Microstructural effects on small fatigue crack initiation and growth in Ti6A14V alloys

The nucleation and growth of small naturally initiated cracks have been investigated on Ti 6 Al 4V alloys, with bimodal or globular microstructures. Tests have been performed in air at a stress amplitude near 0.75 of the yield stress. The influence of microstructure on fatigue damage is described and the differences in fatigue life of some titanium alloys are explained. The effect of microstructure on fatigue resistance is mainly related to the early stages of damage including initiation and small crack growth. Coalescence processes favoured by a high density of initiated surface microcracks in fine a grain material, or rapid initiation of large cracks in coarse alpha colonies, explains low fatigue resistance or lifetime scattering in globular alloys. Small crack growth retardation due to alpha/beta barriers associated with a low surface crack density, limiting coalescence processes, explains the highest resistance of bimodal structures.