L'application d'un temps de maintien, même de faible durée, lors d'un chargement cyclique, modifie de façon très sensible à la fois le comportement contrainte-déformation et le nombre de cycles à amorçage dans l'alliage base titane TI-6242. Ceci est lié à un régime de fluage cyclique, conduisant à de la déformation progressive d'une part, et à une forte interaction fatigue-temps de maintien pour ce qui concerne le nombre de cycles à amorçage. Les différents phénomènes sont pour le moment assez mal analysés, si bien qu'il n'est pas possible d'effectuer une conception optimale des pièces, de larges marges de sécurité étant nécessaires. Le but du travail est de mieux comprendre les mécanismes locaux qui régissent le comportement et l'amorçage des fissures, dans le but de suggérer des microstructures optimales, et de calibrer des modèles macroscopiques utilisables en bureau d'études. En s'appuyant sur une base expérimentale fournie par Snecma et l'ENSMA, une approche multiéchelles a été mise en place pour représenter les hétérogénéités locales qui ont un rôle significatif sur les comportements observés. Dans les calculs des microstructures, faisant intervenir une étape d'évaluation statistique, on se focalise sur la représentation explicite des ”plumes”, grains de taille exceptionnelle, qui sont à l'origine des premières microfissures en raison du contraste cristallin qu'ils introduisent avec l'environnement. Une revue des différentes configurations de plumes, afin de retenir celles qui sont le plus critique, a été établie. Cette analyse a permis de mettre en évidence la présence de plumes simples, doubles ou triples, les domaines se présentant sous formes de bandes. Les configurations à étudier comportent comme paramètres critiques l'orientation géométrique de la bande par rapport à la direction du chargement macroscopique, mais surtout l'orientation cristallographique au sein de cette (ces) bande(s). Des calculs systématiques ont été effectués afin de mener une étude statistique et de déterminer les configurations les plus sensibles. / The application of a dwell period, even of short length, during a cyclic loading, simultaneously changes the stress-strain behaviour and the number of cycles to failure in a very sensitive way. This phenomenon is connected to a cyclic creep regime, generating progressive deformation, and to a strong interaction between the fatigue process and dwell periods for the number of cycles to failure. All these phenomena are poorly analysed nowadays, so that engineers hardly perform optimal design of the components, since large security margins are necessary. The aim of the work is to better understand the local mechanisms which govern both behaviour and crack initiation, having in view optimal microstructures, and to calibrate manageable macroscopic models for the design department. Using an experimental data set given by Snecma and ENSMA, a multiscale approach has been developed to represent the local heterogeneities that play a significant role on observed behaviour. In the calculations of microstructures that are performed for a statistical evaluation, the focus is made on the explicit representation of the so called "plumes", that are grains of exceptional size, which are at the origin of the first microcracks due to crystal contrast they introduce with the environment. A review of various "plume" configurations is made, in order to investigate the most critical ones. This analysis allowed to shed the light on the presence of simple, double or triple "plumes", the domains being in band shapes. The critical parameters are the geometric arrangement of the band with respect to the direction of the macroscopic loading, but essentially the crystal orientation within this (these) band(s). Systematic calculations were carried out in order to do a statistical study and to determine the most critical configurations.

Dans ce travail, basé sur une approche semi empirique de conception d’alliages de titane à propriétés mécaniques contrôlées, un alliage modèle binaire Ti-12Mo (% massique) et des alliages ternaires sur la base du système Ti-Mo ont été élaborés, combinant des effets TRIP et TWIP lors de la déformation. (TRIP – Transformation Induced Plasticity : plasticité induite par transformation ; TWIP – Twinning Induced Plasticity : plasticité induite par maclage).Les résultats des essais mécaniques montrent que ces alliages présentent une haute résistance mécanique (1000-1200 MPa), une grande plasticité (entre 0,3 et 0,4) et un écrouissage amélioré grâce aux effets simultanés TRIP/TWIP. Différentes techniques de caractérisation telles que la diffraction de rayons X conventionnelle (XRD), la diffraction in-situ sur Synchrotron (SXRD), la diffraction d’électrons rétro-diffusés (EBSD), les mesures de résistivité électrique (ERM), la microscopie électronique en transmission (TEM) et les mesures et traitements automatiques des orientations cristallographiques associées (ACOM/TEM), ont été mis en œuvre pour étudier les mécanismes de déformation, les transformations de phases et l’évolution microstructurale.Différents mécanismes de déformation, tels que le maclage mécanique {332}<113> et la transformation martensitique induite sous contrainte α˝ ont été identifiés à l’issue des essais mécaniques, permettant d’expliquer l’excellente combinaison de propriétés mécaniques obtenue. L’optimisation microstructurale de ces alliages a été conduite à partir de recuits basses températures dans le domaine de précipitation de la phase ω avec pour objectif d’améliorer les propriétés mécaniques afin de contrôler la formation de la phase ω sans modifier de manière excessive la composition chimique de la matrice β, et conserver les effets combinés TRIP/TWIP. / In this work, based on combination of the ‘d-electron alloy design method’ and controlling of electron/atom ratio (e/a), a model of binary Ti-12Mo (wt. %) and ternary Ti-Mo based alloys were designed, induced combined TRIP and TWIP effects (TRIP for Transformation Induced Plasticity and TWIP for Twinning Induced Plasticity). The tensile results show that so-designed alloys exhibit true stress-strain values at uniform plastic deformation, of about 1000-1200MPa and between 0.3 and 0.4 of strain, with a large strain-hardening rate. Several characterization techniques, such as conventional X-ray diffraction (XRD), In-situ Synchrotron X-ray diffraction (SXRD), electron backscatter diffraction (EBSD), electrical resistivity measurements (ERM), transmission electron microscopy (TEM) and automatic crystal orientation measurements (ACOM) TEM, were carried out to to investigate the deformation mechanisms and microstructure evolution sequence. Various deformation mechanisms, i.e. {332}<113> mechanical twinning, deformation induced ω phase and stress-induced α’’ martensite, were identified after mechanical testing, resulting in a combination of high strength, large ductility and improved strain-hardening rate. Furthermore, low temperature aging (LTA) treatments were performed on the Ti-12Mo alloy to improve the mechanical property through controlling the ω phase transformation without excessive modification of β matrix chemical composition, keeping the possibility for combined TRIP and TWIP effects to occur. The influence of LTA treatment on the mechanical behavior and microstructural evolution of Ti-12Mo alloy was discussed in detail.

No description available.

No description available.

In the present work, phase transformations in TIMETAL LCB tita- nium alloy and their influence on mechanical properties were studied. Different initial conditions were prepared by solution treating above β-transus immedi- ately followed by heat treatment in α/β temperature regime. These resulted in different grain boundary α thicknesses and contiguities at a fixed α vol- ume fraction. The subsequent ageing response of this material was studied by low temperature ageing at 400 ◦ C, 450 ◦ C and 500 ◦ C. Phase transformations were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), differential scanning calorimetry (DSC) and resistivity measurements. Mechan- ical properties were investigated using microhardness measurements and tensile tests. It has been proved that metastable ω phase is formed during annealing at 400 ◦ C and 450 ◦ C. ω particles further transform to very fine precipitates of α phase when exposed to annealing for longer time periods. These fine precip- itates significantly contribute to increase of microhardness and achieving high value of yield stress. Keywords: Metastable beta Ti alloys, phase transformations, microstructure changes, ω phase 1

Titanium alloys are widely used for manufacturing of bone implants. Recent studies proved superior mechanical and chemical properties of TiNb alloys. The performance of TiNb is analyzed on Ti39Nb alloy and on evaporated layers of Ti, Nb and TiNb. Performance is compared to Ti6Al4V and pure Ti. This work is focused on the early stage of a bone growth process (studied in vitro). An investigation of this early stage has not been found in any available literature. At this early stage, CaHPO4 compound is formed. This compound is accompa- nied by Mg oxide formation. It is a difference to later stage of this process (as observed in many studies) where the layer is created solely by hydroxylapatite, Ca10(PO4)6(OH)2. The thermal oxidation of surface leads to a grain structure and to a rougher surface. The surface with different roughness effects growth rate depending on material. Generally, the best performance is achieved with TiNb (only polished as well as subsequently thermally oxidized), followed by Ti. 1

No description available.

Commercially available titanium (Ti) alloys, such as Ti–6Al–4V and c.p. Ti, even though established in clinical use as load-bearing bone implant materials in orthopedics and dentistry, possess significant drawbacks that may lead to implant failure: i) presence of alloying elements with harmful health effects, ii) high Young’s modulus (E > 100 GPa) compared to human cortical bone (Ebone = 10 – 30 GPa), and iii) lack of antibacterial activity against multidrug-resistant bacteria, which may lead to implant-associated infections. To overcome the first two drawbacks, a new generation of biocompatible metastable β-type Ti alloys has been developed, in particular β-type Ti–Nb alloys, which are versatile candidates due to their low Young’s modulus, high strength-to-weight ratio and improved corrosion resistance. The present work aims to tackle all three aforementioned issues by developing novel β-type Ti–45Nb-based alloys with potential intrinsic antibacterial activity by adding antibacterial gallium (Ga) and copper (Cu) in minor amounts (up to 8 wt.%) via metallurgical route. Nine alloys with the following chemical compositions: (100-x)(Ti–45Nb)–xGa, (100-x)(Ti–45Nb)–xCu (where x = 2, 4, 6, 8 wt.%), and 96(Ti–45Nb)–2Ga–2Cu, based on alloy design approaches, were produced by controlled casting and homogenization treatment. The effect of antibacterial alloying additions on phase constitution, mechanical characteristics, corrosion, and tribocorrosion response in a simulated physiological environment has been investigated. All nine alloys in the homogenized state display a single-phase β (BCC) phase microstructure, whose lattice parameter is proved to be sensitive to Ga and Cu content, with an almost linear contraction. The mechanical characteristics are strongly influenced by Ga and Cu addition, with a general strengthening effect mainly attributed to substitutional solid solution strengthening, and to grain boundary strengthening in case of Ga. Deformation behavior indicates high mechanical stability of the β phase, suggesting dislocation slip as dominant deformation mechanism. The results demonstrate that strategic alloy design is an effective method to significantly increase strength without adversely affecting the Young’s modulus, which remains in the range of good biomechanical compatibility (E = 64 – 104 GPa). Evaluation of the corrosion response and metal ion release in simulated physiological environment demonstrates the high corrosion resistance of the nine alloys, whereas tribocorrosion wear resistance increases upon Ga addition. Further thermal (aging) treatments, carried out on a specific Cu-containing alloy, proved the feasibility of tailoring enhanced mechanical, chemical and potentially antibacterial properties by thermally-induced precipitation of Ti₂Cu intermetallic compound. These novel developed alloys are considered to be promising candidates for biomedical bone implant applications.

Titanium alloys have outstanding mechanical properties such as high hardness, a good strength-to-weight ratio, excellent fatigue properties and high corrosion resistance. However, several inherent properties including their low thermal conductivity and high chemical affinity to tool materials impairs severely their machinability with conventional machining techniques. Conventional machining of Ti-based alloys is typically characterized by low depths of cuts and relatively low feed rates, thus adversely affecting the material removal rates during the machining process. Recently, a non-conventional machining technique known as ultrasonically assisted turning (UAT) was introduced to machine modern alloys, in which low-energy, high-frequency vibration is superimposed on the movement of a cutting tool during a conventional cutting process. This novel machining technique results in a multi-fold decrease in the level of cutting forces with a concomitant improvement in surface finish of machined modern alloys. Also, since the late 20th century, machining of wear resistant materials that soften when heated has been carried out with hot machining techniques. In this work, a new hybrid machining technique called Hot Ultrasonically Assisted Turning (HUAT) is introduced for processing of a Ti-based alloy Ti-15V3Al3Cr3Sn. In this technique, UAT is combined with a traditional hot machining technique to gain combined advantages of both schemes for machining of intractable alloys. HUAT of the studied alloy was analysed experimentally and numerically to demonstrate its benefits in terms of reduction in cutting forces over a wide range of industrially relevant speed-feed combinations. Thermal evolution in the cutting process was assessed, and the obtained results were compared with FE simulations to gain knowledge of temperatures reached in the cutting zone. The developed novel turning process appeared to improve dry turning of the Ti alloy with significant reduction of average cutting forces without any substantial metallurgical changes in the workpiece material. Nano-indentation, light microscopy and SEM studies were performed to get an insight into the development of hardness in a zone near the machined surface in the workpiece. Backscatter electron microscopy was also used to evaluate the formation of α-Ti during the novel HUAT. No grain changes or α-precipitation were observed in machined workpieces in conventional and hybrid turning processes. 3D elasto-plastic thermomechanically coupled finite-element models for the orthogonal turning process were developed for conventional turning (CT), hot conventional turning (HCT), UAT and HUAT, followed by a more realistic novel 3D finite-element model for the oblique turning process. These 3D models were used to study the effects of cutting parameters (cutting speed, feed rate and depth of cut, ultrasonic vibration, ultrasonic frequency, rake angle and tool nose radius) on cutting forces, temperature in the process zone and stresses. The later model was used to analyse the effect of vibration and heat on the radial and axial components of cutting forces in HUAT, which was not possible with the developed 3D orthogonal-turning model. Comparative studies were performed with the developed CT, HCT, UAT and HUAT finite-element models and were validated by results from experiments conducted on the in-house prototype and in literature. The HUAT for the Ti-15333 was analysed experimentally and numerically to demonstrate the benefits in terms of a significant reduction in the cutting forces and improvement in surface roughness over a wide range of industrially relevant speed-feed combinations.

Durante as últimas décadas, um dos desafios da indústria aeroespacial é em relação ao aumento da eficiência dos motores de turbinas a gás. A eficiência dos motores é limitada principalmente pela temperatura dos gases de combustão, que não pode ser aumentada devido às limitações intrínsecas relacionadas ao uso das superligas à base de Ni nas partes quentes da turbina, onde as temperaturas podem atingir valores acima de 1000 °C. Este trabalho visa caracterizar novos materiais do sistema Nb-Ti para aplicações aeronáuticas, materiais de baixa massa específica que podem substituir as superligas de Ni. As ligas foram produzidas através de fusão a arco, tratadas termicamente a 1200 °C durante 48 h e expostas a temperaturas semelhantes às encontradas na seção de baixa pressão de um turborreator. Os materiais foram caracterizados em termos de composição química, propriedades mecânicas e microestrutura. Foram utilizadas as seguintes técnicas: difração de raios; microscopia eletrônica de varredura, espectrometria de raios X por energia dispersiva e Microscopia Eletrônica de Transmissão. A caracterização microestrutural revelou que as ligas expostas a 1000 °C durante 168 h apresentam uma microestrutura de duas fases composta principalmente de uma matriz ?0-BCC (Nb/Ti) com precipitados de uma segunda fase rica em titânio. Microestruturas de duas fases também foram observadas para as ligas expostas a 800 °C durante 168 h, na qual uma matriz ?0-BCC (Nb/Ti) com precipitados de uma segunda fase identificada como O2-Ti2NbAl foi observada. As ligas estudadas apresentaram massa específica inferior às superligas à base de Ni normalmente utilizadas na indústria aeronáutica. Em termos de propriedades mecânicas, as amostras expostas e testadas a 1000°C apresentaram valores baixos de resistência à compressão (100 MPa) quando comparado as amostras expostas e testadas a 800 °C (565 MPa). Pelos resultados de oxidação observou-se uma maior resistência a oxidação das ligas testadas a 800 °C, porém tanto a 1000 °C como a 800 °C não houve a formação de um filme protetor. / During the last decades, one of the challenges in the aerospace industry is with respect to increase the efficiency of gas turbine engines. The efficiency of the engines is a function of temperatures of the fluel gas, which cannot be increased because of intrinsic limitations related the use of Ni-based superalloys in the hot parts, where temperatures can reach values above 1000 °C. This work aims to investigate new materials in the Nb-Ti system, with low-density materials that may substitute Ni superalloys. The alloys were processed via arc melting, heat treated at 1200°C for 48h and exposed at temperatures similar to those encountered at the low-pressure section in a turbojet engine. The materials were characterized in terms of chemical composition, mechanical properties and microstructure. The following techniques have been used: X-ray diffraction; Scanning Electron Microscopy; Energy Dispersive X-ray Spectrometry and Transmission Electron Microscopy. The microstructural characterization have revealed that the alloys exposed at 1000 °C for 168 hours present a two-phase microstructure composed mainly of a ?0-BCC (Nb/Ti) matrix with precipitations of a second phase rich in titanium. Two-phase microstructures were also observed for the alloys exposed at 800 °C for 168 hours, where a ?0-BCC (Nb/Ti) matrix is observed with precipitates of a second phase identified as O2-Ti2NbAl. The studies alloys reported a lower density when comparing with the Ni based superalloys normally used in the aeronautical industry. In terms of mechanical properties, specimens exposed and tested at 1000 °C showed lower values of compressive strength (100 MPa) than those exposed and tested at 800 °C (565 MPa). The oxidation results allowed to observe a higher oxidation resistance of the alloys tested at 800 °C, however there was no protective film formation at 1000 °C as at 800 °C.