Three-dimensional characterization of fatigue cracks in Ti-6246 using X-ray tomography and electron backscatter diffraction

In the present study, fatigue crack propagation was imaged non-destructively in three dimensions during in situ fatigue loading of Ti-6246 using X-ray microtomography on beam line ID19 at the European Synchrotron Radiation Facility. Phase contrast enabled the visualization of the two-phase microstructure and in combination with absorption contrast the crack chronology was recorded in situ during the fatigue experiment. In order to obtain the crystallographic orientation of individual grains along the crack path, a 3D electron backscatter diffraction volume was subsequently recorded. By combining both techniques it was possible to identify the role of prior β grain boundaries and the crystallographic orientation of the α lamellae on the crack path. It is shown that a near-surface crack that cuts across a prior β grain boundary is often diverted and crack bifurcation takes place. This results in local retardation of crack propagation and a first-order undulation of the crack front. In addition, the lamellar grain orientation and morphology causes a second-order crack front undulation with the tendency of large misorientations between α lamellae preferably orientated for prismatic 〈a〉 slip or pyramidal 〈c + a〉 slip deflecting the crack path.