Design and development of \(\beta\)-titanium alloys using iron ass a low cost \(\beta\)-stabiliser

O'Kelly, Paraic (2024). Design and development of \(\beta\)-titanium alloys using iron ass a low cost \(\beta\)-stabiliser. University of Birmingham. Ph.D.

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Abstract

Two key imperatives of 21st Century aviation are an improvement in fuel consumption of turbine jet engines and thus a reduction in environmental impact. It has been widely reported that improvements can be made in both thermal and propulsive efficiency of the gas turbine. One pathway to achieving this objective is to utilise materials capable of operating in a higher temperature regime in the hottest sections of the engine, leading to improvements in efficiency. To replace the incumbent nickel-based superalloys will require development of a cost-effective, competitor material which can exceed the balance of properties in one or more domain. The bcc-\(\beta\)-Ti phase offers significant density improvements over high temperature alloys based on Ni, Fe or Co and therefore substantial weight savings, while for structural applications the high specific strength of titanium alloys is well documented. These are the core reasons for the use of titanium alloys operating in higher temperature regimes. In addition to being a low-cost \(\beta\)-Ti stabiliser, alloying with Fe can also promote formation of a \(\beta\)’-TiFe superlattice structure as the strengthening precipitate. This is an attractive concept as it offers scope to develop a microstructure based on a disordered-bcc matrix with a high phase fraction of a coherent, ordered-bcc second phase.

Chapter 1 presents an overview of the incumbent superalloys utilised in turbine jet engines, in addition to selected commercial titanium alloys and their composition – microstructure – property relationships. The benefits of a bcc \(\beta\)-Ti alloy are then discussed comparatively and strategies to impart strength, ductility and oxidation resistance are outlined. Consideration is given to the influence of the metastable \(\omega\)-phase which forms from the \(\beta\)-Ti phase and its impact on the mechanical response of the alloys.

An experimental investigation of phase equilibria in the dual \(\beta\)-Ti + ’-TiFe phase field of the base Ti-Fe system is presented in Chapter 2. This binary experimental investigation was crucial as the literature review highlighted key uncertainties in this system. A key finding from this work was the minor oxygen levels required to stabilise the \(\alpha\)-Ti phase in the supposed dual \(\beta\)-Ti + \(\beta\)’-TiFe phase field at near eutectoid temperatures.

In Chapter 3 the evolution of the metastable \(\omega\)-phase was studied in Ti-Fe alloys as a function of bulk composition and temperature. This work revealed that for Ti-Fe alloys in the range 14-22Fe (at%) neither the athermal-\(\omega\) nor the isothermal-\(\omega\) can form, instead retaining the parent \(\beta\)-matrix and nano-scale domains of distorted-\(\beta\) phase on quenching from a high temperature. This was an important finding as it indicates the phase separation associated with the isothermal \(\omega\)-phase does not contribute to the inherent brittle nature of the high solute content alloys.

A notable challenge in development of \(\beta\)-Ti alloys based on the Ti-Fe system is designing for oxidation resistance. In Chapter 4 a diffusion couple approach was utilised to rapidly assess the effects of Al and Cr additions to the binary Ti-Fe system with a focus on composition – microstructure – property relationships. The microstructural gradient was evaluated to identify a dual \(\beta\)-Ti + \(\beta\)’-TiFe microstructure and this was coupled with an oxidation investigation of the entire composition space bounded by the diffusion couple. This work enabled identification of a suitable composition range to manufacture discrete bulk alloys for onward investigation.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):