Correlation of microstructure and fatigue crack growth resistance in Ti-6Al-4V alloy

Abstract

The effect of the microstructure on fatigue crack growth resistance of the Ti-6Al-4V alloy was investigated. Various microstructures were produced by solution treatment above the beta transus temperature followed by cooling at different rates. The compact tension specimen was used to measure fatigue crack growth rate and to determine the Paris constants. The lamellar microstructural morphology showed a better fatigue crack growth resistance than the fully martensitic microstructure. The threshold stress intensity and the critical stress intensity factors were higher for the lamellar morphology ( Kth = 15.2, Kc = 68) but lower for the fully martensitic morphology ( Kth =11.6, Kc = 63). Random orientation of the a/ß colonies in the lamellar microstructure led to crack branching and formation of secondary cracks. The fully martensitic morphology had cleavage fractures leading to a smoother fracture surface. Furthermore, the presence of brittle and fine martensitic needles could not hinder the crack propagation.