Effect of microstructure and texture on the dwell fatigue behaviour of titanium alloys
Ever since the failure of the fan blades of the Rolls Royce gas turbine engine that powered the Lockheed Martin tristar aircraft in 1972, ambient temperature dwell fatigue (DF) in titanium alloys has received considerable research attention. Now, it is well known that titanium alloys (alpha, near-alpha and few alpha-beta) are sensitive to a dwell period at the peak load of an otherwise normal fatigue loading cycle. Many industrially relevant α and near α Ti alloys are susceptible to this rather unusual phenomenon. The phenomenon is very interesting in the fact that it is most pronounced at ambient temperature (~25 °C) and vanishes above 200 °C. This study is aimed to develop a comprehensive understanding of the effect of microstructure and texture on the dwell fatigue of α and near α titanium alloys. Firstly, the temperature sensitivity of dwell fatigue was investigated with the help of phenomenological modelling using Kocks Mecking approach. The key role played by the strain hardening and strain rate sensitivity of the material was understood. The conditions that result in the highest dwell fatigue sensitivity was also identified. Further, the in-plane anisotropy in the dwell fatigue behavior of commercially pure titanium (cp-Ti) was investigated using Electron backscattered diffraction (EBSD) and crystal plasticity FFT simulations. The role of texture in regulating the strain hardening and strain rate sensitivity and the subsequent control over the anisotropy in dwell fatigue response was vividly elucidated. Also, the microstructural heterogeneity in deformation of cp-Ti and its implications on dwell fatigue was studied. The conditions that result in incompatibilities at grain boundaries resulting in the formation of grain boundary affected zones and grain boundary sliding which in turn affected the dwell fatigue life was identified. A detailed investigation of the crack nucleation mechanisms in a near alpha titanium alloy using 2D and 3D EBSD techniques was carried out. Site-specific TEM using FIB-TEM lamella preparation and TEM-OIM was also done to understand finer microstructural features resulting in crack nucleation. Finally, the effect of thermomechanical processing on the dwell fatigue behavior of this near alpha alloy was studied. Volume fraction of primary alpha was identified as the most important parameter that affects the dwell fatigue response of this alloy.