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International audience; In this this work, superelastic NiTi and Ni-free Ti-23Hf-3Mo-4Sn biomedical alloys were investigated by tensile tests in relationship with their microstructures. To follow the stress-induced martensitic transformations occurring in these alloys, in situ tensile tests under synchrotron beam were conducted. In NiTi, an intermediate trigonal R phase, which is first stress-induced before the B19 ' martensitic phase, was identified. However, the Ti-23Hf-3Mo-4Sn alloy does not present a transitional phase, and a direct beta into alpha '' reversible stress-induced martensitic transformation was observed. With NiTi, all the applied strain is recovered after unloading, and no residual plastic deformation occurs. However, the strain is not completely recovered with the Ti-23Hf-3Mo-4Sn alloy, and residual plastic strain was observed to prevent a complete recovery, thus explaining why the strain recovery is lower for Ti-23Hf-3Mo-4Sn compared with NiTi. We also showed that the maximum strain recovery depends on the texture in the Ti-23Hf-3Mo-4Sn alloy. The favorable texture leading to the highest strain recovery (4.6 pct) is the {111}< 110 and rang;(beta) texture, which can be obtained by a short-time solution treatment (0.3 ks) at 1073 K with this alloy.

International audience; The main goal of this paper is to propose some analytical or pseudo-analytical tools facilitating the study and modeling of SMA helical spring-based actuators. Particular attention is paid to R-phase SMA helical spring-based actuators (i.e., springs made in NiTi and concerned by R-phase transformation phenomena). First, simplified models (i.e., based on beam theory framework) of R-phase SMA helical springs taking into account detwinning and assisted two-way memory effect are proposed and validated. Then, these models are used to determine the thermo-mechanical behavior of a linear bias-spring actuator. An actuator is built and used to validate the proposed models.

International audience; We report on the crystallographic phases and their epitaxial relationships observed during the metal-insulator transition (MIT) of a VO2 film deposited on Al2O3(0001). A key feature of this work is to establish two possible in-plane epitaxial relationships for the VO2 film, introducing an “on-axis” and a “diagonal” model. These models have distinctive signatures in reciprocal spaces in the form of multiplet reflections and can, therefore, be easily differentiated by diffraction. They serve as a basis for interpreting the results gained by high-resolution X-ray diffraction, and complemented by Raman spectroscopy, on a 420-nm-thick VO2 film grown by microwave plasma-assisted reactive sputtering on Al2O3(0001). We address its orientation and follow its structure through the MIT. The film is oriented according to the diagonal model and exhibits a MIT involving three phases with different temperature domains of (co)existence. The room- and high-temperature phases are, respectively, identified as the monoclinic M1 and rutile R polymorphs of VO2. Additionally, we observe that when the sample resistivity starts to decrease, the insulating M1 phase turns into an intermediary phase, which then partially transforms into the metallic R phase when the MIT is complete. The intermediary phase is similar to the M2 polymorph of VO2, according to its lattice parameters, Raman signature, and its known proximity with the M1 and R phases in tensile conditions. The mixing of the M2 and R phases is still detected at 110∘C, i.e., far above the MIT; the M2 polymorph is being probably stabilized at the interface between the film and the substrate, where the interfacial strain is higher.

This Ph.D. dissertation thesis addresses the conditions and mechanisms that lead superelastic NiTi wires to fail under cyclic mechanical loads. NiTi shape memory alloys exhibit functional thermomechanical properties (superelasticity, shape memory effect, thermal actuation) due to martensitic phase transformations caused by a change of the applied stress and temperature. These phase transformations are though as fully reversible damage-free processes, however, when NiTi is subjected to repetitive stress-induced phase transformations its fatigue performance drops drastically in comparison to non-transforming NiTi. Most of fatigue S-N curves reporting this drop were measured on straight NiTi wires in which martensitic transformations proceed heterogeneously through nucleation and propagation of shear bands. Moreover, from our experience fatigue testing straight wire samples results in undesired failure inside the testing machine clamps. Hence, the reported stress-strain values in S-N curves are not necessarily representative of the critical mechanical conditions that lead the material to failure. With the aim of better characterize the fatigue performance of NiTi wires, we started by carrying out a series of pull-pull fatigue tests using hourglass-shaped samples. This sample geometry allowed us to confine all martensitic transformation and related material fatigue processes into a well-defined gauge volume. The samples’ characterization was performed by combining several experimental and analysis techniques such as Digital Image Correlation, Infrared Thermography, Synchrotron-source X-ray diffraction, Optical Microscopy, Scanning Electron Microscopy and Finite Element Analysis. A special attention was paid to the High Cycle Fatigue (HCF) performance of NiTi in which the material shows elastic behavior and/or an intermediate phase transformation (so-called R-phase). The results from HCF tests allowed us to distinguish crack nucleation and crack propagation stages during the total life of our NiTi samples. In order to get a better understanding of the mechanisms that lead to crack nucleation, we applied the nonconventional Self-Heating fatigue assessment method, which has shown efficiency in the case of aluminum and steel alloys. This method correlates the temperature elevation of a sample subjected to different cyclic load amplitudes with energy dissipating mechanisms that contribute to accumulating local damage in the material. The Self-Heating method was performed using full-field thermal measurements of cyclically loaded NiTi hourglass-shaped samples., Ce travail de thèse aborde les conditions et les mécanismes qui conduisent des fils superélastiques de NiTi à la rupture sous chargement mécanique cyclique. Les alliages à mémoire de forme du type NiTi présentent des propriétés thermomécaniques fonctionnelles comme la superélasticité et l’effet de mémoire de forme simple et double, lesquels sont générées grâce aux transformations de phase martensitiques provoquées soit par un changement de la contrainte ou de la température. Ces transformations de phase sont en principe des processus totalement réversibles et sans endommagement. Cependant, lorsque le NiTi est soumis à des transformations de phase induites par des contraintes cycliques, la performance en fatigue de l’alliage chute considérablement par rapport au NiTi non-transformant. La plupart des courbes S-N de fatigue rapportant cette chute ont été mesurées sur des fils NiTi a section constante dans lesquels les transformations martensitiques se développent de façon hétérogene par nucléation et propagation de bandes de cisaillement. De plus, d'après notre expérience, des essais de fatigue sur des échantillons de fils à section constante entrainent la rupture à l'intérieur des mors de la machine d'essai. Par conséquent, les valeurs de contrainte-déformation rapportées dans les courbes S-N ne sont pas nécessairement représentatives des conditions mécaniques critiques qui conduisent le matériau à la rupture. Dans le but de mieux caractériser les performances en fatigue des fils NiTi, nous avons effectué une série de tests de fatigue en traction-traction, tout en utilisant des échantillons sous forme ≪ diabolo ≫. La géométrie de ces échantillons nous a permis de confiner tous les processus de transformation martensitique et de fatigue dans un volume utile bien défini. La caractérisation du comportement thermomécanique de ces échantillons a été réalisée en combinant plusieurs techniques expérimentales et d'analyse telles que la corrélation d'image numérique(DIC), la thermographie infrarouge, la diffraction des rayons X à source synchrotron, la microscopie optique, la microscopie électronique à balayage et l'analyse par éléments finis. Une attention particulière à été portée à la performance de NiTi dans le régime à grand nombre de cycles (HCF) dans laquelle le matériau présente un comportement élastique ou une transformation de phase intermédiaire (appelée R-phase). Les résultats des tests de fatigue nous ont permis de distinguer les étapes de nucléation et de propagation des fissures pendant la durée de vie totale de nos échantillons. Afin de mieux comprendre les mécanismes qui conduisent à la nucléation des fissures, nous avons appliqué la méthode de l’auto-échauffement, qui a démontré son efficacité dans la prédiction de fatigue dans les cas des alliages d'aluminium et des alliages d'acier. Cette méthode corrèle l'élévation de température d'un échantillon soumis à différentes amplitudes de charge cyclique avec des mécanismes de dissipation d'énergie. Ces mécanismes dissipatifs sont après associés à l’accumulation d’endommagement locale dans le matériau. La méthode d'autoéchauffement a été réalisée en utilisant des mesures de champs thermiques des d'échantillons de NiTi sous forme diabolo pendant de chargement cyclique.

In this article the structural fatigue performance of superelastic NiTi wires is assessed using hourglass-shaped samples. This non-traditional geometry of NiTi fatigue test samples permits to avoid the random deformation localization processes upon wire deflection, which makes it difficult to interpret correctly S-N and Srain-N curves. Force controlled fatigue tests were performed at different temperatures and were complemented by self-heating measurements. The obtained results aims to shed light on the role of the phase transformations of the material into R-phase and B19' in the cyclic damaging of NiTi wires subjected to repetitive loadings., Dans cette étude, les performances en fatigue structurelle de fils de NiTi est estimée en utilisant des éprouvettes en diabolo. Cette forme d'éprouvette, non-traditionnelle pour des essais de fatigue du NiTi, permet d'éviter une localisation aléatoire de la déformation au cours de l'allongement qui rend difficile l'étude de la fatigue. Des essais de fatigues pilotés e force ont étés réalisés à différentes températures et ont été complétés par des mesures d'auto-échauffement. Les résultats obtenus permettent de mettre en évidence le rôle des transformations de phase du matériau en R-phase et martensite sur la fatigue des fils de NiTi.

International audience; The well-known martensitic transformation is not always the unique solid-solid phase change in NiTi shape memory alloys (SMA). For this material, R-phase can occur from both austenite and martensite. In some applications, macroscopic strain of the material can be limited to 2%. In these cases, R-phase contribution can not be neglected anymore when compared with martensite. Furthermore, different thermomechanical couplings have to be taken into account to carefully predict strain rate effects and to better describe application conditions. In this paper, a new model taking into account various phase transformations with thermomechanical couplings is presented. This model is based on several transformation criteria. In most applications, SMA are used as wires, submitted to tensile-tensile loadings, in the superelasticity working range. Consequently, a uniaxial reduction of the model is presented for its simplicity. A thermodynamic framework is proposed. It enables to describe the internal variables evolution laws. The simple and fast identification process of model parameters is briefly presented. To verify the validity of the proposed model, simulation results are compared with experimental ones. The influences of testing temperature and strain amplitude on the material behavior is discussed. The damping capacity is also studied, using an energy-based criterion.

International audience; NiTi shape memory alloys (SMAs) wires exhibit R-phase frequently. This intermediate phase has interesting properties under cyclic loadings and fatigue. These wires have a good potential in civil engineering applications for their damping capacity. In accordance with these applications, in this study, a complete characterisation of NiTi wires is proposed. First the pseudo phase diagram concerning R-phase and its existence comparing to austenite or to martensite is obtained by realising tensile tests at different temperatures and strain amplitudes. Then, the cyclic behaviour under tension at various amplitudes is studied. Based on these loadings, the significant observations are: a small residual strain, the decrease of transformation yield stresses and the evolution of the hysteresis size with the number of cycles. To consider the real use conditions, the effect of strain rate has been studied at two temperatures. The obtained results show that, due to thermo-mechanical coupling, the mechanical behaviour is affected by the evolution of strain rate. Eventually, the damping effect of the studied wires has been evaluated. All the obtained results confirm the potential use of NiTi wires exhibiting R-phase in civil engineering applications. They also constitute an interesting reference for models development describing R-phase occurrence and ease their identification and validation procedures.

Shape memory alloys (SMA) have many interesting properties due to solid-solid phase transformations (usually between austenite and martensite), such as super-elasticity and/or shape memory effects. More particularly, Nickel-Titanium (NiTi) based SMA are currently used in many industrial fields for their oxydation resistance and their fatigue resistance. Furthermore, their dissipation capacities make them particularly suitable for using as dampers dedicated to Civil Engineering issues. Indeed, several phenomena (road traffic, earthquakes, wind, rain...) which are the main causes of structure vibrations, affect the sustainability of bridge cables. Current solutions, consisting in setting-up hydraulic dampers, are not satisfactory out of their working range in terms of amplitude and frequency. A new device based on NiTi wires is studied in this thesis. However, this material adapts its thermomechanical response according to input loading rates or amplitudes and thermal surroundings.In the thesis, we have developed a numerical tool which enables to predict the NiTi wires based damper influence on the dynamical response of the cable. Thus, the specimens are characterized using experimental tests in order to build a numerical thermomecanical model taking into account the superelasticity effect, at the REV scale. The model is validated according to an energetical criterion. The model is then extended to the scale of the structure, using a non-local finite elements model, in order to investigate heterogeneity effects.A damping device, developed during the thesis, is tested successfully on a full-scale bridge cable. Furthermore, the NiTi wires based damping device shows a real damping power effectiveness. The study of several test configurations provides recommendations for optimisation of the system. A related finite elements model is used to realize a transient dynamic analysis. The model, which lies on the superelastic law, is validated. It justifies, afterwards, the consideration of phenomena assumed to be sources of dissipation, such as intrinsic dissipation, thermomechanical coupling and R-phase transformation. These phenomena can be numerically isolated, to evaluate how they take part in the mitigation of cable vibrations., Les Alliages à Mémoire de Forme (AMF) possèdent, entre autres, des propriétés de superélasticité et de mémoire de forme remarquables dues à une transformation de phase solide-solide entre l'austénite et la martensite. En particulier, les AMF à base de Nickel-Titane (NiTi) sont aussi utilisés dans l'industrie, pour leur résistance à l'oxydation et leur tenue en fatigue. Leur capacité à dissiper l'énergie incite à leur utilisation au sein de dispositifs amortisseurs dédiés au Génie Civil. En effet, les sources de vibration (trafic routier, séismes, vent, pluie...) affectent la durabilité, vis-à-vis de la fatigue, des câbles de pont. Les amortisseurs de type hydraulique, utilisés jusqu'à aujourd'hui, peuvent transmettre des contraintes néfastes à la structure hors de leur domaine d'utilisation (en fréquence et amplitude). Un nouveau dispositif à base de fils NiTi est étudié au cours de cette thèse. Toutefois, ce matériau adapte son comportement thermomécanique aux conditions de chargement et à l'environnement thermique. Cette étude a permis de mettre au point un outil numérique destiné à caractériser l'influence d'un tel dispositif sur la réponse dynamique d'un système "câble+amortisseur". Pour cela, le matériau est caractérisé expérimentalement afin d'alimenter un nouveau modèle numérique du comportement thermomécanique en superélasticité, à l'échelle du VER. Le modèle proposé est validé selon un critère énergétique. Celui-ci est ensuite étendu à l'échelle du fil par un modèle non-local, afin d'explorer les effets d'hétérogénéité de comportement, dans le but d'une utilisation concrète. Un prototype d'amortisseur, développé au cours de cette thèse, a été testé avec succès sur un câble de pont à l'échelle 1. Le dispositif montre une réelle efficacité à réduire les amplitudes de vibration du câble. L'analyse de différentes configurations d'essais sur le câble fournit des pistes d'optimisation du système. Un modèle d'éléments finis associé, intégrant la loi de comportement du fil, permet de réaliser une analyse dynamique transitoire. Celui-ci est validé, justifiant ainsi la prise en compte des différentes sources de dissipation observées expérimentalement, à savoir : la dissipation intrinsèque, le couplage thermomécanique et la présence d'une phase solide intermédiaire (R-phase). Ces dernières peuvent être découplées, afin d'évaluer leur contribution à l'amortissement du câble.}

Colloque avec actes et comité de lecture. Internationale.; International audience; Les alliages à mémoire de forme de type NiTi, mettent en jeu des phénomènes complexes lors des transformations entre les différentes phases Austénitique (A), Martensitique (M) et phase R (R). Ces phénomènes sont observés à l’aide de nombreux dispositifs expérimentaux ayant pour objectif de déterminer les caractéristiques nécessaires pour la modélisation et la simulation. Ce travail présente les résultats expérimentaux issus d’une analyse par un dispositif non conventionnel, qui couple un spectromètre mécanique et thermique plus connu sous le nom DMTA (Dynamical Mechanical and Thermal Analysis) à une machine de traction classique. Des mesures dynamiques du module de conservation (E’) et de l’amortissement (tan ) ont été effectuées pendant deux types d’essais. Les essais du premier type sont des tractions isothermes pour des températures allant de -100°C à +100°C, en charge/décharge et en charge jusqu’à rupture. Les essais de second type sont effectués sous contrainte constante s’échelonnant de 50 MPa à 1000MPa, au chauffage et refroidissement entre 200°C et -140°C. En analysant l’évolution de E’ sur l’ensemble des essais, il est possible de détecter les changements de phase, caractérisés par une diminution importante de E’. Il est ainsi possible de déterminer les domaines d’existence des différentes phases, séparés par les lieux de transformations directes et inverses. Ces résultats sont en cohérence avec ceux obtenus à contrainte nulle par DSC. De plus, il a été effectué une analyse de l’évolution de E’ en fonction de la température et de la contrainte dans les domaines où l’alliage est monophasique, totalement austénitique ou martensitique. Les évolutions de E’ sont importantes. Pour l’austénite, E’ augmente avec la température, contrairement aux alliages métalliques usuels.

International audience; Heat sources estimations from temperature field measurements deduced from infrared imaging are increasingly used to study thermo-mechanical coupling during materials deformation. These estimations are based on approximations of the derivative terms with respect to time and space which are involved in the heat diffusion equation. This paper proposes a first experimental validation of this method by applying it to an experimental uniform air cooling of a NiTi Shape Memory Alloy thin plate. In the studied cooling temperate range, heat sources are due to Austenite to R phase transformation. Transformation temperatures, heat sources and energies are estimated from infrared temperature measurements and compared to differential scanning calorimetry results. The small 2 discrepancies are mainly explained by errors in DSC measurements due to thermal inertia and baseline determination.

International audience; The principal focus of this work is to identify the microstructural changes occurring in the Fe-27Co-8Mo (mass%) alloy on aging. The constitution and microstructure of the alloy have been studied for aging treatments between 800 degrees C and 600 degrees C using X-ray diffraction, transmission electron microscopy and energy dispersive X-ray spectroscopy. For a long aging duration and for the highest temperatures, micrometric precipitates of mu phase are found preferentially at grain boundaries of the alloy. At lower temperatures, nanometric precipitates of R phase are observed inside the grains in addition to mu sub-micron precipitates at the grain boundaries. The mu precipitates show lattice parameters slightly different from the Fe7Mo6 phase and contain a high density of crystalline defects. The first phases to come out from solid solution on heating the alloy at 600 degrees C are Mo rich precipitates with a cube-cube orientation and ordered domains. These nanometric phases bring a very high hardness to the alloy at the onset of aging.

A revival of the interest on the morphotropic phase boundary of the PbZr 1 - x Ti x O 3 started after a neutron diffraction study revealed the existence of a monoclinic phase (M) bridging the tetragonal (T) and rhombohedral (R) ones. In addition, the M phase has been found to exhibit a low temperature transformation, possibly involving rotations of the oxygen octahedra, raising a debate on its nature. Finally, the rotational instability has been predicted to extend also in the T phase. We present measurements of the complex dielectric susceptibility and elastic compliance of PbZr 1 - x Ti x O 3 with 0.455 ⩽ x ⩽ 0.53 which bring new information on the low temperature phase transformations. The T–M transition gradually changes from first- to second-order increasing the Ti content from the boundary with the R phase to x = 0.48 , confirming that the coexistence of T and M phases found by neutron diffraction is intrinsic and not due to compositional fluctuations. Strikingly, the T–M transformation causes a peak in the elastic compliance, whose amplitude may even exceed that of the jump at the cubic-to-tetragonal transition, as if it were a ferroelastic transformation; it is shown that indeed there is linear coupling between shear strain and order parameter, when the T–M transition mainly consists of a rotation of the polarization. The instability involving rotation of the octahedra below 200 K is visible both in the dielectric and anelastic losses, and it extends in the tetragonal phase, as predicted by recent first-principle calculations.

International audience; The lattice parameters of epitaxial barium strontium titanate films with various thicknesses (from 6 to 960 nm) were measured as a function of temperature in the normal and tangential directions with respect to the film plane using x-ray diffraction. The films were grown through the layer-by-layer mechanism by rf cathode sputtering under elevated oxygen pressure. A critical film thickness (similar to 50 nm) was found to exist, below and above which the films are subjected to compressive and tensile stresses, respectively. As the temperature varies from 780 to 100 K, the films undergo two diffuse structural phase transitions of the second order over the entire thickness range. The transitions in the films under tensile stresses are likely to be transformations from the paraelectric tetragonal to aa phase and then to r phase, whereas the transitions under compressive stresses are transformations from the tetragonal paraelectric to ferroelectric c phase and then, with further decreasing temperature, to r phase.

The martensitic transformation of Ti–Ni shape memory alloys is very sensitive to thermomechanical treatments (cold work and annealings). In the present paper, a Ti–Ni alloy close to the equiatomic composition has been 40% cold rolled and then submitted to various annealing treatments, each one characterized by temperature and time (Ta, ta). A large range of annealing temperatures (538–778 K) and times (10 s–123 d) has been covered. From differential scanning calorimetry measurements, three zones have been identified with reference to (Ta, ta) values. Zone I, at low temperatures and times, is characterized by badly defined transformations on cooling as well as on heating; it corresponds to a recovery–reverse martensitic transformation stage. Zone II exhibits large changes of the transformation features which occur in two steps on cooling (R phase then martensitic) and in one step on heating; this zone corresponds to the recrystallization and growth of very small stressed grains. Similar transformation behavior can be obtained with different (Ta, ta) annealings, which can be characterized by an activation energy of ≈3.4 eV in the range 698–778 K. Zone III relates to a well defined martensitic transformation taking place in one step with almost no further evolution; it corresponds to the growth of stress-free grains.

The aim of this presentation is to analyse and discuss some recent advances in shape memory and pseudoelastic properties of different alloys. Experimental work in connection with theoretical ones will be reviewed. The first part is devoted to the microstructural origin of shape memory properties of Fe-Mn-Si based alloys (γ - e tranformation); the second part is a synthetic analysis of the effects of thermomechanical treatments on shape memory and pseudoelastic effects in Ti--Ni alloys, with some focus on the behaviour of the R phase introduced.

Superelasticity phenomena and Impact behaviour have been studied on Ti 50 Ni 50 and Ti 50 Ni 48 Fe 2 shape memory alloys. Both alloys have been made by powder metallurgy (combustion synthesis). The main objective of the present work was to induce superelastic properties for temperatures ranging from -25°C to 0°C on a TiNi-based alloy by the addition of 2% Fe. It is well known that superelasticity properties are enhanced by the occurence of premartensitic R phase transition. On the other hand, Fe addition decreases the martensitic transformation temperature (Ms) promoting the occurence of intermediate premartensitic R phase. For binary TiNi alloys, cold working and partial recovery heat treatment is necessary to obtain such as properties. This paper describes the microstructure, superelasticity and Charpy Impact behaviour of both Ti 50 Ni 50 and Ti 50 Ni 48 Fe 2 . Results have shown that: Impact energy curves display unexpected trend for testing temperatures ranging from - 196 to + 50°C. In particular, for TiNiFe alloy, typical Impact energy values are: 25J/cm 2 at -196°C, 4J/cm 2 at -25°C (the minimal value obtained) and 20J/cm 2 at +20°C. Complementary fractographic examinations have shown a brittle fracture mode for test temperatures ranging from -25 to 0°C. On the other hand, these temperatures correspond to the maximum of superelasticity, that is 1.5 to 2% of reversible strain measured by loading and unloading during tensile test. These results show a high brittleness probably induced by the intermediate R phase. Consequently, the optimization of superelasticity should be detrimental to the ductility/toughness of the TiNi alloys.

Electric transport measurements were performed as a function of temperature across the B2 to R phase transition in a TiNi-5wt% Fe alloy, by means of electric resistivity, thermoelectric power, magnetic susceptibility and point contact spectroscopy. The first derivative of resistivity, magnetic susceptibility and thermoelectric power with temperature shows a peak at the same characteristic temperature. At lower temperatures, the resistance vs. temperatures curves revealed a small minimum at about Tc≈9K, that could be associated with some electronic transition. The dV/dl vs V curves, taken from the point contact experiments, also show asymmetric behavior at low temperatures (about 10K), indicating a kind of electronic scattering. No martensitic transformation occurred in this high Fe content alloy.