β-Ti alloys are widely used in airframe and biomedical applications due to their high ductility, high hardenability, and low elastic modulus. The phase transformations in β-Ti alloys are rather complex due to formation of metastable phases during various thermo-mechanical treatments. One such critical metastable phase, the hexagonal omega (ω) phase, can form in β-Ti alloys under quenching from the high temperature β phase and/or isothermal aging at intermediate temperature. Despite a substantial amount of reported works on the ω phase, there are several critical issues related to the ω formation need to be resolved, e.g. role of alloying elements and oxygen content. Therefore, this dissertation has attempted to provide insights into ω transformation in low misfit (Ti-Mo) and high misfit (Ti-V) binary systems as well as multicomponent (Ti-Nb-Zr-Ta) alloys. The evolution of ω structure, morphology and composition from the early stage (β-solution+quenched) to later stages after prolonged aging are systematically investigated by coupling transmission electron microscopy (TEM), atom probe tomography (APT) and high-energy synchrotron X-ray diffraction techniques. The influence of aging temperature and duration on characteristic of ω phase in Ti-Mo, and Ti-V alloys is addressed in details. It is found that compositional changes during aging can alter the structure, size and morphology of ω precipitates. In low misfit alloys, the ellipsoidal morphology of ω phase was retained during isothermal aging, while in high misfit alloys it changed from ellipsoidal to cuboidal morphology after prolonged aging. Secondly, ω transformation in biomedical Ti-Nb-Zr-Ta alloy is probed in which the micro-hardness was sensitive to microstructural changes. Furthermore, the evolution of oxygen concentration in ω precipitates during various aging conditions in binary Ti-Mo and Ti-V alloys are reported. It has been accepted that interstitial elements such as oxygen can largely alter mechanical behavior and the microstructure of Ti-alloys. Recently, oxygen is intentionally added to some biomedical alloys to improve their performances. However, a careful understanding of the effect of oxygen on ω phase transformation is still lacking in the literature. In this work, the role of oxygen on ω phase formation in biomedical TNTZ alloys is investigated. Although it is traditionally accepted that oxygen suppresses ω transformation, our observations revealed contradictory results during isothermal aging of TNZT alloys. The results of our investigations provide a novel insight into understanding the effect of interstitial elements on metastable phase transformation in β-Ti alloys. It is concluded that depending upon the nature of alloying elements and/or the applied thermo-mechanical treatments, oxygen may play a different role in ω transformations.

Orientador: Ana Paula Rosifini Alves Claro / Coorientador: Maria Cristina Rosifini Alves Rezende / Banca: Monica Beatriz Mathor / Banca: Estevão Tomomitsu Kimpara / Resumo: Neste estudo, foi avaliada a superfície da liga experimental Ti-30Ta após tratamento. A técnica empregada foi o tratamento de superfície biomimético com incorporação de alendronato de sódio. Os lingotes da liga Ti-30Ta foram obtidos em forno a arco voltaico, seguido de tratamento térmico e forjados a frio. O tratamento de superfície foi dividido em duas etapas: primeiramente foi realizado o tratamento de superfície biomimético com imersão em SBF 5x (Simulated Body Fluid) durante 24 horas. Em seguida as amostras foram imersas em solução formada por SBF 5x e alendronato de sódio por uma semana. Difração de raios X, microscopia eletrônica de varredura, microscopia de força atômica e ângulo de contato foram utilizados para análise de superfície. Para análise da osseointegração, os implantes foram colocados no fêmur de ratos e a análise histológica realizada após 30 dias. Os resultados mostraram uma melhora na osseointegração devido à incorporação de alendronato de sódio ao revestimento de apatita / Abstract: In this study, the surface of Ti-30Ta experimental alloy was evaluated after treatment. Biomimetic surface treatment with sodium alendronate incorporation was technique employed. Ingots of Ti-30Ta alloy were obtained in arc melting furnace, followed heat treatment and cold work by swaging. Surface treatment was divided into two steps: first was realized Biomimetic surface treatment with immersion in SBF 5x (simulated body fluid) for 24 hours. Followed samples were immersed in solution formed by SBF 5x plus sodium alendronate for one week. X- rays, SEM, AFM and contact angle were used for surface analysis. For analysis of osseointegration, the implants were implanted in the rat's femur and histological analysis was realized after 30 days. The results showed improved of osseointegration due to the incorporation of sodium alendronate in the apatite coating / Mestre

Orientadora: Ana Paula Rosifini Alves Claro / Banca: Marcelo Santos Pereira / Banca: Roberto Zenhei Nakazato / Banca: Estevão Tomomitsu / Banca: Cristiane Aparecida Assis Claro / Resumo: Materiais biometálicos são usados para aplicações biomédicas, como cardiovasculares, ortopédicas e odontológicas, devido as suas propriedades, como biocompatibilidade e adequadas propriedades mecânicas, como resistência à corrosão em ambiente corpóreo. E pela combinação de resistência ao desgaste e baixo módulo de elasticidade. Entretanto, apesar da sua excelente resistência a corrosão estes materiais podem ser corroídos quando expostos em ambiente oral. A liberação de ions metálicos pode produzir toxicidade indesejável, principalmente em ligas contendo níquel como NiTi. Nesse estudo, uma nova liga quaternária contendo elementos não citotóxicos Ti, Ta, Nb e Sn, foi processada e avaliada. As ligas foram processadas em forno de arco voltáico em atmosfera de argônio, a partir de chapas dos elementos comercialmente puros. Os lingotes foram fundidos, refundidos por pelo menos dez vezes e homogeneizados (1000 ° C para 86,4 ks) para eliminar a segregação, após trabalho a frio em prensa rotativa. As fases foram avaliadas por análises de XDR (Difração de raios X). Teste de microdureza e de tração foram realizado para avaliar as propriedades mecânicas. Comportamento de corrosão foi investigado em solução de fluoreto em polarização eletroquímica. Módulo de elasticidade e citotoxicidade da nova liga também foram avaliados. Neste estudo, para a liga quaternário Ti25Ta25Nb3Sn a estabilização da fase beta foi mantida. O módulo de elasticidade de Ti25Ta25Nb3Sn (65GPa) foi menor do que do titânio (105 GPa) e Ti6Al4V (110 GPa), sendo ligeiramente maior do que Ti25Ta25Nb liga (55GPa). A adição de Sn inibiu o duplo escoamento verificado na liga ternária Ti25Ta25Nb. Estudos eletroquímicos mostraram que o filme de óxido passivo formado ficou estável sobre a superfície da liga Ti25Ta25Nb3Sn e a não-toxicidade foi ... ( Resumo completo, clicar acesso eletrônico abaixo) / Abstract: Metallic biomaterials have been used for biomedical applications, such as cardiovascular, orthopaedics and orthodontics, due to properties. However, although their corrosion resistance these materials can be corrode when exposed oral environment. Metals ion release can produce undesirable toxicity, principally in alloys containing Ni, as NiTi. In this study, a new quaternary alloys with non-cytotoxic elements Ti, Ta, Nb and Sn was processed and evaluated. Alloys were processing in arc melting furnace with argon atmosphere from sheets of commercially pure elements. Ingots will be melted and re-melted ten times at least and homogenized (1000°C for 86.4 ks) for eliminated segregation chemical after cold-worked by swaging. Phases were evaluated by XRD (X-ray diffraction) analysis. Microhardness and tension test was carried out to evaluate mechanical properties. Corrosion behaviors was investigated in fluoride solution by electrochemical polarization. Young's modulus and cytotoxicity of the new alloy were also evaluated. In our study, for quaternary alloy Ti25Ta25Nb3Sn the stabilization of beta phase was maintained. It was observed that the elastic modulus of Ti25Ta25Nb3Sn (65GPa) was lower than CP Ti (105 GPa) and Ti6Al4V (110 GPa) and slightly higher than Ti25Ta25Nb (55GPa) alloy. The addition of Sn suppressed the double yielding verified on ternary alloy Ti25Ta25Nb.Electrochemical studies showed that stable passive oxide film was formed on the Ti25Ta25Nb3Sn surface and non-toxicity was observed after MTT assays. Results obtained showed that Ti25Ta25Nb3Sn alloy is indicated for biomedical applications / Doutor

Orientador: Ana Paula Rosifini Alves Claro / Coorientador: Luiz Rogério de Oliveira Hein / Banca: Angelo Caporalli Filho / Banca: Maria Ismênia Sodero Toledo Farria / Resumo: Ligas de titânio são utilizadas para aplicações biomédicas devido as excelentes propriedades como a alta resistência à corrosão, biocompatibilidade e propriedades mecânicas. No presente trabalho, foram analisadas as microestruturase as propriedades mecânicas daliga Ti-30Ta e em amostras deformadas com diferentes graus de compressão. A liga foi obtida pela fusão dos metais puros em um forno a arcovoltaico com atmosfera inerte de argônio. O material obtido foi homogeneizado em um forno tubular a 1000 °C por 24 h e deformadas a friopelo processodeforjamento rotativo para obter barras de 13 mm de diâmetro. A liga foi avaliada utilizadastécnicas de difração de raios-X (DRX) em temperatura ambiente e com aquecimento, calorimetria diferencial de varredura(DSC), microscopia eletrônica de transmissão (TEM), microdureza e compressão de corpos de provas em diferentes graus de deformação. As amostras foram deformadas com taxas graus de 19%, 22% e 43% e analisadas, usando as técnicas de DRX,microscopia óptica, microdureza. A liga apresentou transformação alotrópica a uma temperatura próxima de 650°C, e microestrutura do tipo α', a deformação da amostra resultou em cisalhamento na amostra de 43%, as amostras apresentaram microdureza de 250, 269, 286 e 280HV para as amostras sem deformação, 19%, 22%e 43% de deformação / Abstract: Titanium and its alloys have been used for biomedical applications due their excellent properties, such as high corrosion resistance, biocompatibility and mechanical proprieties. In this work, the microstructure and mechanical proprieties of Ti-30Taand deformeds amples with different degrees of compression were analyzed The in gots were obtained in a arc melting furnace inan argon atmosphere. In gots were homogenized at 1000 ºCin a tubular furnace with argon atmosphere for 24 hours and than they were cold worked by swaging. and samples with 13 m mindiameter were obtained. The alloy was evaluated by X-raydiffraction (XRD), Heat treatment X ray diffraction (HTXRD)and differential scanning calorimetrical (DSC), transmission electron microscopy(TEM), micro hard nessand compression test with different deformations. The samples were forging at reductionratiosof 19%, 22% and 43%. After this deformation,its microstructure were evaluated by optical microscopy, X-ray diffraction(XRD)and micro hardness in each condition. The alloy show a allotropic transformation at 650 °C, and α' microstructure. The sample that was compressed with 43% shear and the others samples ( 19%, 22%) don't change their microstructure. the samples without deformation, 19%, 22% and 43% result 250, 269, 286, 280 HV / Mestre

Submitted by Luciana Ferreira (lucgeral@gmail.com) on 2018-04-24T11:46:58Z No. of bitstreams: 2 Dissertação - Ana Carolina Da Silva Araújo Rezende - 2018.pdf: 3974974 bytes, checksum: d44cb7153672b2c65486e4563be18a44 (MD5) license_rdf: 0 bytes, checksum: d41d8cd98f00b204e9800998ecf8427e (MD5) / Approved for entry into archive by Luciana Ferreira (lucgeral@gmail.com) on 2018-04-24T11:52:16Z (GMT) No. of bitstreams: 2 Dissertação - Ana Carolina Da Silva Araújo Rezende - 2018.pdf: 3974974 bytes, checksum: d44cb7153672b2c65486e4563be18a44 (MD5) license_rdf: 0 bytes, checksum: d41d8cd98f00b204e9800998ecf8427e (MD5) / Made available in DSpace on 2018-04-24T11:52:16Z (GMT). No. of bitstreams: 2 Dissertação - Ana Carolina Da Silva Araújo Rezende - 2018.pdf: 3974974 bytes, checksum: d44cb7153672b2c65486e4563be18a44 (MD5) license_rdf: 0 bytes, checksum: d41d8cd98f00b204e9800998ecf8427e (MD5) Previous issue date: 2018-03-23 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES / Titanium is a metal of first choice for use as biomaterial. Titanium alloys of type β are the most suitable for orthopedic implants. This study aims to obtain, characterize structurally and evaluate the bioactivity properties of three types of Ti-Nb-Sn alloys: a) 61% Ti-18% Nb-11% Sn of crude fusion; B) 61% Ti-35% Nb-4% Sn with thermo mechanical treatment; C) 61% Ti-35% Nb-4% Sn with thermomechanical treatment and chemically modified surface. Materials and Methods: samples were obtained by melting in an arc furnace. After melting the Ti-35Nb-4Sn samples were subjected to reduction, treatment and ther malaging. A portion of these Ti-35Nb-4Sn samples had the surface chemically and thermally modified through NaOH, CaCl, heat and water. After this process the three samples were submitted to bioactivity assay through the Kokubo procotol. The samples were immersed in PBS during different periods (1, 3, 7 and 14 days), during this period they were kept in anoven at 36.5 ± 0.03 ° C. It was verified the formation of apatite through XRD and MEV / EDS. The experimental techniques used in the characterization were: MO, DRX, FRX, MEV, and Hardness Test. Results: Structurally the Ti-18Nb-11Sn alloys howed a structure composed of the orthorhombic martensitic phase α ", with Vickers hardness of 189,5HV, MEV images present the formation of Hydroxyapatite in 14 days after immersion in PBS, XRD analysis demonstrates peaks In 2θ = 31,8 ° referring to Hydroxyapatite. The Ti-35Nb-4Sn alloy is formed by β phase, exhibits Vickers hard ness of 270HV, SEM images demonstrating the formation of hydroxyl apatite after 7 days confirmed by XRD. Conclusion: The Ti-18Nb-11Sn alloy, shows composed of orthorhombic martensitic phase, thus presents structural and mechanical characteristics that are not considered appropriate for implants. The bioactivity assay has shown that Ti- 35Nb-4Sn alloys have a more bioactive surface treatment than the others. / O Titânio é um metal de primeira escolha para utilização como biomaterial. As ligas de Titânio do tipo β são as mais indicadas para implantes ortopédicos. Este estudo teve por objetivo obter, caracterizar estruturalmente e avaliar a bioatividade de três tipos de ligas de Ti-Nb-Sn: a) 61%Ti-18%Nb-11%Sn de fusão bruta; b) 61%Ti-35%Nb-4%Sn com tratamento termomecânico; c) 61%Ti-35%Nb-4%Sn com tratamento termomecânico e superfície modificada quimicamente. Materiais e Métodos: as amostras foram produzidas através da fusão em forno arco voltaíco. Após a fusão, às amostras de Ti-35Nb-4Sn foram submetidas à redução, tratamento e envelhecimento térmico. Uma parte destas amostras de Ti-35Nb-4Sn tiveram a superfície modificada quimicamente e termicamente, através de soluções de NaOH, CaCl, Calor e água. Após este processo as três amostras foram submetidas a ensaios de bioatividade, seguindo o procotolo de Kokubo. As amostras foram imersas em um Fluído Corpóreo Simulado (FCS), durante diferentes períodos, (um, três, sete e quatorze dias). Durante este período, foram mantidas dentro da estufa à 36,5 ±0,03°C. Averigou-se a formação de apatita através de DRX e MEV/EDS. As técnicas experimentais utilizadas na caracterização foram: MO, DRX, FRX, MEV, e Ensaio de Dureza. Resultados: estruturalmente a liga Ti-18Nb-11Sn relevou uma estrutura composta pela fase martensítica ortorrômbica α”, com dureza Vickers de 189,5HV, as imagens de MEV apresentam a formação de Hidroxiapatita no 14° dia após a imersão em FCS. A análise de DRX mostra a presença de picos de difração para o ângulo 2θ=31,8°, o qual é referente à formação de Hidroxiapatita. A liga Ti-35Nb-4Sn é apresenta a formação da fase β, a qual apresenta microdureza Vickers de 270HV. As imagens de MEV confirmam a formação de hidroxiapatita após 7 dias de imersão de FCS. Os resultados de DRX também confirmam a presença de hidroxiapatita sobre a superfície da liga. E para as amostras modificadas termoquimicamente, a formação de HA é evidenciada pela MEV, no terceiro dia após imersão em FCS. Conclusão: A liga Ti-18Nb- 11Sn demonstrou ser composta por fase martensítica ortorrômbica, assim, apresenta características estruturais e mecânicas que não são consideradas apropriadas para implantes. O ensaio de bioatividade demonstrou que a liga Ti-35Nb-4Sn com tratamento de termoquímico da superfície, apresenta maior bioatividade que as demais ligas.

Biomateriais para uso em implantes devem exibir propriedades como biocompatibilidade, biofuncionalidade e resistência à corrosão. O uso do titânio e suas ligas em aplicações biomédicas vem continuamente aumentando devido à sua maior taxa resistência-peso, superior biocompatibilidade e resistência à corrosão, boas propriedades mecânicas e baixo módulo de elasticidade quando comparado a outros materiais biomateriais metálicos como aço inox e ligas Co-Cr. Um importante aspecto para o sucesso de um implante é a utilização de materiais com módulo de elasticidade baixo, próximo ao do osso, pois estudos indicam que a transferência insuficiente de carga entre implante e osso pode resultar em reabsorção óssea e afrouxamento do implante, que é chamado efeito de blindagem de tensão (stress shielding effect). A liga ternária Ti13Nb13Zr apresenta em sua composição somente elementos considerados biocompatíveis, possui menor módulo de elasticidade que ligas Co-Cr, aço inox e Ti6Al4V, e superior resistência à corrosão. Neste trabalho foi estudado o efeito do teor de nióbio e zircônio nas propriedades mecânicas da liga ternária TiNbZr numa faixa de composições com quantidade de titânio constante em 74 %peso e teor de Nb e Zr variando entre 6 e 20 %peso, produzida por metalurgia do pó a partir de pós hidrogenados e método de mistura elementar, após diferentes condições de processamento e tratamentos térmicos. Os resultados de caracterização microestrutural obtidos por MEV e difração de raios X mostraram que o aumento do teor de Nb não produziu alterações significativas na fração volumétrica das fases e e, o aumento do teor de Zr proporcionou o aumento da fração de fase , favorecendo a formação da estrutura de Widmanstätten. O aumento dos teores de Nb e Zr produziram alterações nas microestruturas dos materiais que levaram ao aumento dos valores de módulo de elasticidade e dureza, e com aumento do teor de Zr houve aumento da susceptibilidade à corrosão. Nas condições de processamento utilizadas, a liga Ti13Nb13Zr apresentou as propriedades mecânicas e microestruturais mais indicadas para utilização como biomateriais. / Biomaterials for use in implants must be biocompatible, biofunctional and resistant to corrosion. Utilization of titanium and its alloys is continuously increasing due to their larger strength-weight ratio, superior biocompatibility e corrosion resistance, good mechanical properties and low elastic modulus when compared to other metallic biomaterials such as stainless steel and Co-Cr alloys. Using materials with low elastic modulus, close to bone values, is important to reduce stress shielding effect, which can cause implant loosening. Ti13Nb13Zr ternary alloy shows only elements considered biocompatible and has a lower elastic modulus than Co-Cr alloy, stainless steel and Ti6Al4V, and superior corrosion resistance. In this work TiNbZr alloy was prepared in different compositions, maintaining Ti content at 74%wt with Nb and Zr contents ranging amongst 6 and 20%wt, produced by powder metallurgy using different processing conditions and heat treatments. Characterization via SEM and X-Ray diffraction showed that the Nb increase didnt produce significant alterations on volume fraction of and phases in the material, and the increase in Zr content led to an increase in phase amounts and formation of Widmanstätten patterns. Nb and Zr content increasing produced microstructural modifications, leading to an increase in elastic modulus and hardness values, as well as corrosion susceptibility variations, where higher Zr contents leaded to a surface with less corrosion resistance. Amongst compositions and for the designed processing method, Ti13Nb13Zr presented the most indicated mechanical and microstructural properties for utilization as biomaterials.

Thesis (Ph. D. (Physics)) -- University of Limpopo, 2013 / A combination of ball milling experiments and ab initio calculations in this study successfully yielded results that shed light into understanding the fundamental basis for immiscibility and the concept of mechanical alloying in Ti-Mg system. In addition, the conditions for achieving extended solid solubility in elements that usually do not dissolve in each other under thermodynamic equilibrium conditions have been predicted using ultrasoft (US) and norm-conserving (NC) pseudopotentials. Hydostatic pressures required to stabilize ordered phases were determined. Our new systematic representation of martensitic transformation (MT) paths as a result of dislocation necessary to induce α→FCC, α→BCC and α→ω phase transitions led to, for the first time, a direct determination of CRSS and tensile strength for Ti and Mg HCP metals. Furthermore, a new ω phase which is less stable than α phase at 0 GPa is proposed. Based on this phase, α→ω deformation path which yielded the onset of uniaxial transition pressure of 4.167 GPa is reported. Attempts of synthesizing Ti-Mg solid solutions by means of Simoloyer high energy ball mill were not successful; however, nanocrystalline Mg-TiH2-x composites were instead formed. These results were attributed to quick formation of metastable Ti hydrides or cold welding at early stages of BM prior to alloying, thus serving as possible obstacles to forming such solid solutions. The deformed Ti crystals adsorbed H+ from the stearic acid leading to formation of metastable orthorhombic TiH2-x phase which later transformed to a tetragonal TiH2-x or even cubic TiH2 when stoichiometric amount of H2 had been adsorbed. Although the yield was significantly lower, the product of milling a mixture of coarse Mg and fine Ti particles was comprised of Ti particles adhering around ductile Mg particles in a core shell manner. The adhesion of the fine hard titanium particles on the surface of the large ductile magnesium particles impeded the further plastic deformation of the titanium particles, thus suppressing the formation of the faults necessary for mechanical alloying. Nanocrystalline Ti powder of about 40 nm was produced by 30h ball milling. During BM of Ti powder, solid-state transformation from HCP to FCC occurred in the presence of PCA with lattice parameters of 4.242 and 4.240 Å after 24 and 30 h, respectively, v due to protonation. When Ti powder was milled in the absence of PCA, no phase transformation was observed for both uninterrupted and interrupted milling cycles. In addition, nanocrystalline Mg powder with crystallite size varying between 60 and below 40 nm was produced by ball milling. However, no solid-state transformation took place even if the powder was milled for 90 h. Therefore, we evidently report for the first time that the interstitial H+ is the driving force for α → FCC phase transformation in ball milled Ti powder. Our theoretical results predicted the ω phase to be the ground-state structure of Ti at 0K and P=0 GPa, in support of other previously reported calculations. We noticed that the stability of the α phase was surpassed by that of the FCC lattice at ~ 100 GPa, corresponding with sudden sharp rise in c/a ratio, hence attributed to α → FCC phase transition. Similar results were obtained for Mg at 50 GPa, although in this case the crossing of lattice energies coincided with minimum c/a. However, using our proposed HCP→BCC MT path mechanism for Mg, it is evident that the minimum c/a at 50 GPa corresponds to a change in the preferred deformation slip from basal (below 10 GPa) to prismatic rather than phase transition. Nonetheless, the proposed MT model predicts that both elemental Ti and Mg prefer to deform via prismatic slip as indicated by lower shear stress as well as CRSS values compared to those calculated for basal slip. Theoretical findings from ab initio calculations on hypothetical ordered Ti-Mg phases indicated absence of intermetallic phases at equilibrium conditions, in agreement with experimental data. However, the formation becomes possible at 80 GPa and above with respect to c/a ratio but requires at least 200 GPa with respect to stable lattices. Using calculated heats of formation, elasticity and DOS, it has been possible to show that L12 TiMg3 could not form even at high pressure as 250 GPa. Nonetheless, both approaches indicate that forming an intermetallic compound between Ti and Mg requires a crystal structure change, α→FCC for Ti and HCP→BCC for Mg. Proposed DFT-based solid solution model for predicting phase stability and elastic properties of binary random alloys, with Mg-Li system serving as a test case, successfully yielded reliable results comparable to experimental data. This method was successfully applied to study an immiscible Ti-Mg system and the solubility limit vi was for the first time theoretically established. Based on formation energy of Ti-Mg solid solutions, our calculations predicted for the first time that the solubility of up to 60 and 100 at.% Mg into Ti with the use of USP and NCP, respectively, to be thermodynamically favourable with necessary lattice kinetics being the main challenge. Nonetheless, NCP proved to be reliable in predicting structural and elastic properties of disordered alloys.

Le travail consiste en l'optimisation de la microstructure d'un alliage usuel TA6V par obtention de phases métastables. Ces nouvelles microstructures, obtenues par traitements thermiques, permettent d'améliorer la tenue mécanique (statique et en fatigue) de l'alliage sans pour autant changer la biocompatibilité. / This PhD aims at optimizing the microstructure of the usual TA6V alloy by using metastable phases. These new microstructures, obtained by thermal treatments, permit to improve the mechanical strength (static but also the fatigue ones) without modifying the biocompatibility.

β-grain size is an influential microstructural parameter on the properties of titanium components. A reduction of β-grain size is generally associated with improvements to ductility, strength, corrosion and fatigue resistance of many α, α/β and β titanium alloys. During production of wrought titanium components, the β-grain size is carefully controlled during thermomechanical processing but there is currently no control of the β-grain size during solidification of cast components. As such, this inability to control the β-grain structure during solidification may limit the applications for solidification based technologies including casting, welding and direct metal deposition. Due to the limited knowledge of grain refinement practices and the lack of commercial grain refiners for the titanium system, this thesis investigates the mechanisms of β-grain refinement during solidification of cast titanium alloys. In this thesis, generalized theories for grain refinement that have been developed from research into other metallic systems are applied to the titanium system. Similar to the findings from aluminium and magnesium research, it is shown that grain refinement of cast titanium alloys requires the addition of growth restricting solutes which provide constitutional undercooling as well as the presence of potent nucleant particles. It is demonstrated that commercially pure titanium contains a natural distribution of nuclei particles which may originate from the mould wall and when powerful growth restricting solutes are introduced, significant prior-β grain refinement is achievable. All solutes investigated do not interact or poison the naturally occurring nucleants enabling the grain size of the titanium alloys to be predicted by an empirically determined relationship based on the growth restriction factor. A full list of growth restriction factors for various elements in titanium is determined and it is proven that growth restriction theory is valid in the titanium system. A further reduction in β-grain size is achievable by introducing additional nucleant particles to titanium castings in conjunction with growth-restricting solutes. Using a novel technique, titanium powder was introduced to the melt stream prior to solidification and was mixed throughout the liquid. The powder particles partially melted and the oxide surface layer dissolved allowing intimate substrate-liquid contact, enabling the titanium substrates to act as sites for heterogeneous nucleation. Using this technique, it was possible to grain refine commercially pure titanium without foreign elemental addition and when growth restricting solutes were present it was possible to obtain approximately an order of magnitude grain size reduction. The results and concepts developed from this work may aid the future development of a commercial grain refiner for titanium. If a grain refiner is developed, its application will not just be limited to the titanium casting industry but may also benefit other solidification based technologies such as welding, direct metal deposition and wrought billet production.

Mechanical properties of Nb-Ti composite superconducting wires were tested at room temperature. The results were analysed using simple composite theory, the rule of mixtures. The objective is to predict the mechanical properties of Nb-Ti superconducting composite wires as a function of volume ratio and geometry of the components, the composite wire size and the effect of heat treatment at final drawing wire sizes. To understand the mechanical behaviors of the Nb-Ti composite, mechanical testing of the individual composite components, Nb-Ti filament and copper matrix, was performed, and the geometry of the composite was also studied. The results indicate that for the monofilamentary composite simple composite theory with two components, Nb-Ti filament and copper matrix, can be used as the prediction of the UTS of the composite. For the multifilamentary composite three components make up the composites; a high strength Nb-Ti fiber, a low strength, high ductility bulk copper matrix and a mid-strength (between the Nb-Ti fiber's and bulk copper matrix's) interfilamentary copper matrix. After heavy cold work the UTS of Nb-Ti filaments and bulk copper matrix in the composite saturate, while the UTS of the interfilamentary copper increases as the interfilamentary spacing decreases. The UTS of the interfilamentary copper matrix as a linear function of the reciprocal of interfilamentary spacing is found. The controlling parameters in the manufacturing which determine the mechanical properties of Nb-Ti composite superconducting wires include superconductor to composite ratio, UTS of the Nb-Ti filament and copper matrix, wire final drawing size, and geometry of the composite such as size and number of the filaments, interfilamentary spacing, volume fraction of fringe and core bulk copper in multifilamentary composites. / Graduation date: 1991