The origins of abnormal grain structures in Beta annealed Ti-64 aerospace components
N Byres, P Shanthraj, B. Dod, J Quinta da Fonseca, PB Prangnell
Abnormal grain size distributions in wrought titanium, 6% aluminium, 4% vanadium (Ti64) aerospace components can unexpectedly arise during the β-annealing step of the thermomechanical processing route. β-annealing is the final step of the thermomechanical processing route and is performed to obtain a transformed-β microstructure that improves fatigue crack growth resistance. As these microstructures render the components unfit for purpose, the phenomenon has the potential to cause the rejection of nearly completed forgings, resulting in a large waste in energy and resources. The ultimate goal of the research presented here is to improve understanding into the origins of abnormal grain structures in β-annealed Ti64 forgings and the pre-requirements (e.g. texture and stored energy) for such explosive growth. It is now understood that a very strong cube β-grain texture component provides a matrix for uninhibited β-grain growth whilst the subgrain structure, compromising the cube component, provides the necessary driving force through a subgrain, boundary driven, recrystallisation process. The current unknowns lie in the origins of the larger and faster growing grain orientations. Recent advances in electron back scatter diffraction (EBSD) microscopy technology, that facilitates faster data acquisition, has helped improve understanding into the development of microscale texture bands that has helped elucidate macroscale texture development in hot-worked Ti64 aerospace components. Large area EBSD mapping has revealed the dominant recrystallisation behaviour of components along the theta-fibre, namely the (001)  or “rotated cube” component in rolled plate and the (001)  or “cube” component in forgings. Subsequent in-situ heating experiments, coupled with large scale EBSD mapping, produced quasi real-time crystallographic orientation (texture) data during the α + β phase transformation, with respect to changing α and β-phase volume fractions during heating through the α + β-phase window. This data was fed into phase field simulations to predict the starting conditions that lead to abnormal grain microstructures and control their development during processing.