Nitrogen and fatigue on Ti-6Al-4V

Abstract

The work in this thesis was designed to aid the understanding of the effect of the ingress of nitrogen on diffusion bonded super plastically formed structures manufactured from the ubiquitous titanium alloy - Ti-6Al-4V. This wasn’t an abstract question but one originating from a desire to improve the communities knowledge of the integrity of in-service components that may have been affected by an ingress of air/nitrogen during manufacture. To achieve this, that aim was split down into smaller questions with experiments designed to answer each of those smaller questions allowing for a picture of the answer to the overall aim to be established. Those smaller questions were: • What effect does nitrogen have on the mechanical properties of Ti-6Al-4V? • What effect does a vacuum environment have on the fatigue properties of Ti-6Al-4V? • What effect does a vacuum environment coupled with a diffused gradient of nitrogen have on the fatigue properties of Ti-6Al-4V? The main takeaway conclusions for the reader should be the following: • A gradient of nitrogen diffused into an internal surface in a super-plastically formed component will lead to a reduction in the mechanical capability of that surface. • The nature of the gradient has a significant effect, a shallow deep diffusion profile will have less of a deleterious effect than a steep shallow diffusion profile as the higher concentrated content in the second case will lower the required localised strain to failure at the surface causing a crack to be initiated. However the depth of the diffusion profile will have a large bearing on the capability of the component to survive that initial starter crack. • Nitrogen content, the presence of a vacuum and the dominant underlying crystallographic texture of the material have significant effects on mode of fracture and the subsequent observable fractographic features. Care needs to be taken when examining fracture faces from super-plasctically formed titanium components or test samples to ensure the underlying crystallographic texture is understood and the presence of any diffused interstitial species is determined and characterised appropriately to enable more accurate conclusion to be drawn from any fractographic work.