Influence of sharp microstructural gradients on the fatigue crack growth resistance of α plus β and near-α titanium alloys

The present study focuses on the effect of microstructural gradients on the fatigue crack growth resistance of Ti-6Al-4V and Ti-6242 titanium alloys. Sharp microstructural gradients from fine-grained bimodal to coarse-grained lamellar microstructures were obtained by heat treating only a portion of fine-grained plates in the beta single-phase field using a high-frequency induction coil. For fatigue crack growth from a bimodal into a lamellar microstructure, it was found that the initial crack extension past the microstructural transition within the lamellar microstructure shows the same crack growth resistance as the reference bimodal microstructure. Similarly, for fatigue crack growth from a lamellar into a bimodal microstructure, the initial crack extension past the microstructural transition within the bimodal microstructure shows same crack growth resistance as the reference lamellar microstructure. Based on detailed crack front profile investigations using optical light and scanning electron microscopy as well as heat tinting procedures, these findings can be mainly attributed to the effect of the crack front geometry.