasticity and the numerous cyclic pressing.