{"id":{"repo_id":"cambridge","oai_identifier":"oai:www.repository.cam.ac.uk:1810/363782"},"canonical_url":"https://search.dev.ndltd.org/etd/cambridge/oai:www.repository.cam.ac.uk:1810/363782","repository":{"repo_id":"cambridge","name":"Cambridge University","base_url":"https://api.repository.cam.ac.uk/server/oai/request"},"display":{"title":"Understanding the formation and influence of the omega phase in metastable beta Ti-Nb based alloys","abstract":"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<sub>ath</sub> and isothermal ω<sub>iso</sub>. 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<sub>iso</sub>, 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<sub>iso</sub> was found to be suppressed by the addition of Zr, which reduced both the intragranular strain and interphase misfit of the evolving ω<sub>iso</sub> phase. Whilst the addition 4 at.% Sn to Ti-24Nb prevented ω<sub>iso</sub> growth entirely, potentially due to the electronic structure of Sn. Crucially, ω<sub>ath</sub> did not prevent superelasticity. Instead ω<sub>ath</sub> 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<sub>ath</sub>. In contrast, ω<sub>iso</sub> 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<sub>iso</sub> evolution, especially at low temperatures. This suppressive effect was subsequently utilised to study the effect of smaller ω<sub>iso</sub> 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<sub>ath</sub> 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<sub>iso</sub> formation can be controlled or prevented, opening new avenues for alloy design and improving the tolerance of this class of alloys to ω formation.","abstract_html":"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&lt;sub&gt;ath&lt;/sub&gt; and isothermal ω&lt;sub&gt;iso&lt;/sub&gt;. 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&lt;sub&gt;iso&lt;/sub&gt;, 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&lt;sub&gt;iso&lt;/sub&gt; was found to be suppressed by the addition of Zr, which reduced both the intragranular strain and interphase misfit of the evolving ω&lt;sub&gt;iso&lt;/sub&gt; phase. Whilst the addition 4 at.% Sn to Ti-24Nb prevented ω&lt;sub&gt;iso&lt;/sub&gt; growth entirely, potentially due to the electronic structure of Sn. Crucially, ω&lt;sub&gt;ath&lt;/sub&gt; did not prevent superelasticity. Instead ω&lt;sub&gt;ath&lt;/sub&gt; 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&lt;sub&gt;ath&lt;/sub&gt;. In contrast, ω&lt;sub&gt;iso&lt;/sub&gt; 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&lt;sub&gt;iso&lt;/sub&gt; evolution, especially at low temperatures. This suppressive effect was subsequently utilised to study the effect of smaller ω&lt;sub&gt;iso&lt;/sub&gt; 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&lt;sub&gt;ath&lt;/sub&gt; 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&lt;sub&gt;iso&lt;/sub&gt; formation can be controlled or prevented, opening new avenues for alloy design and improving the tolerance of this class of alloys to ω formation.","abstract_has_math":false,"creators":["Talbot, Christian"],"institution":"University of Cambridge","degree_name":"Doctor of Philosophy (PhD)","degree_level":"Doctoral","degree_discipline":null,"degree_department":null,"school":null,"contributors":[],"advisors":["Jones, Nicholas"],"committee_chairs":[],"committee_members":[],"year":2023,"date_issued":"2023-09-01","date_published":"2023-09-01","updated_at":"2026-07-22T22:24:04Z","subjects":["Omega phase","Superelasticity","Synchrotron X-ray diffraction","Ti-Nb"],"languages":["eng"],"rights":[],"rights_urls":["https://apollo8-f-pro.lib.cam.ac.uk/bitstreams/e2fe65e7-a81a-4780-a9c7-af05ae4a81e2/download","https://creativecommons.org/licenses/by/4.0/"],"identifier_entries":[{"key":"dc:creator.authoridentifier","label":"Author Identifier","values":["0000000316195423"],"render_values":[{"text":"0000-0003-1619-5423","href":"https://orcid.org/0000-0003-1619-5423","code":true}]}]},"links":{"outbound_url":"https://doi.org/10.17863/CAM.105689","outbound_label":"DOI","outbound_source":"dc:identifier.doi"},"metadata_groups":[{"id":"people","label":"People","entries":[{"key":"dc:contributor.advisor","label":"Advisor","values":["Jones, Nicholas"]},{"key":"dc:contributor.sponsor","label":"Sponsor","values":["Funding for the project was provided by the EPSRC (EP/R513180/1) and Rolls-Royce plc. 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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<sub>ath</sub> and isothermal ω<sub>iso</sub>. 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<sub>iso</sub>, 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<sub>iso</sub> was found to be suppressed by the addition of Zr, which reduced both the intragranular strain and interphase misfit of the evolving ω<sub>iso</sub> phase. Whilst the addition 4 at.% Sn to Ti-24Nb prevented ω<sub>iso</sub> growth entirely, potentially due to the electronic structure of Sn. Crucially, ω<sub>ath</sub> did not prevent superelasticity. Instead ω<sub>ath</sub> 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<sub>ath</sub>. In contrast, ω<sub>iso</sub> 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<sub>iso</sub> evolution, especially at low temperatures. This suppressive effect was subsequently utilised to study the effect of smaller ω<sub>iso</sub> 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<sub>ath</sub> 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<sub>iso</sub> formation can be controlled or prevented, opening new avenues for alloy design and improving the tolerance of this class of alloys to ω formation."]},{"key":"dc:format.checksum.md5","label":"Dc Format Checksum Md5","values":["87eda9de84448d1f82354d60eee3eb5f","64a663d77f875245ae35077f58b08b56"]},{"key":"dc:title","label":"Title","values":["Understanding the formation and influence of the omega phase in metastable beta Ti-Nb based alloys"]}]}],"canonical_facts":{"dc:contributor.advisor":["Jones, Nicholas"],"dc:contributor.sponsor":["Funding for the project was provided by the EPSRC (EP/R513180/1) and Rolls-Royce plc. 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These issues are compounded by the existence of two crystallographically identical, but mechanistically distinct, forms; athermal ω<sub>ath</sub> and isothermal ω<sub>iso</sub>. 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<sub>iso</sub>, 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<sub>iso</sub> was found to be suppressed by the addition of Zr, which reduced both the intragranular strain and interphase misfit of the evolving ω<sub>iso</sub> phase. Whilst the addition 4 at.% Sn to Ti-24Nb prevented ω<sub>iso</sub> growth entirely, potentially due to the electronic structure of Sn. Crucially, ω<sub>ath</sub> did not prevent superelasticity. Instead ω<sub>ath</sub> 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<sub>ath</sub>. In contrast, ω<sub>iso</sub> 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<sub>iso</sub> evolution, especially at low temperatures. This suppressive effect was subsequently utilised to study the effect of smaller ω<sub>iso</sub> 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<sub>ath</sub> 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<sub>iso</sub> formation can be controlled or prevented, opening new avenues for alloy design and improving the tolerance of this class of alloys to ω formation."],"dc:format.checksum.md5":["87eda9de84448d1f82354d60eee3eb5f","64a663d77f875245ae35077f58b08b56"],"dc:identifier.doi":["https://doi.org/10.17863/CAM.105689"],"dc:identifier.uri":["https://apollo8-f-pro.lib.cam.ac.uk/bitstreams/f10ab9bd-ca63-420b-bd04-5466a0a95523/download"],"dc:language":["eng"],"dc:publisher.institution":["University of Cambridge"],"dc:relation.isreferencedby.uri":["https://www.repository.cam.ac.uk/handle/1810/363782"],"dc:rights":["https://apollo8-f-pro.lib.cam.ac.uk/bitstreams/e2fe65e7-a81a-4780-a9c7-af05ae4a81e2/download","https://creativecommons.org/licenses/by/4.0/"],"dc:subject":["Omega phase","Superelasticity","Synchrotron X-ray diffraction","Ti-Nb"],"dc:title":["Understanding the formation and influence of the omega phase in metastable beta Ti-Nb based alloys"],"dc:type":["Thesis"],"dc:type.qualificationlevel":["Doctoral"],"dc:type.qualificationname":["Doctor of Philosophy (PhD)"]},"updated_at":"2026-07-22T22:24:04Z"}