Microstrain analysis of titanium aluminides

International audienceThe aeronautic and automotive industries have shown a renewed interest in TiAl based alloys. The main reasons for such an interest are their low density (~3,8g/cm3), a good stiffness and a high strength for temperatures up to 750°C. However, these alloys exhibit, in their polycrystalline form, a poor ductility at room temperature with widely scattered values. The aim of this study is therefore to characterise their mechanical behaviour with a multiscale methodology, coupling microstructure analysis and strain field measurements. This methodology employs orientation imaging microscopy as well as digital imaging correlation techniques with an intragranular step size of a few micrometers. Two chemical compositions (47 at. % Al and 48 at. % Al) and two processing routes (casting and powder metallurgy) are studied. Thus, four different types of final microstructures are considered, from fully lamellar Ti3Al (a2) + TiAl (g) microstructure to bimodal ones composed of two-phase (a2+g) lamellar grains and monolithic g grains. Firstly, the microstructure is characterised crystallographically and morphologically. This allows the identification of a representative volume element (RVE) inside the analysed volume. Then, uniaxial mechanical tests are performed for each microstructure, and the strain fields are analysed with a multiscale approach, which determines the spatial distribution of the strain field heterogeneity with respect to the different microstructures

Mise en place d'un modèle de zone cohésive à base physique dans un bicristal de gamma-TiAl

Titre du résumé joint : Étude numérique par éléments cohésifs et remaillage de la compétition entre la rupture intragranulaire et intergranulaire dans un bicristal de TiAlNational audienceCes travaux proposent d'étudier numériquement l'effet d'une fissure intragranulaire modélisée par des éléments cohésifs dans un bicristal de γ-TiAl. Les effets de la fissure, insérée dans le premier grain, sont étudiés dans le second grain à la suite d'un calcul de traction monotone en plasticité cristalline. Des critères de fissuration intragranulaire fonction de la déformation plastique équivalente induite par les macles sont étudiés afin de prédire la rupture du bicristal. L'état de contrainte normal au joint de grains est ensuite examiné afin d'évaluer la compétition entre la rupture intergranulaire et intragranulaire.See http://hal.archives-ouvertes.fr/docs/00/59/29/23/ANNEX/r_7L09K124.pd

Multi-scale viscoplastic behaviour of Halite: In-situ SEM full field measurements, a micro-mechanical approach

Halite geological formations are already extensively used for underground storage of hydrocarbons. For example, the entire USA federal reserve of petrol resides in deep (500 - 1000 m) artificial salt caverns, which are realized by controlled dissolution. In France, many such salt caverns are used for storage of natural gas by GDF. Salt caverns and carries are also intended to become nuclear waste repositories. At this point, salt caverns are also seriously envisaged for the daily storage of energy from renewable, but intermittent sources (photovoltaic, Aeolian), under the form of compressed air. Halite mechanical behaviour was extensively studied for the purpose of safe geothechnical applications. Halite is a ductile type rock. Its differed (time-dependent) mechanical response dominates by far, and therefore deep salt caverns experience convergence (closure), which may result in catastrophic subsidence of the overlaying geological layers. Hence, a particular attention was drawn to characterize salt single crystal creep properties (active slip systems and critical resolved shear stresses), and the rheology of poly-crystalline salt, at various temperatures, pressures, differential stresses and water contents (Ter Heege et al., 2007). But, most studies were concerned with macroscopically derived flow laws, corresponding to rather high differential stresses (as compared with those experienced on site), where crystal slip plasticity (CSP) dominates. But, many studies have also shown that halite is very sensitive to solution-precipitation creep (SPC) mechanisms, which may result in solution transfer accommodated grain boundary sliding (GBS). Conversely, some recent studies report that halite is able to flow at ambient conditions, and under very small loads, with strain rates much faster (four orders of magnitude) than those extrapolated from high stress experiments (Bérest et al., 2005). Though, the specific creep micro-mechanisms were not identified, Bérest et al. (2005) invoked possible SPC. Additionally, the effects on long term behaviour of cyclic loading (fatigue) are still poorly known. It is therefore still questionable weather it is really possible to safely extrapolate the laboratory data to the long term envisaged geotechnical applications. To answer we need i) additional experimental work in order to up date the deformation mechanism maps on the basis of better identified micro-physical mechanisms and quantification of their respective activity; and ii) numerical modelling at the scales of the material, and of the underground storage structures, in respect with the appropriated thermo-hygro-mechaniclal loadings. In the present work, we present our preliminary investigation of viscoplastic global and local responses of synthetic fine grained (50 - 500 m) halite by the means of full field measurements (FFM) of local strain by digital image correlation (DIC) during simple compression in-situ SEM (Doumalain et al., 2003). Figure 1 shows a typical loading curve obtained incrementally at the constant strain rate of c.a. 5x10-5 s-1. CSP evidenced by the development of slip lines on the free grain surfaces, and characterized by quasi-linear strain hardening, dominates the overall response up to several % of strain (microfracturing did not appear before 8 % strain). Yet, at the scale of the microstructure, the development of viscoplastic strain is heterogeneous, as shown by the strain maps obtained by DIC and corresponding to four incremental stages of the loading sequence. The heterogeneity of the strain field relates to the loading boundary conditions and to the local microstructure, such crystal size and orientation (which is characterized by electron back scattering diffraction, EBSD). Such micromechanical approach aims to provide the basis for the development of FE (finite element) computational CSP of polycrystalline halite

Full field investigation of salt deformation at room temperature: cooperation of crystal plasticity and grain sliding

International audienceWe observed with optical and scanning electron microscopy halite samples during uniaxial compression. Surface displacement fields were retrieved from digital images taken at different loading stages thanks to digital image correlation (DIC) techniques, on the basis of which we could 1) compute global and local strain fields, 2) identify two co-operational deformation mechanisms. The latter were 1) crystal slip plasticity (CSP), as evidenced by the occurrence of slip lines and computed discrete intracrystalline slip bands at the grain surfaces, 2) interfacial micro-cracking and grain boundary sliding (GBS), as evidenced by the computed relative interfacial displacements. The heterogeneities of the strain fields at the aggregate and at the grain scale, and the local contributions of each mechanism were clearly related to the microstructure, i.e. the relative crystallographic orientations of neighboring grains and the interfacial orientations with respect to the principal stress

Modélisation du comportement des verres métalliques par une approche phénoménologique

L'étude porte sur le développement d'une loi permettant de décrire le comportement macroscopique de verres métalliques dans le mode de déformation homogène. Le choix s'est porté sur un verre métallique à base zirconium de composition Zr52,5Cu22Al10Ni13Ti2,5. Des premiers tests de compression ont été réalisés pour mettre en évidence les effets significatifs à prendre en compte dans le modèle. Une stratégie de couplage essais/simulations a été mise en place pour identifier les différents paramètres du modèle

Heterogeneity in tribologically transformed structure (TTS) of Ti-6Al-4V under fretting

Fretting wear is a surface degradation process caused by oscillatory motion and contact slipping. During gross slip, high local stresses and plastic deformation in the surface and subsurface can lead to the creation of a nanosized grained structure called Tribologically Transformed Structure (TTS). The current paper studies the formation of TTS in an alpha-beta Ti-6Al-4V alloy under fretting loading while changing the contact pressure and the number of fretting cycles.Cross-sections of wear scars are observed after polishing and chemical etching. Above a threshold pressure of 300 MPa, TTS appears early in the contact (before 1000 cycles) along with two other structures: a Third Body Layer (TBL) made of compacted debris and a General Deformed Layer (GDL) which is the plastic zone under the TTS. TTS first appears as islands and merges in the middle of the contact after enough cycles. Below 200 MPa, only TBL and GDL are formed. At 200 MPa, only small, localized TTS is found. All structures have the same chemical compositions as the initial bulk material except for the nitrided TBL. TTS has a very high hardness compared to the bulk. TTS was carefully extracted using a Focused Ion Beam (FIB) and its microstructure was observed with a Transmission Electron Microscope (TEM). It shows extreme grain refinement and is composed of two alternated zones. The first zone I is composed of $\alpha$ grains with a size of 20 to 50 nm with crystallographic texture. Zone II comprises nanosized equiaxed grains whose sizes range from 5 to 20 nm without texture. The results made it possible to establish a scenario of the appearance of the TTS according to the conditions of contact pressure and number of fretting cycles

Multiscale correlated analysis of the Aguas Zarcas CM chondrite

In this paper, we report the results of a campaign of measurements on four fragments of the CM Aguas Zarcas (AZ) meteorite, combining X‐ray computed tomography analysis and Fourier‐transform infrared (FT‐IR) spectroscopy. We estimated a petrologic type for our sampled CM lithology using the two independent techniques, and obtained a type CM2.5, in agreement with previous estimations. By comparing the Si‐O 10‐µm signature of the AZ average FT‐IR spectra with other well‐studied CMs, we place AZ in the context of aqueous alteration of CM parent bodies. Morphological characterization reveals that AZ has heterogeneous distribution of pores and a global porosity of 4.5 ± 0.5 vol%. We show that chondrules have a porosity of 6.3 ± 1 vol%. This larger porosity could be inherited due to various processes such as temperature variation during the chondrule formation and shocks or dissolution during aqueous alteration. Finally, we observed a correlation between 3D distributions of organic matter and mineral at micrometric scales, revealing a link between the abundance of organic matter and the presence of hydrated minerals. This supports the idea that aqueous alteration in AZ’s parent body played a major role in the evolution of the organic matter

Méthode de couplage multi-échelles entre simulations numériques polycristallines et mesures de champs pour l'identification des paramètres de lois de comportement et de fissuration des matériaux métalliques. Application à l'étude des alliages TiAl.

The aim of this study is to better understand the influence of microstructure on the mechanical behaviour of titanium aluminides, and especially their poor ductility at room temperature. In order to reach this objective, a coupling between full-field measurements and polycristalline finite element calculations has been developed and analysed. With this methodology, strain field measurements cans be compared directly to the results of finite element simulations on the real microstructure, thus permitting the identification of the parameters of the constitutive law. This thesis presents the results of multi-scale strain measurements obtained by compressive tests on four kinds of titanium aluminides. These measurements show influence of chemical composition and processing route on strain measurements depending on the measurement scale. Crack propagation analyses at the grain scale are also performed on the four microstructures by using four point bending tests inside a scanning electron microscope. The validation of the methodology is partially carried out. It is shown that a mesh created by an extrusion of the surfacic microstructure can be sufficient for the identification of the constitutive law when the experimental displacements are imposed as boundary conditions. The applicability of the same methodology to the analysis of crack propagation inside microstructures is also studied by introducing cohesive zone elements in the finite element simulations.L'objectif général de cette étude consiste en la meilleure compréhension de l'influence de la microstructure sur les propriétés mécaniques des aluminiures de titane et notamment de leur faible ductilité à température ambiante. Pour cela, une méthodologie de dialogue entre mesures de champs et calculs polycristallins a été développée et analysée en détails. Celle-ci permet une comparaison directe entre les mesures de champs cinématiques à l'échelle des grains et les résultats des calculs par éléments finis sur microstructure réelle en vue de l'identification des coefficients de la loi de comportement cristalline. Ce document présente les résultats d'analyses multi-échelles de quatre alliages de titane-aluminium soumis à des essais de compression en mettant en avant l'influence respective du mode d'élaboration et de la composition chimique sur les mesures de champs cinématiques aux différentes échelles. L'analyse de la propagation de fissures au sein des différentes microstructures est également étudiée à l'aide d'essais de flexion sous microscopie électronique à balayage. Parallèlement à cette étude expérimentale, la validation de la méthodologie permettant un dialogue entre ces mesures et les calculs par éléments finis sur les microstructures associées est en partie validée, en démontrant notamment la suffisance d'un maillage de surface extrudée ainsi que l'utilisation des conditions aux limites expérimentales. L'application de cette méthodologie à l'étude de la propagation de fissure est également étudiée en utilisant, dans les calculs par éléments finis, les modèles de zones cohésives

Numerical simulation of the third body in fretting problems

International audienceThis study is devoted to the computation of realistic stress and strain fields at a local scale in fretting. Models are proposed to improve surface and volume modelling, by taking into account the heterogeneity of stress fields due to the irregular interface. This gives a new view toward damage mechanisms. The surface heterogeneity which is considered here, results from the third body trapped in the contact zone. This third body is known to drastically influence the contact conditions. The competition between wear and crack initiation is investigated with respect to local stress fields. The first model is used to study the evolutions of particles and the contact stress according to the loading conditions. Then, Dang Van's multiaxial fatigue model is used to predict cr

FIB manufactured microstructures with low Coefficients of Thermal Expansion

International audienc