Investigation on the dwell fatigue behavior of Ti-6.5Al-3.5Mo-1.5Zr-0.3Si alloy

In this work, the dwell sensitivity of the TC11 alloy was evaluated as a function of alpha(p) contents obtained by two different heat treatment routes and peak applied stresses. The equiaxed microstructure had higher alpha(p) content and sharper macrozones, with consequently exhibiting higher dwell sensitivity than the bimodal microstructure. In order to elucidate the dwell fatigue cracking mechanism, the electron back-scattering diffraction and quantitative tilt fractography techniques were employed to determine the crystallographic and spatial orientations of the facets on the fractures. The results indicated that the initiation facets nearly cracked along the basal plane, whereas the propagation facets deviated from the basal plane with an average angle of similar to 13 degrees for both microstructures. Based on the measurements of the facet orientation, a method was proposed that the macrozone filtered with a threshold of 35 degrees could be used as a reliable microstructure characteristic for assessing the dwell sensitivity and predicting the crack initiation sites in TC11 alloy. Specifically, a basal plane of soft-oriented grain in this hard macrozone, which had the largest c-axis inclination and basal Schmid factor compared to its surrounding grains, was more prone to the crack initiation.