Understanding the role of microstructure on fatigue properties of titanium alloys

Titanium alloys are well established as important materials for aero-engine application. Their high specific strength at temperatures up to 600C makes them advantageous for compressor components, such as fan blades and critical rotating discs. However, their capability can be limited, in some cases, due to reduction of fatigue properties under cold dwell conditions and under medium to high mean loads. Recent advances in experimental techniques, crystal plasticity modelling and fundamental understanding have highlighted the importance of crystallographic texture, generated during conversion of titanium components, on the likelihood and level of performance debit under such loading conditions.

It is necessary to understand the physical basis for such reductions in fatigue properties. This is to ensure that such effects are captured in component validation testing but also so that manufacturing process routes can be optimised, where practical, for both development and production parts. To this end a study is required of the significance of crystallographic texture on both the level of cold dwell debit and the reduction in notched fatigue performance at high R-ratios. This should investigate whether the two phenomena are connected or indeed result from the same strain accumulation mechanism associated with stress relaxation between combinations of hard and soft grains.

Ti6-4 plate will be used as the study material. Ti6-4 plate made by rolling at temperatures high in the alpha-beta phase field show a preferred transverse texture relative to the rolling direction. For cross rolled plate this gives preferential basal poles in both the longitudinal and transverse direction and maximises the combinations of hard and soft orientation in these directions. Regions of similar crystallographic orientation, known as macro-zones, are a common feature of plate microstructures. The role these features play in the failure mode at notches and their orientation relative to the point of fatigue initiation is of particular interest. The project should also test the hypothesis that the effect of this preferred texture can be minimised by testing at close to 45 degrees relative to the rolling directions and that the debit in fatigue life under dwell and / or under high R-ratio is reduced in this direction.

In summary, the project involves the metallurgical and crystallographic characterisation of Ti6-4 plate and mechanical testing under appropriate dwell and mean load conditions at various orientations to elucidate the influence of texture and the failure mechanisms involved in each case.