An investigation of the effects of colony microstructure on fatigue crack growth in Ti–6Al–4V

This paper presents the results of an experimental study of the effects of colony microstructure on fatigue crack growth in Ti–6Al–4V. Colony microstructures with controlled lath widths and colony sizes are produced by controlled cooling from the β phase field. The micromechanisms of fatigue crack growth are then studied at low and high positive stress ratios of 0.1 and 0.8, respectively. For colony microstructures with approximately 20 vol.% of β phase, the fatigue crack growth rates are shown to decrease with increasing α lath and colony size. The effects of stress ratio are rationalized largely by crack closure arguments. The differences in the closure-corrected growth rates of the different microstructures are attributed primarily to differences in the fatigue crack growth mechanisms revealed by scanning electron microscopy (SEM) analyses of the fracture surfaces.