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International audience; The bulk quaternary equiatomic CoCrFeNi alloy is studied extensively in literature. Under experimental conditions, it shows a single-phase fcc structure and its physical and mechanical properties are similar to those of the quinary equiatomic CoCrFeMnNi alloy. Many studies in literature have focused on the mechanical properties of bulk nanocrystalline high entropy alloys or compositionally complex alloys, and their microstructure evolution upon annealing. The thin film processing route offers an excellent alternative to form nanocrystalline alloys. Due to the high nucleation rate and high density of defects in thin films synthesized by sputtering, the kinetics of microstructure evolution is often accelerated compared to those taking place in the bulk. Here, thin films are used to study the phase evolution in nanocrystalline CoCrFeNi deposited on Si/SiO 2 and c-sapphire substrates by magnetron cosputtering from elemental sources. The phases and microstructure of the films are discussed in comparison to the bulk alloy. The main conclusion is that second phases can form even at room temperature provided there are sufficient nucleation sites.

The invention relates to a process for preparing single-crystal thin films of pure metals or alloys, by deposition and growth of grains on a (0001) sapphire having a miscut comprised between 0.2° and 5°, and oriented towards a specific plane. It also relates to a single-crystal thin film obtainable by such a process and uses thereof., L'invention concerne un procédé de préparation de films minces monocristallins de métaux ou d'alliages purs, par dépôt et croissance de grains sur un saphir (0001) présentant une mauvaise découpe comprise entre 0,2° et 5°, et orientés vers un plan spécifique. L'invention concerne également un film mince monocristallin pouvant être obtenu par un tel procédé et ses utilisations.

International audience; A systematic variation in the orientation relationship (OR) of Ag films grown on Ni substrates previously discovered by a combinatorial approach is analyzed using concepts of grain boundaries, surface science and phase transformations. On roughly half of all Ni substrate orientations, Ag adopts a "special" OR that varies systematically from a twin OR, which develops on substrates lying along the (111)-(210) line of the standard stereographic triangle (SST), to the so-called oct-cube OR, which arises exclusively on (100) substrates. On the other half of Ni substrate orientations, Ag adopts the standard cube-on-cube OR. The special ORs are modeled by a) linear interpolation, b) 1D edge-to-edge matching and c) 2D transformation strains, using the twin relationship as a reference. The 1D model shows that the systematic variation in the special ORs can be understood in terms of a growth mechanism by attachment at steps and a lattice rotation due to the difference between the step heights in the substrate and the film. The 2D model explains why this mechanism does not apply to substrates on which Ag displays the cube-on-cube OR.

International audience; Molecular dynamics simulations have been performed to compare the orientation relationships (ORs) that develop between Ag films/particles equilibrated with Ni, depending on whether the Ag is subjected to three-dimensional (3-D) confinement by being embedded in Ni, or to 2-D confinement by equilibration with a Ni substrate. Previous results of both simulations and experiments have shown that Ag films equilibrated with planar Ni substrates display a large number of different ORs; in particular, Ag equilibrated on Ni{100} displays an OR of Ag{111} // Ni{100} , often referred to as an oct-cube OR. Here it is shown that a Ag particle embedded in Ni displays an OR of Ag{111} // Ni{111} , i.e., a cube-on-cube OR. It has also been shown that as the confinement of a Ag particle is gradually increased from 2-D to 3-D, by equilibrating a Ag particle on Ni(100) substrates with progressively deeper dimples, a transition in OR occurs from oct-cube to cube-on-cube. This result contradicts the conventional notion which supposes that the ORs displayed in the presence of 3-D confinement (e.g., during phase transformations) will also tend to be displayed in epitaxy on a substrate.

International audience; The equilibrium configuration of compound sessile drops has been calculated previously in the absence of gravity. Using the Laplace equations, we establish seven dimensionless parameters describing the axisymmetric configuration in the presence of gravity. The equilibrium axisymmetric configuration can be either stable or unstable depending on the fluid properties. A stability criterion is established by calculating forces on a perturbed Laplacian shape. In the zero Bond number limit, the stability criterion depends on the density ratio, two ratios of interfacial tensions, the volume ratio of the two drops, and the contact angle. We use Surface Evolver to examine the stability of compound sessile drops at small and large Bond numbers and compare with the zero Bond number approximation. Experimentally, we realize a stable axisymmetric compound sessile drop in air, where the buoyancy force exerted by the air is negligible. Finally, using a pair of fluids in which the density ratio can be tuned nearly independently of the interfacial tensions, the stability transition is verified for the axisymmetric configuration. Even though the perturbations are different for the theory, simulations and experiments, both simulations and experiments agree closely with the zero Bond number approximation, exhibiting a small discrepancy at large Bond number.

International audience; Molecular dynamics simulations of Ag films equilibrated on Ni have been performed for 12 different Ni substrate orientations. The results show that well-equilibrated films display several different orientation relationships (ORs) depending on the Ni substrate orientation: cube-on-cube, twin, oct-cube, and a series of special ORs that are a consequence of the oct-cube OR that develops on Ni(100). It is found that an important feature displayed by the observed ORs is the alignment of the step edges of the Ag film with those of the Ni substrate at the interface. Such alignment is initiated during film formation, but also tends to produce minimum energy interfaces between film and substrate. This feature of hetero-epitaxy and of the resulting ORs has not previously been emphasized.

International audience; The orientation relationships (ORs) that develop when Ag films are equilibrated on more than 200 different Ni substrate orientations distributed over the standard stereographic triangle have been determined by electron backscatter diffraction. Four different types of OR have been observed, including cube-on-cube, twin-related, oct-cube (that develops only on Ni(100)), as well as a family of more complex ORs (referred to as "special") that arise during the gradual transition from the oct-cube to the twin OR. A mechanism of OR development, based on the alignment of the step edges on the Ag side of the interface with those of the Ni substrate, has been validated by comparison with the results obtained on the large range of substrate orientations investigated in this study. The mechanism proposes that step alignment occurs in the early stages of film formation, but is also consistent with equilibrium ORs as it tends to produce minimum energy interfaces. This feature of hetero-epitaxy and of the resulting ORs has not previously been emphasized.

Molecular dynamics simulations have been performed to study the interaction between a faceted pore and an anisotropic grain boundary (GB). Nickel was chosen as a convenient model system. In order to establish the equilibrium crystal shape (ECS) of the pore, studies were also conducted on isolated pores. Isolated pores were found to be subject to the nucleation inhibition of equilibration that has been predicted by Rohrer et al. (J Am Ceram Soc 2000;83:214, 2001;84: 2099). This work shows that configurations close to the ECS can be obtained if supersaturation within a pore is artificially increased by adding mobile adatoms to the internal surfaces of the pores. In the case of pores located at GBs, the nucleation energy barriers to facet displacement are not present for facets in contact with the GB at the triple line, but may still persist for facets that have no contact with the GB. This problem can be overcome by approaching the equilibrium shape from different initial configurations. The configuration of the GB in the vicinity of the pore has been found to be essentially planar, indicating that GB puckering in the vicinity of anisotropic pores is not generally necessary. The present calculations show that incompatibilities between misoriented pore facets that meet at the triple line with the GB are easily accommodated by local atomic rearrangements at the disordered region of intersection with the GB.

International audience

Existing methods to rapidly compute interface-energy minimizing shapes with anisotropy are collected and clarified, and new methods are introduced. A description of freely available, platform-independent software for the computation and display of equilibrium geometries is provided. The software relies on a new computational method to rapidly find equilibrium geometries. It also features a graphical user interface and includes the 32 crystallographic point groups to simplify inputting interfacial energies and their associated orientations. When a particle is completely enclosed within a single interface (isolated), the software computes and provides visualization for Wulff shapes. When a particle is enclosed by two interfaces, such as a particle at a grain boundary, the software minimizes their collective interfacial energy; if one of the interfaces is planar, the computation reproduces the Winterbottom construction. When both interfaces are deformable, the software provides a new tool for calculating the particle shape and the distortions of boundaries that are attached to it, even for highly anisotropic interfaces. The properties of particles bounded by two deformable interfaces are discussed, and applications of the software are illustrated. In some cases, the software can be used as a method to infer values of relative interfacial energies from a microscopic observation.

Nanometer-thick films at interfaces and surfaces exist in various materials and can substantially influence their properties. Whether these films are an equilibrium or transient state is debated. To address this question, we equilibrated 1.2-nanometer-thick films at gold-sapphire interfaces in the presence of anorthite glass and measured the solid-solid interface energy. The equilibrated film significantly reduced the interfacial energy and could be described by the Gibbs adsorption isotherm expanded to include structure in addition to chemical excess. Unlike artificially made conventional thin films, these films do not break up during equilibration and offer an alternative design criterion for thin-film technology. These results demonstrate that nanometer-thick films at interfaces and surfaces can be an equilibrium state and included in phase diagrams with dedicated tie-lines.

Copper particles have been grown on sapphire (0 0 0 1) substrates by dewetting a copper film either in the solid or the liquid state. After equilibration the particles adopt different orientation relationships (ORs). Solid state dewetting produces a single OR: Cu(1 1 1) [ 1 1 ¯ 0 ] ||Al2O3(0 0 0 1) [ 1 0 1 ¯ 0 ] . In contrast, dewetting in the liquid state (followed by solid state equilibration) produces four additional ORs, Cu(1 1 1) [ 1 1 ¯ 0 ] ||Al2O3(0 0 0 1) [ 2 1 ¯ 1 ¯ 0 ] , Cu(1 1 0) [ 1 1 ¯ 0 ] ||Al2O3(0 0 0 1) [ 2 1 ¯ 1 ¯ 0 ] , Cu(3 1 1) [ 0 1 1 ¯ ] ||Al2O3(0 0 0 1) [ 2 1 ¯ 1 ¯ 0 ] and Cu(2 1 0)[0 0 1]||Al2O3(0 0 0 1) [ 2 1 ¯ 1 ¯ 0 ] , which have been found to have a similar interfacial energy. All of the ORs observed in this study are consistent with the Fecht and Gleiter lock-in model, from which one would expect that densely packed directions in the interface plane of the metal crystals will tend to align with relatively dense directions in the substrate surface. The 30° change in alignment direction on the c-sapphire side of the interface, from Al2O3 [ 1 0 1 ¯ 0 ] for solid state dewetted samples to Al2O3 [ 2 1 ¯ 1 ¯ 0 ] for samples dewetted in the liquid state, appears to be related to a reconstruction of the copper–c-sapphire interface that occurs at a temperature between those at which the two types of samples are processed.

International audience; Ultrasonic inspection of sodium-cooled fast reactor requires a good acoustic coupling between the transducer and the liquid sodium. Ultrasonic transmission through a solid surface in contact with liquid sodium can be complex due to the presence of microscopic gas pockets entrapped by the surface roughness. Experiments are run using substrates with controlled roughness consisting of a network of holes and a modeling approach is then developed. In this model, a gas pocket stiffness at a partially solid-liquid interface is defined. This stiffness is then used to calculate the transmission coefficient of ultrasound at the entire interface. The gas pocket stiffness has a static, as well as an inertial component, which depends on the ultrasonic frequency and the radiative mass.

A model of O-adsorption at liquid metal surfaces has been constructed, using liquid Cu as an example. The modeling approach used is similar to the regular solution scheme previously used successfully for modeling the adsorption/segregation behavior of metal alloys, in that the internal energy of the system is evaluated by nearest neighbor bond energies. In the model, the adsorption of oxygen in the near-surface region is allowed to occur at both surface and sub-surface sites. The model predicts a variety of possible adsorption characteristics, including the possibility of first order adsorption transitions which involve the formation of a 2-dimensional surface oxide, and different sequences for the occupancy of surface and sub-surface adsorption sites. In particular, by fitting the model to the experimental dependence of Cu surface energy on O-partial pressure, it is possible to conclude that O-adsorption in that case most likely occurs by the occupancy of sub-surface sites.

The contact angle of liquid Cu on monocrystalline and polycrystalline Co surfaces is found to be close to zero, and some of the grain boundaries of the Co polycrystal are completely wetted by liquid Cu. The consistency of these results is discussed in terms of interfacial, grain boundary and surface energies in the Co–Cu system. The Co solid/Cu liquid interfacial energy is found to be equal to or smaller than 390 mJ m−2 at 1383 K.

In the present study, we compare available experimental surface tension data with two theoretical concepts: the Butler equation and a multilayer model in which the surface is described as a stacking of atomic layers. The experimental data were measured systematically over a wide temperature and concentration range using electromagnetic levitation and the oscillating drop technique. Three classes of alloys are studied: alloys with rather weak or no segregation, alloys with strong segregation, and associate forming alloys.

International audience; A model of anisotropy of surface energy, resulting from surface segregation, has been used to calculate the equilibrium crystal shape (ECS) of FCC alloy crystals. The parameters of the model, including (a) surface energy differences between pure solvent and solute, (b) the regular solution constant, and (c) solute strain energy, have been varied in order to evaluate their effects on the ECS. Whereas the model predicts that the ECS of pure metals should only display {111} and {100} facets, the effects of segregation can produce several new facets, e.g. at {110}, {311}, {331} and {210} orientations. In particular, new cusps in the γ-plot can only appear if the solute strain energy parameter has finite values. Also, it has been found that for any pair of orientations, the ratio of surface energies displays a minimum at the temperature corresponding to the crossover in adsorption for those two surface orientations. These crossovers occur over a limited temperature range, and new cusps that are deep enough to produce new facets on the ECS also tend to occur in that restricted temperature range. Some comparisons with experimentally determined ECSs of alloy crystals are provided, and show that the model can provide valuable though qualitative guidance for experiments.

A previously developed model of the dependence of grain boundary (GB) segregation on GB character has been exercised to investigate solid-state wetting transitions at GBs, and their anisotropy. In the case of binary systems displaying a solid-state miscibility gap, it is shown that the wetting transition temperature for precipitates at a GB is anisotropic, and is inversely related to GB energy. The model also allows calculation of prewetting transitions and associated excursions in adsorption off phase coexistence. These transitions are first order below a prewetting critical point ( T PWC ), and higher order at temperatures above T PWC . Investigation of the prewetting behavior provides the means for construction of the two-dimensional phase diagram of a GB.

This chapter presents a comprehensive overview of wetting phenomena which take place at high temperatures. It relies in part on the chapter "Introduction to interfaces and diffusion", elsewhere in this volume, as a foundation. The topics addressed include: definitions of intrinsic wetting and associated quantities, wetting anomalies due to diffusion and anisotropy of solid/fluid interfaces, wetting hysteresis on real surfaces, and orders of magnitude of the energies that determine wetting in metal/metal, metal/ceramic and glass systems. At the end of this chapter some examples of evolving contact between liquid and solid metal illustrate the complexity of wetting phenomena at high temperature.

Various factors that can affect the coverage dependence of diffusivity for the case of surface hetero-diffusion have been analyzed. Simple Monte Carlo modeling of the diffusion process shows that diffusivity can either increase or decrease with coverage, depending on whether the interaction between diffusing atoms is repulsive or attractive. The results of more detailed simulations show that these simple predictions are reasonable. If the diffusing species undergoes first-order phase transitions as a function of coverage, then important perturbations of the diffusion profiles will result. One final factor in coverage dependence of diffusivity is considered, namely, the possible effects of coverage on the surface step structure. An analysis of diffusion profiles obtained for the spreading of Bi over Cu(100) shows that the major effects of coverage on diffusivity come about from the presence of phase transitions, and from the inherent coverage dependence of diffusivity in systems with short-range repulsion.

International audience; Energetics and chemistry of liquid surfaces and liquid/liquid interfaces of binary A B alloys are calculated using a subregular solution model. In this model, two macroscopic energetic parameters are used to produce an asymmetric miscibility gap. They are related to two microscopic parameters which describe the interaction energy between two atoms as a function of the composition of the first coordination shell of each atom. The impact of the asymmetry of the A B interactions on the surface and interfacial energies and adsorption are analyzed by comparing the results obtained with this subregular model to those calculated for a regular solution. The role of the asymmetry on the prewetting and wetting transitions are also discussed. Calculations performed in the Co Cu system are in good agreement with experimental data of surface energy.

Since the beginning of materials science research under microgravity conditions immiscible alloys have been an interesting subject. New possibilities to investigate such systems are offered by containerless processing techniques. Of particular interest is the ternary system Cu-Fe-Co, and its limiting binaries, Cu-Co and Cu-Fe. They all show a metastable miscibility gap in the regime of the undercooled melt. Within the ESA-MAP project "CoolCop", different aspects of this alloy have been investigated; results obtained so far are reported here.

Scanning tunneling microscopy (STM) has been used to study the various possible structures of adsorbed Bi on the Cu(1 0 0) surface, after equilibration at a temperature of 520 K. All of the structures previously identified by X-ray diffraction (lattice gas, c(2 × 2), c(9√2 × √2) R 45°, and p(10 × 10), in order of increasing Bi-coverage) were found to be present on a single sample produced by diffusing Bi onto the Cu(1 0 0) surface from a 3-d source. By investigating the possible coexistence of various pairs of phases, it was demonstrated that the c(2 × 2) phase transforms to the c(9√2 × √2) R 45° phase by a first order transition, whereas the transition from c(9√2 × √2) R 45° to p(10 × 10) is continuous. In addition, the structure of surface steps was studied as a function of Bi-coverage. The results showed that the presence of Bi changes the nature of the step–step interactions at the Cu(1 0 0) surface from repulsive to attractive. The attractive step–step interactions transform any small deviations from the nominal (1 0 0) orientation of the Cu substrate into (3 1 0) microfacets. When compared with the known equilibrium crystal shape (ECS) of Bi-saturated Cu, the observed microfaceting may imply that the ECS of Cu–Bi alloys is temperature dependent.

International audience; Wetting of ìm-sized Pb droplets on thin polycrystalline films of decagonal Al13Co4 and cubic crystalline AlCo phases is reported. The sample preparation is crucial to have Pb droplets lying on a clean surface. Decagonal and cubic films were prepared under high vacuum conditions, by sequential deposition and annealing of specific stackings of Al and Co layers of nanometric thicknesses. A 300nm-thick Pb slab was then deposited on the top of the films. Dewetting experiments were performed in situ in a scanning Auger microprobe where the surface chemistry can be monitored: the Pb slab dewetts into droplets. The wetting of Pb on both substrates was found to be similar with contact angles around 45°.

The purpose of this article is to review analytical models of the anisotropy of segregation to grain boundaries (GBs) and surfaces, and to evaluate their predictions. A summary of Gibbsian interfacial thermodynamics is provided as an introduction to the topic. This is followed by a historical overview of previous analytical models. A recently developed model of the dependence of GB segregation on the five macroscopic parameters of GB orientation is outlined, and illustration of how this formulation reduces to the particular cases of segregation to simpler types of interfaces is provided. In addition, some specific aspects of interfacial segregation, which have either been problematic or have lacked satisfactory explanation, are addressed. These include (a) the relationship between the compositions on the two sides of a given GB; (b) the difficulty of meaningful definitions of segregation-free energy (and related thermodynamic quantities such as enthalpy and entropy); (c) the so-called compensation temperature, at which the anisotropy of interfacial segregation seems to vanish; (d) the relationship between surface and GB segregation; and finally (e) an attempt to determine whether segregation increases or decreases interfacial energy anisotropy, and the consequences thereof on the equilibrium crystal shape of alloys. Where possible, comparisons are made with the results of experiments or computer simulations.

The interfaces between liquid or solid Pb islands and single- or poly-crystalline Cu substrates are investigated. EBSD (Electron backscattered diffraction) analyses show that when Pb is solidified from droplets, a cube-on-cube orientation relationship prevails between the Pb crystals and each Cu grain surface, whatever its surface orientation, even though the lattice parameter of Pb is 1.37 times larger than that of Cu. Scanning electron microscope (SEM) and atomic force microscope (AFM) analyses show that during annealing of Pb droplets, ridges develop at the droplet-substrate triple lines, indicating that the interfaces evolve towards equilibrium by diffusional processes. These features are discussed in comparison with experiments and calculations performed on Pb nanocrystals embedded in Al, another fcc solid with a lattice parameter smaller than that of Pb. It is proposed that the cube-on-cube orientation relationship results from the solidification of Pb along the {111} planes of the solid Cu/liquid Pb interface which have developed by diffusion during the process of equilibration of the interface shape.

The spreading of Pb and Bi adsorbed layers from pure bulk sources, and their intermixing, on a Cu substrate, has been studied by scanning Auger microscopy at 473 K. The pure Pb layer spreads faster than the Bi layer. When the pure layers meet, two phenomena are observed. The junction line of the two adsorbed layers drifts due to a replacement of adsorbed Pb by Bi. Superimposed on this drift, the Pb and Bi layers undergo interdiffusion. The interdiffusion coefficient has been analyzed by the Boltzmann–Matano method.

Recently developed models of grain boundary segregation and grain boundary energy, as a function of the 5 macroscopic parameters of grain boundary orientation, have been used to predict the behavior of a simple face centred cubic alloy. It is shown that the equilibrium composition, as well as the composition profile, on one side of a boundary with a given terminating plane, depends on the crystallographic orientation of the terminating plane of the other side of the grain boundary. These predictions are confirmed by recent measurements of Nb segregation to TiO2 grain boundaries. In addition, the models show stronger solute segregation to higher energy grain boundaries of the pure solvent. This result is also confirmed by the experimental distribution in orientation space of Nb segregation to TiO2 grain boundaries, when it is compared to the orientation distribution of the grain boundary energy of pure TiO2. The overall results are consistent with expectations from the Gibbs adsorption isotherm, and indicate that grain boundary energy anisotropy is decreased by grain boundary segregation.

The spreading of metallic liquid drops on flat solid metal or ceramic substrates ranges from milliseconds — comparable to room-temperature spreading of water on glass — to hours. This varying timescale can be understood by what happens — or doesn't happen — at the foot of the drop where the liquid surface contacts the substrate.

The Wulff construction yields the equilibrium shape of a particle of fixed volume embedded in a single phase at constant temperature. When the particle attaches to an interface (boundary) between two distinct phases or grains of the same phase, the Wulff construction must be modified to account for the abutment of Wulff shapes at the boundary as well as the boundary energy that is replaced by the particle. If the boundary is non-deformable, a portion of the particle interface is constrained to replace the image (i.e. the exact position) of the boundary that is removed, and the Winterbottom construction yields the equilibrium particle shape. If the boundary can deform, the particle is not constrained to replace the image of removed boundary, and the particle shape is determined by a more general modification of the Wulff construction. In two dimensions, the particle attaches to an initially flat boundary and creates two disjoint segments that remain flat at equilibrium. In three dimensions, the boundary su...

Ga drops on a Ge(111) surface have been investigated by scanning Auger electron spectroscopy. Surface segregation of both Ga on Ge, and of Ge on Ga, was observed at about 600 K. These results are found to be consistent with the various driving forces of surface segregation. Following ion sputtering of the Ge substrate at about 400 K, the Ga drops spontaneously move over the substrate, thereby healing sputter-induced damage at the Ge surface.

Anisotropic Bi segregation, at surfaces of a Cu polycrystal saturated with Bi at 1223 K, changes the Cu equilibrium shape from a faceted spheroid displaying all orientations to an almost polyhedral shape with missing orientations. Surface cleaning, followed by annealing at ∼573 K, leads to Bi readsorption from some grain boundary sources.

The equilibrium crystal shape (ECS) of copper has been studied by scanning electron microscopy on μm-sized copper crystallites supported on single-crystals of α-alumina. In addition, the orientation relationships between copper crystals and the sapphire substrate were investigated by X-ray techniques. A detailed discussion of the kinetic factors that can inhibit equilibration is provided, and it is shown that only crystals ranging in radius from 3 to 4.5 μm can achieve equilibrium shapes under the conditions of the experiment. The maximum anisotropy of surface energy was found to be about 1.02, which is significantly lower than that of the other two fcc metals (lead and gold) for which reliable data are available. Another distinction between copper and those other fcc metals is that its ECS displays {110} facets, and possibly {311} facets, in addition to the commonly observed {111} and {100} facets, at temperatures where equilibration is possible. The observed facets connect tangentially to the curved parts of the ECS, so that all possible surface orientations are present on the copper ECS.

L'ensemble des travaux sur la fragilisation par les metaux liquides que nous avons realises dans le cadre du programme GEDEON est resume ici. Nous avons choisi de focaliser l'attention sur le role du mouillage des joints de grains sur leur envahissement rapide par un metal liquide, et nous mettons en evidence de nouveaux parametres thermodynamiques et cinetiques impliques dans ce phenomene. Depuis l'article de Mullins (1957) et les etudes experimentales realisees dans les annees 60 sur le gravage des joints de grains, de nouveaux outils ont ete developpes pour l'observation du gravage (et du mouillage) des joints de grains a des echelles bien inferieures au micrometre. Nous montrerons sur des systemes a base de cuivre, fer et nickel que les mesures precises autorisees par ces nouveaux instruments permettent de mettre en lumiere le role du facettage des joints de grains sur leur penetration et de proposer un nouveau modele decrivant ce phenomene.

International audience; This paper presents a molecular dynamics simulation of an experiment on the stability of a pile of glass beads in a rotating drum filled with a “humid” atmosphere. The experimental avalanche angles were found to be different whether the humidity came from water or heptane. These experimental results allow us to set the basic hypothesis of the modeling: in the heptane atmosphere the glass beads are connected by deformable liquid bridges, whereas in the case of a water atmosphere solid bridges form between the beads. The numerical calculations of the avalanche angle as a function of the undersaturated pressure of heptane or water are in good agreement with the experimental results.

This paper examines the factors that determine the position occupied by a void associated with an embedded precipitate particle. Analytical expressions are developed for the energy changes associated with positioning the void and the precipitate at various possible locations within a cubic cavity bounded by interfaces of a single crystallographic type. The calculated energies include consideration of the void volume fraction as well as the wetting criteria to account for interfacial energetics. The results of the analysis are applied to the interpretation of previous experimental results obtained on voids associated with Pb precipitates in a Si matrix, where the precipitate and void are confined within a more complex cubo-octahedral cavity. By taking into account the volume fraction of the void and the wetting conditions for Pb on Si, it is verified that voids in Pb precipitates embedded in Si should be convex and occupy corner sites. A computer graphics analysis of three-dimensional voids and pa...

A model is developed for describing rapid penetration of a liquid phase along a grain boundary. It is based on the assumption of a highly faceted solid/liquid interface. Experiments showing the faceting of the solid/liquid interface in grain boundary penetration experiments are presented. The basic hypothesis of the model are an undersaturated solid and a positive spreading coefficient of the liquid phase along the grain boundary. The model explains the apparent concave shape of the tip of the groove and the reason why penetration also occurs if the liquid phase is pre-saturated with the material of the solid. Moreover it predicts a power law with an exponent close to unity for the time dependence of the depth of penetration of the liquid layer along the grain boundary.

This paper reports the effects of Tl additions on Pb adsorption to the surface of liquid Ga–Pb alloys. Adsorption of Pb and Tl was measured by Auger electron spectroscopy, as a function of temperature, for six ternary alloys ranging in composition from Ga–3.33×10−1 at.% Pb–1.65×10−1 at.% Tl to Ga–1×10−4 at.% Pb–5×10−5 at.% Tl, all having a 2:1 ratio of Pb:Tl. The temperature of the prewetting critical point in binary Ga–Pb alloys had previously been estimated to lie at about 240 K. The motivation for this work was to raise the temperature of the prewetting critical point by the addition of Tl. However, the results show that whereas Pb adsorption is enhanced by Tl additions, as expected from simple models, the estimated temperature of the prewetting critical point is lowered even further to about 215 K.

The mechanical behavior of set plaster has been studied in humid atmospheres. This material is made of entangled needle-shaped gypsum crystals. At room temperature, Young’s modulus of set plaster decreases as the coexistence of gypsum and water is approached by increasing the humidity of the surrounding air. Simultaneously, the thickness of the adsorbed water layer at the grain boundaries between the gypsum crystals is shown to diverge. It is suggested that the gypsum crystals are able to slide along certain grain boundaries because the viscosity of the water film approaches that of bulk liquid water. Small additions of d , l -tartaric acid prevent both the adsorption of water at grain boundaries of gypsum and the decrease of the Young’s modulus of set plaster.

Using the sandwich drop method, advancing contact angles were measured for liquid tin on the silica (SiO 2 ) surface under high vacuum (10 −5 Pa) at 1173 K. A drop size dependence for the measured contact angles was observed, i.e. the contact angle decreased as the drop's base radius was increased. In line with explaining similar observations for “low-energy” (i.e. organic) systems, the observed trend for the Sn–SiO 2 (i.e. “high-energy”) system was interpreted using the modified Young equation. It was found that the drop size dependence of contact angles can be interpreted as being due to a positive line tension. The line tension value obtained might seem to be large (155 μJ/m) in comparison with the line tension values of the order of 1 μJ/m for low-energy systems; however, a large line tension for the Sn–SiO 2 system is in line with results from the low-energy systems which indicate a positive correlation between the line tension and the solid–liquid interfacial tension.

Wetting results from a balance of the free energies of two surfaces and an interface meeting at a triple line. Metal-oxide systems are in bulk chemical equilibrium if the activities of each of the components are identical in the metal, oxide and vapor phases which constitute the system. The data available for wetting of oxides by metallic liquids show significant scatter because two principal parameters are not controlled: the chemical equilibrium of the system, i.e. the chemical state of the surfaces and the interface, and the physical state of the oxide surface. Examples are presented to illustrate the manner in which these parameters can influence wetting measurements, and to facilitate comparisons among wetting data reported for various metal-oxide systems.

The surface composition of Ga-rich liquid in equilibrium with Pb-rich solid has been investigated between 508 K and the monotectic temperature (586 K) of the GaPb system using Auger spectroscopy. A Pb-rich layer is shown to thicken up to 9 equivalent monolayers of Pb at the surface of the liquid as the temperature increases. The thickness of the surface layer diverges logarithmically as the composition of the liquid approaches the monotectic composition. This is the signature of the existence of complete wetting in the GaPb system occurring in the metastable region of coexistence of the two liquids.

Wetting of silver on sapphire and surface tension of silver are both determined as a function of oxygen activity in a range corresponding to oxygen content in silver varying from 10 −12 to 10 −2 . The contact angle is shown to decrease at high oxygen activity when simultaneous decreases in surface tension of silver and interfacial energy between silver and alumina occur. This double oxygen adsorption is thermodynamically analyzed and some relevant parameters inducing oxygen segregation at the silver interfaces are proposed

International audience

This paper reviews the fundamental concepts and the terminology of wetting. In particular, it focuses on high temperature wetting phenomena of primary interest to materials scientists. We have chosen to split this review into two sections: one related to macroscopic (continuum) definitions and the other to a microscopic (or atomistic) approach, where the role of chemistry and structure of interfaces and free surfaces on wetting phenomena are addressed. A great deal of attention has been placed on thermodynamics. This allows clarification of many important features, including the state of equilibrium between phases, the kinetics of equilibration, triple lines, hysteresis, adsorption (segregation) and the concept of complexions, intergranular films, prewetting, bulk phase transitions versus “interface transitions”, liquid versus solid wetting, and wetting versus dewetting., Seventh Framework Programme (European Commission) (Grant FP7-NMP-2009-CSA-23348-MACAN)