Metastable b-Ti alloys are titanium alloys with sufficient b stabilizer alloying additions such that it's possible to retain single b phase at room temperature. These alloys are of great advantage compared to a/b alloys since they are easily cold rolled, strip produced and can attain excellent mechanical properties upon age hardening. Beta 21S, a relatively new b titanium alloy in addition to these general advantages is known to possess excellent oxidation and corrosion resistance at elevated temperatures. A homogeneous distribution of fine sized a precipitates in the parent b matrix is known to provide good combination of strength, ductility and fracture toughness. The current work focuses on a study of different mechanisms to engineer homogeneously distributed fine sized a precipitates in the b matrix. The precipitation of metastable phases upon low temperature aging and their influence on a precipitation is studied in detail. The precipitation sequence on direct aging above the w solvus temperature is also assessed. The structural and compositional evolution of precipitate phase is determined using multiple characterization tools. The possibility of occurrence of other non-classical precipitation mechanisms that do not require heterogeneous nucleation sites are also analyzed. Lastly, the influence of interstitial element, oxygen on a precipitation during the oxidation of Beta 21S has been determined. The ingress of oxygen and its influence on microstructure have also been correlated to measured mechanical properties.

The results from this study, on a few commercial and model metastable beta titanium alloys, indicate that the growth restriction factor (GRF) model fails to interpret the grain growth behavior in the additively manufactured alloys. In lieu of this, an approach based on the classical nucleation theory of solidification incorporating the freezing range has been proposed for the first time to rationalize the experimental observations. Beta titanium alloys with a larger solidification range (liquidus minus solidus temperature) exhibited a more equiaxed grain morphology, while those with smaller solidification ranges exhibited columnar grains. Subsequently, the printability of two candidate beta titanium alloys containing eutectoid elements (Fe) that are prone to beta fleck in conventional casting, i.e., Ti-1Al-8V-5Fe (wt%) or Ti-185, and Ti-10V-2Fe-3Al (wt%) or Ti-10-2-3, is further investigated via two different AM processing routes. These alloys are used for high-strength applications in the aerospace industry, such as landing gears and fasteners. The Laser Engineered Net Shaping and Selective Laser Melting (the two AM techniques) results show that locally higher solidification rates in AM can prevent the problem of beta fleck and potentially produce β-titanium alloys with significantly enhanced mechanical properties over conventionally cast/forged counterparts. Further, the detailed investigation of microstructure-mechanical property relationships indicates that the precipitation or formation of non-equilibrium secondary phases like α or ω in these commercial systems can be advantageous to the mechanical properties. The influence of process parameters on the evolution of such secondary phases within the β matrix grains has also been rationalized using a FEM-based multi-physics thermo-kinetic model that predicts the multiple heating-cooling cycles experienced by the layers during the LENS deposition. Overall, the results indicate that Ti-1-8-5 and Ti-10-2-3 are promising β-Ti alloys for AM processing. Further, the results also demonstrate the ability to tune the microstructure (secondary phase precipitation and grain size) via changes in the process parameters to achieve desirable mechanical properties, obviating the need for any secondary post-processing. The understanding obtained through this work can be coupled with the concept of β-phase stability prediction, via parameters like bond order (Bo), the energy level of metal d-orbital (Md), Mo equivalency, etc., to design novel beta titanium alloys with the desired microstructures tailored via AM for structural applications.

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Made available in DSpace on 2017-07-21T20:42:35Z (GMT). No. of bitstreams: 1 rodrigoric.pdf: 9183178 bytes, checksum: 1d5fba078a589cbc1810eea245d1e019 (MD5) Previous issue date: 2006-10-31 / Mechanical alloying is a powder processing technique involving cold welding, fracturing mechanisms and rewelding of powder particles in a ball mill. The present work applied this technique with the purpose of processing titanium, niobium and tin alloys, through planetary ball mill and attritor mill. The atomic percentages of these elements were varied in six differents ways. The niobium hydrate was used on production of some alloys in study, with the purpose to observe the effects during the milling and on the alloys obtained after sintering. Differences between processing in planetary ball mill and attritor mill, as well the cold welding influences on the kinetic parameters of milling process were approached. The use of niobium hydrate result in a decrease of powders average crystallite size, and an increase of micro hardness of sintered alloys. This work obtained beta titanium alloys and alpha-beta titanium alloys in namometric sizes. The average crystallite size, for milled powders, was 7,6 nm. / Mecano-síntese é uma técnica de processamento em pó que envolve a soldagem a frio, mecanismos de fratura e a resoldagem das partículas de pós, em moinhos de bolas. O presente trabalho utilizou esta técnica com o propósito de processar ligas de titânio, nióbio e estanho, através de moinhos de bolas planetário e de atrito. As porcentagens atômicas destes elementos foram variadas de seis maneiras diferentes. O hidreto de nióbio foi utilizado na produção de algumas ligas em estudo, com o propósito de se observar os efeitos durante a moagem e na liga obtida após a sinterização. Diferenças entre o processamento em moinho de bolas planetário e o moinho de atrito, assim como as influências da soldagem a frio nos parâmetros cinéticos do processo de moagem, foram abordadas. O uso do hidreto de nióbio resultou na diminuição do tamanho médio de cristalito dos pós, e em um acréscimo na microdureza das ligas sinterizadas. Neste trabalho foram obtidas ligas de titânio beta e de titânio alfa-beta, em tamanhos nanométricos. O tamanho médio do cristalito, para os pós moídos, foi de 7,6 nm.

The penchant towards development of high performance materials for light weighting engineering systems through various thermomechanical processing routes has been soaring vigorously. Friction stir processing (FSP) - a relatively new thermomechanical processing route had shown an excellent promise towards microstructural modification in many Al and Mg alloy systems. Nevertheless, the expansion of this process to high temperature materials like titanium alloys and steels is restricted by the limited availability of tool materials. Despite it challenges, the current thesis sets a tone for the usage of FSP to tailor the mechanical properties in titanium alloys and steels. FSP was carried out on three near beta titanium alloys, namely Ti6246, Ti185 and Tiβc with increasing β stability index, using various tool rotation rates and at a constant tool traverse speed. Microstructure and mechanical property relationship was studied using experimental techniques such as SEM, TEM, mini tensile testing and synchrotron x-ray diffraction. Two step aging on Ti6246 had resulted in an UTS of 2.2GPa and a specific strength around 500 MPa m3/mg, which is about 40% greater than any commercially available metallic material. Similarly, FSP on an ultra-high strength steel―Eglin steel had resulted in a strength greater than 2GPa with a ductility close to 10% at around 4mm from the top surface of stir zone (SZ). Experimental techniques such as microhardness, mini-tensile testing and SEM were used to correlate the microstructure and properties observed inside SZ and HAZ's of the processed region. A 3D temperature modeling was used to predict the peak temperature and cooling rates during FSP. The exceptional strength ductility combinations inside the SZ is believed to be because of mixed microstructure comprised of various volume fractions of phases such as martensite, bainite and retained austenite.

Metallic biomaterials have an essential portion of uses in biomedical applications. Their properties can be tuned by many factors resulting in their process tuneability. Among metallic biomaterials for biomedical and specifically orthopedic applications, titanium and its alloys exhibit the most suitable characteristics as compared to stainless steels and Co-Cr alloys because of their high biocompatibility, specific strength (strength to density ratio), and corrosion resistance. According to their phase constitution, Ti-alloys are classified into three main groups, namely alpha, beta, and alpha+beta alloys. Depending on the degree of alloying and thermomechanical processing path, it is possible to tune the balance of α and β phases, which permits to tailor properties like strength, toughness, and fatigue resistance. (alpha+beta) Ti alloys, especially Ti-6Al-4V, are widely used alloys in biomedical applications; however, they have some drawbacks like the presence of toxic elements i.e., V and relatively high elastic modulus to that of bones. In view of the lower elastic modulus of body center cubic beta phase (50GPa<100GPa) compared to the alpha+beta, as well as due to their good mechanical properties, excellent corrosion resistance, and biocompatibility, beta-Ti alloys have been recently proposed as a valid alternative to alpha+beta ones. The growing interest in additive manufacturing (AM) techniques opens new and very interesting perspectives to the production of biomedical prosthetic implants. AM will prospectively allow implant customization to the patient and produce it on demand, with large savings on times and costs. Moreover, AM is gaining increasing interest due to the possibility of producing orthopedic implants with functionally graded open-cell porous metals. The main advantages of porous materials are the reduction of the elastic modulus mismatch between bone and implant alloy reducing the stress shielding effect and improving implant morphology providing biological anchorage for tissue in-growth. In this scenario, the first goal of the present PhD thesis work was to identify a high-performance β-Ti alloy formulation suitable to Laser-Powder Bed Fusion (L-PBF) additive manufacturing. In particular, it explores the potential use of a β-metastable Ti alloy, namely Ti-15Mo-2.7Nb-3Al-0.2Si (Beta Ti21S, 21 wt.% of alloying additions, including Silicon) for biomedical applications. Through microstructural, mechanical, and cytotoxicity analyses, it could be shown that this alloy grade exhibits i) an unprecedented ultra-low elastic modulus, ii) improved cytocompatibility due to the lack of Vanadium, and iii) no martensitic transformation responsible for hard and brittle solidification structures. A second goal was to assess the manufacturability of metamaterials made of β-Ti21S via L-PBF. For this purpose, cubic cellular lattice structures of different unit cell sizes (and therefore different strut thickness) have been fabricated and characterized through microstructural analysis using different techniques, and computed tomography combined with linear elastic finite element simulations to identify the minimum cell size that can be printed with adequate dimensional and geometrical accuracy. Samples of the selected unit cell size were also tested to determine their static and fatigue properties. The main results show that i) the suitable manufacturing quality is obtained for strut thickness above 0.5 mm, ii) the mechanical tests place the present cellular structures among the best stretching dominated cellular lattice materials investigated to date in the literature, and iii) the fatigue tests showed a normalized fatigue strength at 107 cycles of close to 0.8, similar to cubic lattices made of Ti-6Al-4V, and higher than most cellular structures in the literature. In the last part of the thesis, a more complex octet truss structure was fabricated in the manufacturable cell size, and its mechanical properties were investigated. The octet truss topology can be beneficial both in terms of mechanical properties and biocompatibility by providing the different types of porosity suitable for bone in-growth.

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Ziel der Arbeit war es in einem pulvermetallurgischen Ansatz gasverdüste Ti-45Nb-Pulver mittels Heißpressen zunächst zu kompakten Formkörpern zu verpressen und über geeignete Gefügeeinstellung und bestmöglicher Partikelverzahnung maximale Druckfestigkeiten bei gleichzeitig niedrigem E-Modul zu erhalten. In einem nächsten Schritt wurden mittels Heißpressen mit Platzhalterphase definierte Porenanteile in die Formkörper eingebracht und der Einfluss dieser auf die mechanischen Eigenschaften untersucht. Die porösen Strukturen sollen als Knochenersatzmaterial in einem osteoporotischen Knochendefekt dienen. In einem solchen Defekt stellen Druckkräfte den dominierenden Belastungsfall dar. Die mechanische Charakterisierung der im Rahmen der Arbeit erzeugten porösen Formkörper erfolgte daher im Druckversuch. Die Oberfläche eines metallischen Knochenersatzmaterials muss chemisch und topografisch modifiziert werden, um damit Einfluss auf das Gleichgewicht zwischen zellbiologischen Prozessen zum Knochenauf- und -abbau an der Grenzfläche zwischen Implantat und Knochengewebe zu nehmen. Im speziellen Fall von Osteoporose, wo dieses Gleichgewicht nachweislich gestört ist, spielt die Stimulation des Knochenaufbaus eine besondere Rolle. Für Strontiumspezies konnte eine das Knochenwachstum stimulierende Wirkung und die Inhibierung des Knochenabbaus in mehreren Studien gezeigt werden. Ein weiteres Ziel der Arbeit stellte daher die Erzeugung von strontiumhaltigen Hydroxylapatitschichten mittels Elektrodeposition dar. Die erzeugten Schichten wurden strukturell, morphologisch und chemisch charakterisiert. Weiterhin wurden die Sr-Freisetzung aus den Schichten und die zellbiologische Wirkung untersucht. Konzepte zur Abscheidung auf planaren Legierungsoberflächen konnten in einem nächsten Schritt im Rahmen einer Machbarkeitsstudie auf poröse Ti-45Nb Strukturen übertragen werden. / Aim of the work was the production of dense Ti-45Nb material by hot-pressing of gas-atomized Ti-45Nb powder. Maximum compression strength and low Young’s modulus values were obtained by means of a tailored microstructure and improved interlinking of the powder particles. In a next step defined amounts of porosity were introduced by hot-pressing the alloy powder with a space holder phase. The produced porous structures should be used as bone substitute material in an osteoporotic bone defect. Compression is the dominating load in such a defect. Accordingly, compression tests were conducted to assess the mechanical properties. The surface state of metallic bone replacement materials plays an important role regarding the osseointegration of the material into the surrounding bone tissue. A chemical and topographical modification of the surface is necessary to influence the equilibrium between the formation and resorption of bone on the interphase of implant and bone tissue. Especially in case of osteoporosis the stimulation bone formation is essential. Several studies have shown that strontium species have a positive effect on the formation of bone tissue and the inhibition of bone resorption. Therefore, a further aim of the work was the electrodeposition of Sr-containing hydroxyapatite layers and the structural, morphological and chemical characterization of the deposited layers. Furthermore, the release of Sr-species from the layers and the effect on hMSC (human mesenchymal stroma cells) were examined. Originating from studies on planar alloy surfaces, the transfer of the deposition approaches was shown in a proof of concept on the porous Ti-45Nb scaffolds.