Phase Transformation and Densification of Nanostructured Alumina. Effect of Seeding and Doping

By following the densification kinetics of nanocrystalline γ alumina and corresponding microstructural evolution we showed that the diffusive transformation gamma-alpha involves several processes such as nucleation of alpha phase, rearrangement of gamma crystallite at alpha seed and grain surfaces, formation of porous alpha alumina monocrystalline colonies. Results concerning the effects of seeding and doping elements on the transformation-densification behaviour of the same γ-alumina raw powder batch are also presented. Doping elements seem to have no influence on nucleation rate but could modify the redistribution rate of the ions during the transformation by short range diffusion of doping elements

Interactions between dislocations and interfaces - consequences for metal and ceramic plasticity

Interactions between dislocations and grain boundaries contribute significantly to plastic behaviour in polycrystalline metals. But a full understanding of the processes and of their influence on plastic response has yet to be achieved. In the present paper, the elementary interaction mechanisms, from the entrance of dislocations in grain boundaries to the relaxation of the resulting intergranular stresses, are briefly reviewed. They are examined for two types of deformation tests and two types of materials: low temperature deformation and relaxation in copper and creep behaviour of alumina. Only the comparison between the response at different scales (nanoscopic, microscopic and mesoscopic) allows us to analyse the response of grain boundaries to an applied stress and to propose an interpretation of their role in macroscopic behaviour. This work may be considered as a preliminary step towards "Interface Engineering"

Analysis of dissociated dislocations in a deformed bicrystal close to the rhombohedral twin orientation in -alumina

International audienceA bicrystal close to a rhombohedral twin orientation in -alumina has been deformed by compression at high temperature. After deformation the twin contained a periodic array of intrinsic dislocations. This configuration is interpreted by the decomposition of incoming basal lattice dislocations into interfacial disconnections and further interactions between sessile and glissile products. The dislocations are widely dissociated and the separation width between partials being close to 30 angstrom. The translation state of the interfacial structure between partials is different from those of the perfect twin. Possible structural models are envisaged and simulated using ab initio calculations and the less efficient Streitz and Mintmire modelling. One model in particular fits the experimental image rather well. The interfacial energies are higher than those estimated using elasticity theory

Determination of the atomic structure of a Σ\Sigma13 SrTiO3_3 grain boundary

New elements of a symmetric [001] 67.4\degres SrTiO$_3$ near $\Sigma$ 13 tilt grain boundary are identified by a quantitative analysis of lattice images, reconstructed electron exit waves, and HAADF images. The analysis reveals local, geometrical variations of structural grain boundary units that relate to the presence of defects introduced by a tilt deviation of 0.65 + 0.02 degrees from the perfect $\Sigma$13 geometry. Sr and TiO columns are discriminated in HAADF images while the reconstructed electron exit wave reveals all oxygen columns in addition. Both methods depict the crystal and boundary structure directly while lattice imaging with a high voltage instrument requires image simulations to link the image intensity to structure. For the first time we observe a Sr column splitting by 90 pm that supports theoretical predictions. An inhomogeneous, preferential etching at the grain boundary is attributed to local charge variations and hampers a quantitative investigation and local stoichiometry. The near $\Sigma$13 boundary forms a dense and compact structure with chemically identical columns in close proximity. Therefore, it is different from the relaxed, bulk like configurations described in previous reports

Potential of Directionally Solidified Eutectic Ceramics for High Temperature Applications

International audienceDirectionally solidified eutectic (DSE) ceramics add new potentialities to the advantages of sintered ceramics: a higher strength, almost constant, up to temperatures close to the melting point and a better creep resistance. The microstructure of melt-growth composites (MGC) of ceramic oxides consists in three-dimensional and continuous interconnected networks of single-crystal eutectic phases. After solidification of binary eutectics, the eutectic phases are alumina and either a perovskite or garnet phase. In ternary systems, cubic zirconia is added as a third phase. For very high temperature structural applications such as turbine blades in future aeronautical turbines or thermal power generation systems, the investigation is focused on both binary (Al2O3-Y3Al5O12 (YAG), Al2O3-Er3Al5O12 (EAG) and Al2O3-GdAlO3 (GAP)) and ternary (Al2O3-YAG-ZrO2, Al2O3-EAG-ZrO2 and Al2O3-GAP-ZrO2) eutectics. Improving the strength and toughness of DSE ceramics being essential for such practical applications, results concerning the mechanical behavior of these eutectics will be reported after a short presentation concerning microstructure and crystallography. This better knowledge of DSE ceramics has led to the development of a specific Bridgman furnace to produce large crystals and investigate possible applications of DSE ceramics to a new generation of very high temperature gas turbines, e.g. hollow non-cooled nozzles, turbine blades or combustor liner panels

High temperature plasticity at twin boundary in Al: An in-situ TEM perspective

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Synthesis and microstructure of bulk nanocrystalline copper

International audienceThe synthesis of bulk nanocrystalline copper (NC-Cu) by powder metallurgy is presented, from compaction of nanocrystalline powders to sintering and differential extrusion. At each step of the process, the microstructure is characterized using X-ray diffraction (XRD) analysis, field-emission gun scanning electron microscopy (FEG-SEM), and transmission electron microscopy (TEM). Particular attention is given to the concept of grain size in nanostructured materials and the comparison of results from the different characterization techniques. The fully dense material has a grain size of 100 nm with a microstructure best described in terms of the distribution of high-angle grain boundaries (GBs), twin boundaries, and low-angle GBs. Dislocations occur in half the grains and at most of the twin boundaries. The GBs are shown to be crystalline, and no evidence is found for amorphous interfacial regions. It is proposed that the grain size be defined only in terms of high-angle GBs, excluding low-angle GBs, for the discussion of mechanical properties. In this respect, the microstructure is compared with the NC-Cu material produced by other synthesis techniques. Powder metallurgy (P/M) processing is revealed as an alternative for the production of large-size submicrocrystalline and NC materials

The role of disconnections in deformation-coupled grain boundary migration

cited By 26International audienceGrain boundary (GB) migration under stress has been recognized in recent years as an important plastic deformation mechanism especially in small-grained materials. It is believed to occur via the motion of disconnections along the interface. However, the origin of these disconnections is a key point for a deeper understanding of this mechanism. In this paper, we consider that GB migration under stress can occur both due to the motion of pre-existing disconnections and due to disconnections resulting from decomposition of lattice dislocations interacting with the GB. High-resolution transmission electron microscopy experiments carried out on an aluminum bicrystal with a Σ41 540 GB indeed confirm the existence of different kinds of disconnections and pure steps prior to deformation. In situ straining experiments performed in the same bicrystal at room and high temperatures reveal the rapid decomposition of lattice dislocations in the GB plane. Theoretical investigation of the possible decomposition reactions shows that different types of disconnections with Burgers vector having both glide and climb components, i.e. parallel and perpendicular to the GB plane, can be produced. Disconnections with a small climb component are likely to move along the GB under stress and induce deformation parallel and perpendicular to the GB plane. Concomitant motion of disconnections with Burgers vectors at right angles to the GB plane is believed to produce GB migration coupled with grain rotation. It is also shown that disconnection interactions in the GB lead preferentially to purely glissile disconnections producing a coupling factor in agreement with the observed coupling factor measured in experiments on macroscopic bicrystals. The idea that shear-coupled GB migration can occur by the continuous feeding of lattice dislocations decomposing in the GB during the migration is also investigated. This process is thought to play a role during recrystallization

Formation of annealing twins during primary recrystallization of two low stacking fault energy Ni-based alloys

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