Understanding the formation and influence of the omega phase in metastable beta Ti-Nb based alloys

Metastable β-Ti alloys have potential applications ranging from low modulus biomedical alloys to vibration damping in aerospace. This is, in part, due to the ability of certain compositions to undergo a stress induced transformation to the martensitic α″ phase, enabling superelasticity. However, the inability to control key properties of the transformation prevents industrial uptake, with the presence of the hexagonal ω phase often reported as a primary cause for such challenges. Despite over 70 years active research, the formation of ω is difficult to predict and there is much contention over its influence on β phase decomposition and subsequent properties. These issues are compounded by the existence of two crystallographically identical, but mechanistically distinct, forms; athermal ω_ath and isothermal ω_iso. This work studied the ω phase within the Ti-Nb alloy system through in situ synchrotron X-ray diffraction, with the aim of investigating its formation, stability and influence, particularly with respect to superelasticity. It was shown that ωath readily formed through a metastable mechanism. The diffusional form, ω_iso, was shown to be a transient of the more stable α phase, with α being the direct decomposition product in alloys with sufficiently high internal strains. The formation of ω_iso was found to be suppressed by the addition of Zr, which reduced both the intragranular strain and interphase misfit of the evolving ω_iso phase. Whilst the addition 4 at.% Sn to Ti-24Nb prevented ω_iso growth entirely, potentially due to the electronic structure of Sn. Crucially, ω_ath did not prevent superelasticity. Instead ω_ath was readily consumed by the growth of α″ during mechanical loading, this is important as it highlights that a number of the issues surrounding superelasticity in these alloys cannot be attributed to the presence of ω_ath. In contrast, ω_iso prevented superelasticity in larger volume fractions. This knowledge was extended into the commercial alloy system, Ti-2448 (Ti-24Nb-4Zr-8Sn, wt%), where the presence of Zr and Sn was shown to significantly suppress ω_iso evolution, especially at low temperatures. This suppressive effect was subsequently utilised to study the effect of smaller ω_iso volume fractions on superelasticity, which altered key characteristics of the transformation, potentially to the benefit of specific applications. These insights significantly expand our understanding of ω. They highlight that, with respect to superelasticity, ω_ath is not as problematic as reported in sections of the literature - a critical observation given the ubiquity of this form of ω. Additionally the addition of Sn and Zr, whose efficacy in suppressing ω has recently been questioned, identified potential mechanisms by which ω_iso formation can be controlled or prevented, opening new avenues for alloy design and improving the tolerance of this class of alloys to ω formation.

Name dc:type.qualificationname

Doctor of Philosophy (PhD)

Level dc:type.qualificationlevel

Doctoral

Grantor dc:publisher.institution

University of Cambridge

Year dc:date.issued

2023

Author dc:creator

• Talbot, Christian

Advisor dc:contributor.advisor

• Jones, Nicholas

• Omega phase
• Superelasticity
• Synchrotron X-ray diffraction
• Ti-Nb

Licence

• https://apollo8-f-pro.lib.cam.ac.uk/bitstreams/e2fe65e7-a81a-4780-a9c7-af05ae4a81e2/download
• https://creativecommons.org/licenses/by/4.0/

Language dc:language

eng

Author Identifier

0000-0003-1619-5423

OAI identifier oai:identifier

oai:www.repository.cam.ac.uk:1810/363782