Control of Abnormal Grain Structures in Titanium Forgings

Titanium forgings are widely used in airframe applications for highly stressed safety critical components. To obtain high damage tolerance, the alloy Ti-6Al-4V is frequently used in a beta annealed condition which results in a coarse lamella transformation microstructure. This microstructure provides low fatigue crack growth rates owing to the high degree of crack deflection it promotes, which ensures a rough fracture surface and decreases the stress range at the crack tip due to 'closure' effects. However, it has been observed that the beta anneal heat treatment can occasionally lead to a highly inhomogeneous beta grain size, or 'abnormal' grain growth in certain forged products. The origin of this phenomenon is currently poorly understood and will be the subject of this project.
The main aims of the PhD project are to understand the mechanisms causing 'abnormal' grain structures to appear in titanium forgings. This will involve the following objectives.
1. Characterising the nature of abnormal grains in a range of parts, to build up a picture of the typical conditions that give rise to this problem in industrial production, as well as their typical features, (e.g. grain size, texture and orientation relative to the matrix etc)
2. Simulation of the forging conditions to investigate the effect of the relationship to the prior deformed state, texture, and level of break-up, recrystallization of the beta and alpha phases during sub-beta transus forging.
3. Simulation of the annealing conditions to investigate their effect on grain growth (e.g. temperature, heating rate). This will be supported with in-situ heating experiments, using direct observation in the SEM.
The project is sponsored by Airbus UK.