Deformation twinning in Cr2AlC MAX phase single crystals: A nanomechanical testing study

In a recent study [1], we observed and characterized for the first time deformation twinning in the Ti2AlN MAX phase deformed at high temperature (800°C) by Berkovich nanoindentation. Since plastic deformation in these nanolayered materials was believed to be governed only by basal plane dislocations involved in kink band mechanisms, this result has shed a new light on the mechanical behavior of MAX phases. In order to go further in the understanding of twinning deformation mechanisms in MAX phases, we performed a study in Cr2AlC single crystal, deformed at room temperature by spherical nanoindentation and by micropillar compression tests, in such an orientation that the basal plane was edge on, to inhibit basal dislocations and to promote twinning. Please click Download on the upper right corner to see the full abstract

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

Nanoindentation cartography in Al/Al-Cu-Fe composites: Correlation between chemical heterogeneities and mechanical properties

During the last two decades, nanoindentation testing has become a commonly used technique for measuring surface mechanical properties such as hardness or elastic modulus. With devices equipped with a motorized X-Y table, it is now possible to perform large regular nanoindentation arrays in order to make an accurate statistics of the mechanical properties. This method is particularly interesting to study heterogeneous materials. The statistical analysis, associated to mathematical deconvolution methods allows identifying the properties of each individual phase. Furthermore, hardness or elastic modulus maps can be then established and compared to other local properties such as microstructure, crystallographic orientation or chemical composition. The nanoindentation cartography method has been used to study the mechanical properties of a metal matrix composite (Aluminum matrix with ω-Al-Cu-Fe reinforcement particles, synthesized by sparking plasma sintering) (cf. figure 1). Emphasize has been placed on the Aluminum matrix properties, where the detailed analysis of the individual nanoindentation curves shows serrated behavior characteristic of Portevin-Le Chatelier effect associated to dislocation pinning by solute atoms. The comparison between chemical (SEM – EDXS analysis) and hardness maps as well as the quantitative analysis of the deformation curves gives evidence of a strong correlation between the chemical heterogeneities and mechanical properties of the Aluminum matrix

Synthèse et caractérisation de la phase -Al7Cu2Fe et de composites Al/Al-Cu-Fe

Dans le contexte des matériaux composites Al/Al-Cu-Fe, la phase -Al7Cu2Fe et des composites Al/ ont été élaborés puis les microstructures et les propriétés mécaniques de ces matériaux ont été étudiées. La phase a été élaborée sous forme monolithique et dense par compression isostatique à chaud et par frittage flash. Des essais de micro-indentation, de frottement intérieur et de compression couplés à des essais transitoires ont été réalisés entre 293 K et 1000 K. Une transition fragile-ductile entre 650 K et 820 K a été mise en évidence. Les courbes contrainte-déformation montrent une limite d'élasticité supérieure suivie uniquement par de l'adoucissement ou un stade à durcissement nul. La limite d'élasticité supérieure présente une forte dépendance en température suggérant des mécanismes de déformation thermiquement activés. L'évolution des volumes d'activation avec la contrainte appliquée présente deux régimes de températures avec une température de transition vers 900 K. Des observations de microscopie électronique en transmission révèlent une distribution hétérogène de dislocations. Ces résultats originaux sur la phase -Al7Cu2Fe montrent de fortes similitudes avec les propriétés bien connues de la phase quasicristalline Al-Cu-Fe associée et apportent de nouveaux éléments de compréhension de la plasticité de ces matériaux complexes. Deux composites Al/ ont été élaborés par compression isostatique à chaud. L'un est obtenu à 673 K à partir de particules initiales tandis que l'autre est obtenu à 823 K à partir de particules Al-Cu-Fe initialement sous forme quasicristalline. La matrice, observée par microscopie électronique en transmission, présente une microstructure complexe avec différentes distributions de particules selon la température d'élaboration. A partir d'essais de compression couplés à des essais transitoires et à la diffraction de neutrons, le renforcement de la matrice Al est attribué au transfert de charge et au durcissement de la matrice par des microstructures différentes.In the general context of study of Al/Al-Cu-Fe composites, -Al7Cu2Fe phase and related Al/ composites have been synthesised and their mechanical properties and microstructure investigated. Dense monolithic -Al7Cu2Fe phase synthesis was achieved for the first time using hot isostatic pressing and spark plasma sintering allowing for mechanical and microstructural investigations. Microindentation, compression and transient tests as well as mechanical spectroscopy have been performed in large temperature ranges. A brittle-to-ductile transition between 650 K and 820 K is evidenced. The stress-strain curves exhibit an upper yield stress followed by a softening or steady state stage only. The upper yield stress shows strong temperature dependence suggesting that deformation mechanisms are highly thermally activated. The evolution of activation volumes with the applied stress exhibits two deformation mechanisms with a transition temperature of 900 K. Transmission electron microscopy observations of deformed material reveal inhomogeneous dislocation distribution. The original results on the crystalline -Al7Cu2Fe phase exhibit a strong similarity with the well-known properties of the related icosahedral Al-Cu-Fe phase and shed a new light on the understanding of plasticity in these complex materials. Two Al/ composites have been fabricated from powder metallurgy, one at 673 K from initial particles, the second at 823 K from initial icosahedral Al-Cu-Fe particles. Transmission electron microscopy observations reveal a complex matrix microstucture with different particles distribution according to the processing temperature. From compression and transient tests performed at various temperatures and in-situ compression test under neutron beam, the reinforcement of Al matrix is attributed to both load transfer and hardening of the matrix resulting from different microstructure.POITIERS-BU Sciences (861942102) / SudocSudocFranceF

Etude de matériaux composites à matrice base Al renforcés par des particules Al-Cu-Fe

Les matériaux quasicristallins (QC) présentent des propriétés mécaniques intéressantes, telles une dureté et un module d Young élevés, ce qui en fait de bons candidats en tant que renforts pour les matériaux composites à matrices métalliques. Ce travail a pour but l élaboration et l étude des microstructures et propriétés mécaniques de matériaux composites à base Al renforcés par des particules QC. Des composites à matrice Al3Mg2, Al, Al-Cu-Mg et Al-Mg-Si renforcés par 50% vol. de particules QC Al-Cu-Fe ont été élaborés par infiltration sous pression de gaz. Les composites obtenus sont complexes et présentent de nombreuses phases résultant de la diffusion de l aluminium et du cuivre. Ces composites sont caractérisés par des contraintes d écoulement élevées et une fissuration précoce, quelle que soit la température de déformation. Deux composites à matrice aluminium renforcés par des particules QC ont été élaborés par compression isostatique à chaud (HIP). En fonction de la température d élaboration, nous avons obtenu des matériaux biphasés, l un renforcé par des particules tétragonales w-Al-Cu-Fe et l autre par des particules i-Al-Cu-Fe. Ces matériaux présentent des propriétés mécaniques améliorées par rapport à la matrice seule. La forte dépendance de s0.2% en température observée dans le composite Al/w suggère que la déformation plastique est contrôlée par des mécanismes thermiquement activés. En revanche, pour le composite Al/i, la contribution des contraintes internes au durcissement du composite est à prendre en compte en plus des mécanismes thermiquement activés.Quasicrystalline (QC) materials exhibit remarkable mechanical properties at low and intermediate temperatures, such as high hardness together with high elastic modulus. One of the potential applications of QC materials is to use them as the reinforcement phase of composite materials. This study reports about processing, microstructures and mechanical properties of Al-based metal matrix composites (MMC) reinforced by QC particles. Five Al-based MMC were produced using the gas pressure infiltration technique. Al3Mg2, Al, Al-Cu-Mg and Al-Mg-Si matrices were used. They were reinforced by 50 % vol. fraction of Al-Cu-Fe QC particles. The as-produced composites are rather complex and various phases are formed during the production process. These phases were identified to result from diffusion of both the aluminium and the copper. These composites are characterised by high flow stresses, that are unfortunately accompanied by numerous cracks whatever the temperature deformation. Two composites with an Al matrix initially reinforced by QC particles were prepared by hot isostatic pressing (HIP). With this technique two-phase composites are obtained, but depending on the processing temperature, the reinforcement particles are either of i-phase or !-phase. It is observed that ! particles contribute more positively to the improvement of the mechanical properties than the QC particles. The temperature dependences of "0.2% suggest that the plastic deformation of the composite Al/!-Al-Cu-Fe is controlled by thermally activated mechanisms. In the composite Al/i-Al-Cu-Fe, in addition to thermally activated mechanism, the contribution of the internal stresses to the hardening of the composite must be taken into account.POITIERS-BU Sciences (861942102) / SudocSudocFranceF

Evidence of dislocation cross-slip in MAX phase deformed at high temperature

International audienceTi2AlN nanolayered ternary alloy has been plastically deformed under confining pressure at 900 degrees C. The dislocation configurations of the deformed material have been analyzed by transmission electron microscopy. The results show a drastic evolution compared to the dislocation configurations observed in the Ti2AlN samples deformed at room temperature. In particular, they evidence out-of-basal-plane dislocations and interactions. Moreover numerous cross-slip events from basal plane to prismatic or pyramidal planes are observed. These original results are discussed in the context of the Brittle-to-Ductile Transition of the nanolayered ternary alloys