Abstract
Rotating bending fatigue tests have been carried out using materials with three different microstructures of a Ti-6Al-4V alloy, prepared with different heat treatments, i.e. annealed at 705°C (AN705) and 850°C (AN850), and solution treated and aged (STA). Emphasis was placed on the effect of microstructure on fatigue behaviour, in particular, fatigue crack initiation and small crack growth. The highest fatigue strength was obtained in STA, followed by AN850 and AN705. Fatigue cracks initiated within α phase in the annealed materials and the crack initiation resistance for AN750 with fine microstructure was superior to that for AN850 with coarse microstructure. For STA, cracks generated within (α+β) phase. Small cracks in all the materials exhibited much higher growth rates just after crack initiation, followed by a retardation and experienced a minimum value, and then showed monotonic increase in growth rate. In the extremely small crack regime below approximately 80μm, the most excellent growth resistance was achieved in AN850, followed by STA and AN705. Based on the observations on fatigue crack initiation and small crack growth, it was concluded that the highest fatigue strength of STA was due to the excellent crack initiation resistance of its microstructure, while the higher fatigue strength of AN850 than AN705 was attributed to higher small crack growth resistance of the former material.
