Dr Beatriz Fernandez's research in Sheffield focused on investigating the effects of thermomechanical processing on the texture and microstructure in Ti834 alloy during compressor disc manufacture. This PhD was sponsored by Rolls-Royce plc working in collaboration with TIMET UK.
The near-α titanium alloy Ti834 shows a good balance between fatigue and creep properties due to its bimodal microstructure formed by primary alpha grains (αp) and secondary alpha colonies (αs) achieved by thermomechanical processing. However, the deformation applied during forging generates local regions in the microstructure where individual alpha grains have similar crystal orientations. These textured regions, known as macrozones, lead to a reduction in fatigue life when the material is exposed to relatively high stress for a period of time. Cold dwell fatigue is a well-established failure mode in titanium and an important factor in controlling the dwell fatigue strength is the presence and size of macrozones. However, the role of thermomechanical processing in controlling the macrozone size is not fully understood.
New billet conversion routes were produced using well-controlled conditions to give a range of strain levels to assess the effect of forging conditions on macrozones formation. During this project, microstructure and texture characterization of compressor disc samples from each route were analysed to understand the effect of the thermomechanical processing parameters in macrozones formation. Fracture surface analysis of dwell fatigue samples was characterised to investigate the relationship between facet formation and crystallographic orientation.
The main objective of this research was to understand the effect of new thermomechanical processing conditions in the texture and micro texture of Ti834 during the fabrication of a compressor disc and their effect in the creation of macrozones. Understanding and reducing macrozones formation would lead to an increment in service life of the component and a reduction in expensive and time-consuming testing.