The microstructure and microhardness of Ti-6Al-4V α-case which experiences thermal exposure at 900℃

930℃ and 960℃ are investigated. After thermal exposure of 0.5-24h

the volume fraction of α is much higher in α-case than in sample interior

and the grains grow larger comparing with the original material. The α-case thickness increases with time and temperature increasing. Due to the strengthening effect of oxygen

the microhardness of α-case thickness also increases. The microhardness value decreases from the surface to the interior of the sample

and it becomes stable below 200μm. The α-case on the fatigue sample induces a multi-site fatigue crack initiation mode

which leads to a significant deterioration of fatigue life.