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Title: Strengthening of metastable beta titanium alloys
Author: Bennett, Joe Mancha
ISNI:       0000 0004 7652 4417
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2018
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Using current technology, it is now possible to probe material at atomic length scales, increasing our fundamental understanding of material behavior and properties. Metastable β titanium alloys are a subset of titanium alloys with huge potential for the aerospace sector. However, they exhibit atomic transformations which, even after 60 years of research, are still disputed. For example, these alloys are strengthened using the ω phase, but the mechanism by which this phase forms and its stability are still in question. The aim of this PhD project was to investigate the strengthening of metastable Ti-15wt.%Mo by understanding the stability and transformation pathways which make the metastable β titanium alloy class unique. Athermal ω shares the same composition as the β matrix and is formed by rapid cooling from the β phase field. The classical theory of athermal ω formation is based upon a diffusion-less mechanism in which consecutive pairs of {111}β planes collapse together. However, latest high-resolution electron microscope observations have suggested chemical alterations occur as well, which give reason to challenge this classical formation mechanism. Two novel methods were explored to determine the nature of the ω phase: 1) electron imaging of thin material at different collection angles and 2) total X-ray scattering analysis of large volumes of material. Complementary techniques are invaluable since thin foil artefacts were identified. In particular, a new B2 structured phase in the Ti-15wt.%Mo alloy was observed only in thin electron transparent material. Experimental data from the two new methods were compared to simulations. It was found that a frozen phonon description of the ω structure provided a best fit in both scenarios. The results are therefore consistent with the classical theory of ω formation but the collapse of the {111}β planes towards the ω phase is not considered complete.
Supervisor: Jones, Nicholas ; Stone, Howard Sponsor: EPSRC ; Rolls-Royce
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
Keywords: Titanium Alloys ; Phase transformations ; Scanning/transmission electron microscopy (STEM) ; High angle annular dark field (HAADF) ; Total Scattering