Ti-6Al-4V drilled plates were subjected to cyclic loading tests in a previous study, which investigated the machining-induced effects on fatigue [1]. The majority of the specimens failed between low to moderate number of cycles, close to the surface of the hole, around the half thickness plane, with no clear crack initiation point. Differences in the performance of the plates were associated to the initiation of cracks from subsurface microstructure defects. However, a clear explanation of the mode of failure and the mechanisms behind it were not established due to the scattering of the fatigue data and the need for a more detailed examination of deformed microstructures and elastoplastic strain fields. This postulated the main research objective for the current study. A selected number of specimens after the fatigue tests were re-examined to identify the critical defects. The preliminary analysis included surface roughness, micro-hardness, LOM, and SEM of the machining-affected layer. Then, advanced characterization techniques, namely EBSD and FIB, were transformed into semi and fully quantitative methods to identify plastic strain gradients and residual stress profiles within the material. Despite the different drilling conditions, the specimens had similar roughness values. However, the plastic deformation and residual stress profiles within the material were directly related to each other and to the drilling conditions. All specimens displayed strain localization within smaller alpha grains of the sheared underlying microstructure, while tensile twins and slip bands were visible below the heavily deformed and strain hardened zone. Tensile, surface or near-surface stresses were measured for all drilling conditions at different locations around the hole. The results indicated that specimens with lower levels of deformation had superior fatigue performance. Since all specimens had identical defects, though to a different depth, it was deemed necessary to examine the fractured surface of the specimens. A closer inspection of the machined surfaces at the locations of the crack initiation revealed that chip fragments, embedded on the surface of the hole, were the critical defect dominating the fatigue life. Surface smearing and intense drilling marks were generating non-critical cracks. Chip cracking, voids within the chip, surface damage and cavities from the embedment of the chip, and shielding of surface twins by the chip were observed in the specimens. An exact description of the failure mechanism(s) was not possible because all of them appeared to be crack nucleation sites. The current study was concluded at this point providing the groundwork and the tools for future work.
