Mechanical hysteresis of the MAX phase Ti2AlN: A nano-mechanical testing study

MAX phases are nano-lamellar ternary carbides and nitrides, with a hexagonal crystallographic structure. These materials combine several properties of metals and ceramics, which give them a high potential for technological applications. Their mechanical properties are characterized by a high stiffness and a relatively low yield strength More surprisingly, deformation tests on MAX phases reveal a mechanical hysteresis. At a macroscopic scale, in polycrystalline samples, several studies have shown that this behavior could be related to load transfers from grain to grain. However, a mechanical hysteresis is also observed in single crystals. In this work, the mechanical hysteresis and the plasticity of the MAX phase Ti2AlN has been studied at small scale by using nanoindentation tests with a spherical tip and micro-pillar compression tests. In both cases, cyclic loadings have been applied in single grains, for different crystallographic orientations, previously determined by EBSD. These cyclic loadings, with partial unloadings (cf. figure 1), have revealed a same behavior in nanoindentation tests and in micro-pillars compression test. In both cases, the unloading curves show an elastic behavior followed by a plastic recovery at low load. Furthermore, this mechanical hysteresis is related to the crystallographic orientation since the energy dissipated during the cycles is shown to be minimum when the basal plane is perpendicular or parallel to the indentation (or compression) axis. Please click Additional Files below to see the full abstract

Mécanismes de déformation des phases MAX (une approche expérimentale multi-échelle)

Il est couramment admis que la déformation plastique des phases MAX est dueau glissement de dislocations dans les plans de base s'organisant en empilements et murs. Cesderniers peuvent former des zones de désorientation locale appelées kink bands. Cependant, lesmécanismes élémentaires et le rôle exact des défauts microstructuraux sont encore mal connus. Cemanuscrit présente une étude expérimentale multi-échelle des mécanismes de déformation de laphase MAX Ti2AlN. A l'échelle macroscopique, deux types d'expériences ont été menés. Des essaisde compression in-situ à température et pression ambiantes couplés à la diffraction neutroniqueont permis de mieux comprendre le comportement des différentes familles de grains dans le Ti2AlNpolycristallin. Des essais de compression sous pression de confinement ont également été réalisés dela température ambiante jusqu'à 900 C. À l'échelle mésoscopique, les microstructures des surfacesdéformées ont été observées par MEB et AFM. Ces observations complétées par des essais denanoindentation ont montré que la forme des grains et leur orientation par rapport à la directionde sollicitation gouvernent l'apparition de déformations intra- et inter-granulaires ainsi que lalocalisation de la plasticité. Finalement à l'échelle microscopique, une étude détaillée par METdes échantillons déformés sous pression de confinement a révélé la présence de configurations dedislocations inédites dans les phases MAX, telles que des réactions entre dislocations, des dipôleset des dislocations hors plan de base. À la vue de ces résultats nouveaux, les propriétés mécaniquesdes phases MAX sont rediscutées.It is commonly believed that plastic deformation mechanisms of MAX phases consistin basal dislocation glide, thus forming pile-ups and walls. The latter can form local disorientationareas, known as kink bands. Nevertheless, the elementary mechanisms and the exact role ofmicrostructural defects are not fully understood yet. This thesis report presents a multi-scale experimentalstudy of deformation mechanisms of the Ti2AlN MAX phase. At the macroscopic scale,two kinds of experiments were performed. In-situ compression tests at room temperature coupledwith neutron diffraction brought new insight into the deformation behavior of the different grainfamilies in the polycrystalline Ti2AlN. Compression tests from the room temperature to 900 Cunder confining pressure were also performed. At the mesoscopic scale, deformed surface microstructureswere observed by SEM and AFM. These observations associated with nanoindentationtests showed that grain shape and orientation relative to the stress direction control formationof intra- and inter- granular strains and plasticity localization. Finally, at the microscopic scale,a detailed dislocation study of samples deformed under confining pressure revealed the presenceof dislocation configurations never observed before in MAX phases, such as dislocation reactions,dislocation dipoles and out-of-basal plane dislocations. In the light of these new results, mechanicalproperties of MAX phases are discussed.POITIERS-SCD-Bib. électronique (861949901) / SudocSudocFranceF

Visualization of Crystallographic Defects in InSb Micropillars by Ptychographic Topography

International audienceInvestigation of the strain field and defects in crystalline materials is essential in materials characterization, fabrication and design, as they are responsible for distinct mechanical, electric and magnetic properties of a desired material. Therefore, the visualization of strain and its relation to the type and density of defects in the crystal at the nanoscale is required. A domain in which such questions are particularly relevant is the fabrication of nanodevices for microelectronics from semiconductors, such as InSb, that are used as fast transistors, detectors and sensors. Classically, transmission electron microscopy (TEM) provides imaging of the crystalline defects with atomic spatial resolution, but due to the thin sections requirement, sample preparation is invasive and can modify the strain fields to be analyzed. A conventional tool to non-invasively study strain is Laue X-ray micro-diffraction [1], which reveals the strain field in crystalline samples averaged over the direction of the beam propagation with a resolution limited by the beam size. X-ray topography (XRT) [2] has been routinely used for imaging defects based on the diffraction contrast, with the resolution being restricted by the detector pixel size. X-ray coherence methods, such as coherent diffraction imaging (CDI) and ptychography, which are based on measuring the sample's far-field diffraction patterns and using phase retrieval algorithms, permit obtaining high resolution images. If the measurements are performed close to a Bragg peak, the resulting image becomes highly sensitive to the presence of strain [3, 4]. We have recently developed ptychographic topography, in which a crystalline sample is rotated with respect to the incident beam such that a certain atomic plane is in the Bragg condition, as shown in Fig. 1a [5]. A pinhole is then placed after the sample in the forward direction and is spatially translated, providing the sufficient overlapping necessary for ptychographic reconstructions [6]. The diffraction patterns are recorded at each pinhole position with a 2D detector downstream of the pinhole and used simultaneously for the reconstruction of the wave front at the pinhole position. So far measurements were performed in forward direction due to the limited space at the beamline to fulfil the ptychographic detector sampling requirement along the Bragg-diffracted beam direction. Numerical backpropagation then enables one to obtain an image of the sample, which is sensitive to the lattice displacements caused by defects

Toward the understanding of the brittle to ductile transition at low size in silicon: Experiments and simulations

While bulk silicon is brittle at temperatures below 600-700K, the compression of nanopillars has shown that a decrease of the diameter below few hundreds of nanometers could change the silicon behavior from brittle to ductile [1,2]. This size effect cannot be explained by the initial defect density like in metals, because pristine silicon nano-objects do not contain residual defects. In these conditions the cracks and/or the dislocations nucleation should take origin at the surface. The identification of the parameters governing the brittle to ductile transition in size and the understanding of the mechanisms are the key points to further develop the MEMS and NEMS technology or to prevent the failure of microelectronic components based on the silicon strained technology. Nowadays the respective improvements in simulations and experiments allow to investigate the mechanical properties of objects of similar sizes, close to hundreds of nanometers. We have then used both approaches - experiments and simulations – to understand the mechanisms at the origin of cracks and dislocations nucleation in such nanopillars. Experimentally,nanopillars with diameters of 100 nm and heights of 300 nm are obtained by lithography. They are deformed in compression by a flat punch nano-indentor under controlled-displacement mode at room temperature, and analyzed by scanning electron microscopy and high resolution transmission electron microscopy. In simulation, nanopillars up to 44 nm in diameter and height are investigated under compression and tension in controlled-displacement too, with a temperature ranging from 1 to 600K. The atomic interactions in silicon are modeled by two different semi-empirical potentials, Stillinger Weber and a Modified Embedded-Atom-Method (MEAM), both fitted to better reproduce the ductile and brittle properties of bulk silicon. Under compressive load (Fig. 1), both approaches reveal a ductile behavior with similar stress-strain curves, and large shear bands of amorphous silicon along the slip plane. In addition the simulations enlighten the formation of stacking fault plane in the anti-twining shear stress direction at the onset of plasticity, not yet confirmed by experiments (work in progress). The simulations under tensile load (Fig. 2) show the nucleation of perfect dislocations from the surface that can lead to cavity opening when they interact [3]. We observe first that the height of the nanopillars must be higher than 20 nm to allow the cavity opening, and second that the brittle to ductile transition is controlled by the diameter of the nanopillars, as observed experimentally in compression. The deformation of pillars with large diameters operates by cavity expansion leading to the brittle fracture, while pillars with smaller diameters are deformed by dislocations gliding leading to ductile fracture. Finally, the simulations in temperature seem to corroborate the fact that the size of the brittle to ductile transition could increase with temperature, as presumed experimentally [2]

Plastic Deformation of InSb Micro-Pillars: A Comparative Study Between Spatially Resolved Laue and Monochromatic X-Ray Micro-Diffraction Maps

International audienc

Multiscale modeling of the elasto-plastic behavior of architectured and nanostructured Cu-Nb composite wires and comparison with neutron diffraction experiments

Nanostructured and architectured copper niobium composite wires are excellent candidates for the generation of intense pulsed magnetic fields ( 100T) as they combine both high strength and high electrical conductivity. Multi-scaled Cu-Nb wires are fabricated by accumulative drawing and bundling (a severe plastic deformation technique), leading to a multiscale, architectured, and nanostructured microstructure exhibiting a strong fiber crystallographic texture and elongated grain shape along the wire axis. This paper presents a comprehensive study of the effective elastoplastic behavior of this composite material by using two different approaches to model the microstructural features: full-field finite elements and mean-field modeling. As the material exhibits several characteristic scales, an original hierarchical strategy is proposed based on iterative scale transition steps from the nanometric grain scale to the millimetric macro-scale. The best modeling strategy is selected to estimate reliably the effective elasto-plastic behavior of Cu-Nb wires with minimum computational time. Finally, for the first time, the models are confronted to tensile tests and in-situ neutron diffraction experimental data with a good agreement

EXPLORATION THEORIQUE ET EXPERIMENTALE DE FILS NANOCOMPOSITES CONTINUS PRESENTANT DES PROPRIETES EXTREMES DE CONDUCTIVITE ELECTRIQUE ET DE LIMITE ELASTIQUE. APPLICATION FUTURE (COILIN 100 T)

TOULOUSE-INSA (315552106) / SudocSudocFranceF

Study of the effects of the hydrostatic pressure on the plasticity mechanisms in semi-conductors (the indium antimony (InSb) case)

La plasticité d InSb est étudiée entre -176C et 400C, i.e. de part et d autre de la température de transition fragile-ductile, Tf-g située autour de 150C. Les techniques de déformation utilisées sont la compression uniaxiale sous confinement gazeux (machine de Paterson), la micro-indentation et la compression sous forte pression dans une cellule multi-enclumes. Pour l analyse des microstructures de déformation, trois techniques sont utilisées : l annihilation de positrons, la MET conventionnelle et la technique du LACBED. L étude macroscopique révèle un changement du comportement mécanique vers 150C, i.e. proche de Tf-g. La technique d annihilation de positrons montre que les défauts formés à 20C et à 300C sont de natures différentes. L analyse par MET confirme la modification des mécanismes dislocationnels en fonction de la température : il est observé, à -176C, des dislocations parfaites vis non dissociées ; à 20C, une majorité de dislocations partielles en interaction et au dessus de 150C, des dislocations parfaites non dissociées en interaction. Ces résultats mettent en évidence un changement de mécanisme de déformation, autour de Tf-g, par glissement dans le système glide de dislocations parfaites à haute température et de dislocations partielles à basse température. L observation de dislocations parfaites non dissociée à très basse température suggère une transition supplémentaire, en dessous de la température ambiante, vers un glissement des dislocations dans le système shuffle. L apparition de la transition fragile-ductile pourrait donc être liée à ces transitions successives de mécanismes de déformation à basse et très basse températureThe plasticity of indium antimony InSb is studied between -176C and 400C, i.e. above and below the brittle to ductile temperature transition BDTT, situated around 150C. The deformation techniques which are used are the uniaxial compression under gaseous pressure (Paterson press), the deformation under localized load (micro-indentation) and the compression under high pressure in a multi-anvils cell. To analyse the deformation microstructures, three techniques are used: positron annihilation, conventional TEM and the LACBED technique. The macroscopic study reveals a change of the mechanical behaviour arround 150C, i.e. close to BDTT. The positron annihilation technique shows that the defects formed at 20C and at 300C have different nature. The TEM microstructural analysis confirms the modification of the dislocation mechanisms according to the temperature: only non dissociated perfect screw dislocations are observed at -176C; a majority of partial dislocations in interaction are observed at 20C and only non dissociated perfect dislocations in interaction above 150C. These results show a change of deformation mechanism around BDTT, by perfect dislocations gliding in the glide set at high temperature and partial dislocations with glide set at low temperature. The observation of non dissociated perfect dislocations at very low-temperature suggests an additional transition, below the ambient temperature, related to the gliding of dislocations in the shuffle set. The appearance of the brittle to ductile transition could be thus connected to these successive transitions of deformation mechanisms at low and very low temperaturePOITIERS-BU Sciences (861942102) / SudocSudocFranceF