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International audience; Tungsten heavy alloys (Co-Ni-W) of different compositions were prepared by powder metallurgy (solid sintering), heat treated at 800°C and 1000°C and subsequently analyzed. The experimental phase identifications and composition measurements emphasized discrepancies compared to the isothermal ternary sections simulated with commercial thermodynamic databases. The isothermal sections at 800°C and 1000°C were found more complex than expected since a new intermetallic phase appeared to be stable (D0a structure-Cu3Ti prototype). Likewise, an interpretation of phase equilibria evolution with temperature was established to reach agreement with high temperature data. Formation enthalpies associated with the new phase were obtained through first-principles calculations and supported experimental results as they confirmed the stabilizing role of Ni addition for the D0a structure.

Diffusion couples of Ti and biphasic alloy Al-7wt%Si were produced and heat-treated at 535 °C for between 1 and 200 hours. The nature and thickness of the phases present in the reaction layer sequence were characterized using conventional techniques such as microprobe analysis and transmission electron microscopy. For short interaction times the interfacial reaction layer consists mainly of silicides, whereas aluminides are formed after times longer than 40h. This variation in the spatial reaction layer sequence over time is explained by the biphasic nature of one of the diffusion couple end-members, with high activity for both elements, and by the huge difference between the composition of the first phase formed at the initial interface and the average composition of the biphasic end-member. This situation, which can be observed in other systems, illustrates the difficulty of confirming that a true diffusion path has been attained in a diffusion couple. Most of the criteria that were first proposed by Kirkaldy and Brown, can apparently be fulfilled, but the system could continue shifting over time towards a more stable configuration. In some systems, as in the case of Al-Si-Ti, the slow kinetics can be problematic, making it impossible to predict a radical change in material properties.

International audience; This paper proposes a novel and simple technique to study secondary wetting for bimetallic systems characterized by almost perfect primary wetting. The main principle is to promote complete dissociation between primary and secondary wettings, avoiding use of the conventional sessile drop technique and its variants. For the first time, evidence is thus obtained that secondary wetting occurs in aluminium alloys.

International audience; The question of material stability through the determination of the thermodynamic properties is of fundamental and technological importance to any analysis of system properties in many applications. For Al 4 C 3 , its experimental heat of formation varies widely, from −0.187 to −0.363 eV/atom, which makes it difficult to use such experimental information for any reactivity assessment. Here, we demonstrate that density functional theory (DFT), with the recently developed strongly constrained and appropriately normed (SCAN) functional, is especially powerful in critically assessing these experimental data. In order to have a more complete description of thermodynamic properties of Al 4 C 3 , we also determine the temperature dependence of its heat capacity using both the harmonic and quasi-harmonic approximation. In addition, and to select the most efficient approximation , we come back to the experimental background by using differential scanning calorimetry (DSC) to measure this quantity at constant pressure up to high temperatures, namely 873 K.

International audience; Several industrial processes involving SiC coupling to Al-Ti alloys (e.g. metallization of SiC components, brazing of SiC parts) require an in-depth knowledge of Al-Ti/SiC interactions occurring at high temperatures. To this end, the surface reactivity between SiC and Al-Ti alloys (Al3Ti and (Al+Al3Ti) systems) was analyzed by specific experiments (wetting, DSC, microstructural examinations) as well as by a thermodynamic approach (CALPHAD method). An Al-C-Si-Ti thermodynamic database was successfully established to calculate several sections and projections in order to compare the computed, expected solid phases formed at the interface with those characterized in wetting experiments. In this way, the change in liquid and solid phases was interpreted and discussed, defining the Ti3(Al,Si)C2 mixed MAX-phase as the main interfacial product created by thechemical reaction, as a function of temperature and alloy composition. This work constitutes a guide for the choice of operating parameters in processes such as brazing or SiC metallization in microelectronic applications, in which the control of interfacial products is one of the most delicate production steps. The approach proposed to monitor the pathway of liquid composition with time and temperature during liquid/solid interaction, successfully applied to interpret the microstructure obtained in wetting experiments, is a promising method for interpreting more complex cases such as homogeneous or even heterogeneous brazing processes.

T2-Al2MgC2 was synthesized from the elements in a Mg-Al melt at 1000 °C using sealed Ta crucibles. Single crystals of T2-Al2MgC2 were extracted by evaporating the Mg-Al matrix. The crystal structure of T2-Al2MgC2 was refined for the first time on the basis of single-crystal X-ray diffraction. The crystal is trigonal (space group P-3m1, Z = 1) with lattice parameters of a = 3.3767(11) A, c = 5.807(2) A and V = 57.34(5) A3. Based on the refined crystal structure, DFT calculations were conducted to evaluate the thermodynamic properties and the electronic structure of the phase. The heat of formation of T2-Al2MgC2 was calculated to be −23.6 kJ/mol of atoms at 298 K. The heat capacity of T2-Al2MgC2 was measured by DSC from 300 to 871 K and calculated by DFT from 0 to 1000 K. Based on the calculated heat capacity, the entropy of formation of the phase at 298 K was determined to be 70.0 J/mol/K. The band structure and the electronic density of state of T2-Al2MgC2 was calculated leading to an indirect band gap value of 1.73 eV.

International audience; The Al–C system is the backbone of wide variety of applications in multicomponent systems, and was therefore studied intensively. Yet, considerable disagreements remain in the reported data, and the phase equilibria in the binary system as well as the standard enthalpy of formation of Al4C3 are still debated. In order to establish a reliable thermodynamic description of the Al–C system a thorough and critical assessment of the literature was conducted. The data on the solubility of carbon in liquid Al and on the thermodynamic properties of Al4C3 were assessed, and the peritectic decomposition temperature of Al4C3 was confirmed at 2425 ± 15 K by thermal analysis. Thermodynamic modeling of the Al–C system is provided and compared to previous works. The proposed description was found to be more robust and to carry more physical meaning than any previous modeling of the system, which imply an easier extrapolation of the binary into higher-order systems.

International audience; In this paper, the electrical properties of Ti3SiC2-based ohmic contacts formed on p-type 4H-SiC were studied. The growth of Ti3SiC2 thin films were studied onto 4H-SiC substrates by thermal annealing of Ti-Al layers deposited by magnetron sputtering. In this study, we varied the concentrations of Ti and Al (Ti20Al80, Ti30Al70, Ti50Al50, and Ti), and the annealing temperature from 900 °C to 1200 °C for each concentration. X-ray diffraction and transmission electron microscopy analyzes were performed on the samples to determine the microstructure of the annealed layers and to further investigate the compounds formed after annealing. Using the transfer length method structures, the specific contact resistance (SCR) at room temperature of all contacts was measured. The temperature dependence up to 600 °C of the SCR of the best contacts was studied to understand the current mechanisms at the Ti3SiC2/SiC interface. The experimental results are in agreement with the thermionic field emission theory. With this model, the barrier height of the contact varies between 0.71 and 0.85 eV. Finally, ageing tests showed that Ti3SiC2-based contacts were stable and reliable up to 400 h at 600 °C under Ar.

International audience; In the scope of supporting the development of Mg–Al alloys and related materials, an experimental study coupled with a CALPHAD thermodynamic modeling of the Al–C–Mg system was conducted. The peritectic decomposition of Al2MgC2 to form Al4C3, graphite and a liquid phase was measured at 1559 ± 10 K by DTA. In order to model the Mg solubility in Al4C3, DFT calculations were performed on the end-member phases Mg4C3, Al2Mg2C3 and Mg2Al2C3 and it was shown that Mg substitutes on the Al2 crystallographic site of the carbide structure. Based on recent literature data and a revised Al–C binary, a model description of the Al–C–Mg ternary is proposed for the first time. More specifically, it is used to calculate the liquidus projection and the phase formation sequence during Scheil solidification of a 91Mg–9Al wt% alloy inoculated with carbon. This work provides a convincing argument that Al2MgC2 is the nucleant responsible for the grain refinement of Mg–Al alloys inoculated by C.

The reaction tending toward thermodynamic equilibrium during the synthesis of Ti/TiC MMC prepared by the powder metallurgy route was studied by in situ synchrotron X-ray diffraction. The carbide composition was found to change rapidly from its initial stoichiometric value TiC0.96 toward a substoichiometric value (TiC0.57) corresponding to thermodynamic equilibrium with the C-saturated Ti matrix. The reaction rate is very fast, and the solid-state reaction is almost complete after only a few minutes at 1073 K (800 °C) for the smallest particles, whereas the rate-limiting step remains the particle size. In addition, modeling of the diffusion processes in MMCs, i.e., initial dissolution of particles and their trend toward equilibrium composition, was performed using three particles size classes and the calculations were performed using the ThermoCalc and Dictra package. First, dissolution of the smallest particles (10% of the initial TiC0.96 particles) is expected to be achieved after only 1 s at 800 °C. Second, the change in TiC composition leads to an increase in the total amount of carbide in the composite from 16 to 19 mass%. The consequences on the industrial process of Ti/TiC MMC synthesis have also been considered. A typical industrial heat treatment of a MMC billet, 1 h at 900 °C, was modeled, and the results showing an increase in the total amount of carbide in the composite from 16 to 22 mass% are in rather good agreement with the experimental value (21 mass%). This highlights the potential of thermodynamic and kinetic modeling to help understand and optimize industrial processes for MMC synthesis.

Grain refinement of Mg–Al based alloys is challenging because it is known that Zr, which is extremely effective in many Al-free alloys, cannot be used. The addition of carbon through various routes by using carbon-containing sources is considered as an option. The grain refinement mechanisms are still under debate. The present work is focused on the ternary base system Mg–Al–C, including the potential nucleants Al4C3 and Al2MgC2, presently without consideration of Al2CO. The ternary carbide Al2MgC2 was synthesized and characterized using sealed Ta crucibles. The decomposition of the carbide was measured at 1290 °C by Differential Thermal Analysis under a pressure of 8 bar. Practical difficulties, including high vapor pressure of Mg and high affinity of Mg with oxygen, as well as rapid hydrolysis of the Al2MgC2 carbide have been overcome.

A metal matrix composite has been obtained by a novel synthesis route, reacting Al 3 Ti and graphite at 1000 °C for about 1 min after ball-milling and compaction. The resulting composite is made of an aluminium matrix reinforced by nanometer sized TiC particles (average diameter 70 nm). The average TiC/Al ratio is 34.6 wt.% (22.3 vol.%). The microstructure consists of an intimate mixture of two domains, an unreinforced domain made of the Al solid solution with a low TiC reinforcement content, and a reinforced domain. This composite exhibits uncommon mechanical properties with regard to previous micrometer sized Al–TiC composites and to its high reinforcement volume fraction, with a Young’s modulus of ∼110 GPa, an ultimate tensile strength of about 500 MPa and a maximum elongation of 6%.

International audience; MoAlB is the first and, so far, the only transition-metal boride that forms alumina when heated in air and is thus potentially useful for high-temperature applications. Herein, the thermal stability in argon and vacuum atmospheres and the thermodynamic parameters of bulk polycrystalline MoAlB were investigated experimentally. At temperatures above 1708 K, in vacuum and inert atmospheres, this compound incongruently melts into the binary MoB and liquid aluminum metal as confirmed by differential thermal analysis, quenching experiments, x-ray diffraction, and scanning electron microscopy. Making use of that information together with heat-capacity measurements in the 4–1000-K temperature range—successfully modeled as the sum of lattice, electronic, and dilation contributions—the standard enthalpy, entropy, and free energy of formation are computed and reported for the full temperature range. The standard enthalpy of formation of MoAlB at 298 K was found to be −132 ± 3.2 kJ/mol. Lastly, the thermal conductivity values are computed and modeled using a variation of the Slack model in the 300–1600-K temperature range.

The evolution of TiC reinforcement during the high-temperature consolidation step of a particulate-reinforced Ti matrix composite has been studied. A four-step scenario has been highlighted starting with the dissolution of the smallest particles to reach C saturation of the Ti matrix, followed by a change in the TiC stoichiometry from the initial TiC0.96 composition to the equilibrium composition (TiC0.57). This change in composition induces an increase in both the total mass fraction of reinforcement and the particle diameter. The diameter increase promotes contact between individual particles in the most reinforced domains and initiates an aggregation phenomenon that is responsible for the observed high growth rate of particles for heat treatment times shorter than 1 h. Finally Ostwald ripening is responsible for the growth of particles for longer heat treatment times.

International audience; An innovative IR irradiation annealing process is used to increase the crystallization ratio of hexagonal boron nitride (h-BN) coatings on pure titanium pellets. Since the polymer pyrolysis route requires heating the green polymer at high temperature to convert it into a ceramic, the use of IR radiation furnace (compared to a resistive furnace) allows achievement of better crystallized h-BN while the substrate remains at relatively low temperature (

The aim of this study was to find conditions allowing the "natural" formation of a regular and controllable step bunched morphology on a 4H-SiC seed without the need of any SiC deposition. This was performed by melting a bulk piece of Si on a 4°off 4H seed in the temperature range of 1500 - 1600°C, for 15 min. After etching the remaining Si, the 4H surface was found to be successfully highly step bunched with steps very parallel and regular. A mechanism of dissolution-precipitation was proposed, which could occur both on a short (step to step) and long (centre to periphery) range. This process is kinetically limited at low temperature (1500-1550°C) and considered to be close to the equilibrium at 1600°C., Materials Science Forum, 858, ISSN:0255-5476, ISSN:1662-9752, Silicon Carbide and Related Materials 2015, ISBN:978-3-0357-1042-7, ISBN:3-0357-1042-2

International audience; In order to reduce the weight in automotive or aeronautic components, aluminum and its alloys are being used more and more. However, their mechanical properties are not as good as those of steel and can sometimes be below the necessary requirement. To overcome this issue, reinforcement of some parts is required. In this case, the mechanical reliability of the interface between the aluminum alloy matrix and its reinforcement has to be insured. This can be done by controlling the process parameters involved in developing a strong metallurgical bond between the two metals. In this work an Fe/Al-7 wt% Si alloy interface was investigated, aimed at linking the chemistry of the reaction layers with the toughness of the bond. First, the chemistry of the interface was characterized after immersion of Fe plates in an aluminum alloy melt. Then, the fracture energy of these assemblies was measured using a modified 4-point bending test for the adhesion measurement. In the case of pure elastic deformation during the loading, an analytical approach based on beam bending theory can be used to quantify the fracture energy. Nevertheless, in bonding between metallic partners, plastic dissipation can be expected. This is why a numerical calculation, based on a cohesive zone mode was proposed. The use of cohesive elements to describe the interface behavior allows for modeling both the threshold of de-cohesion and the progression of damage at each point of the interface. (C) Koninklijke Brill NV, Leiden, 2012

International audience; Ohmic contacts represent a major technological brick for the development of high power devices on Gallium Nitride. Al(200 nm) Ti(70 nm) metallization on n+-GaN, annealed at 650 °C, provides a “Specific Contact Resistivity” (SCR) in the range mid 10-5 Ω.cm², which is low enough for the main switching power applications. However, the Al-Ti metallic compound phases formed during the annealing step result from solid-solid reactions, which may lead to high stress and / or poor cohesion, possibly deleterious to contact reliability. In this work, we have investigated several configurations of Ti-Al-Si based contacts, aiming at favoring liquid-solid reactions and / or Si element diffusion, in order to get better SCR and / or morphology and cohesion of the metallic phase. Surprisingly, only contacts annealed at low temperature (450 °C) provide low contact SCR, comparable to that of Ti-Al only contact, but systematically higher.

International audience; Recently, the ternary carbide Ti3SiC2 has gained much attention due to its unique characteristics combining the properties of metals and ceramics (i.e., a low density, decent thermal and electrical conductivities, an excellent thermal shock resistance, a good machinability, damage tolerance, low friction and so on). The present study describes an investigation of the wettability in high vacuum of bulk Ti3SiC2 by a classical braze alloy based on the Ag-Cu-Ti system. Two techniques, i.e., the sessile drop and dispensed drop methods, were utilized. The results indicated that spreading kinetics is controlled by deoxidation kinetics of Ti3SiC2 surface under vacuum. The final contact angle on clean Ti3SiC2 is very small (~ 10°), testifying the development of strong, metallic interactions across the liquid-solid interface. The reactivity between the ternary carbide and the liquid phase during isothermal heating at 800°C was also considered.

Twin-free 3C-SiC layers were recently obtained by Vapour-Liquid-Solid mechanism on a a-SiC(0001) substrate using Si-Ge melt. The formation of cubic layers is rather unexpected since growth from the melt is known to promote lateral growth and should thus give homoepitaxial layers. The study of the early stage of such growth, after a simple contact between the melt and the substrate (without adding propane), reveals the precipitation of 3C-SiC elongated islands upon the substrate surface. The chemical interactions inside the Ge-Si-C ternary phase diagram suggest an initial dissolution of the SiC seed in contact with a Ge-rich melt (below 1200°C). When the Si content of the melt subsequently increases upon heating, the dissolved carbon atoms precipitate on the seed surface under the form of 3C-SiC islands. When propane is added, these islands enlarge and coalesce to form a complete 3C layer.

The present article addresses the issue of properly modelling the thermodynamic aspect of chemical reactions proceeding at the solid/gas interface in a multi-component system. Attention is more especially paid to the formation of Ti3SiC2 by reactive chemical vapor deposition (RCVD) on a silicon carbide substrate heated at 1100 °C. A deposition diagram has been calculated by Gibbs free energy minimization in the C-Cl-H-Si-Ti quinary system. It is shown that this deposition diagram can account for experimental results obtained by RCVD only for the short period of time during which the reaction layer is thin and discontinuous. For thick, dense and continuous reaction layers, the deposition diagram is no longer appropriate and reaction-diffusion models developed for solid-state diffusion couples have to be used in place of it.

International audience; Mechanical properties of Ti/Al-7Si assemblies produced by insert moulding have been studied with a classical push-out test and a variant that is the circular bending test. Special care has been taken for controlling both the reactivity at theTi/Al-7Si interface and the metallurgical health of the Al-7Si matrix. Mechanical tests until complete debonding have been completed with interrupted tests, metallographic characterizations and FEM analysis of elastic stress state. A mean shear strength of the interface of about 120 MPa has been obtained. When the Ti insert is solely fretted in the matrix, without chemical interaction between Ti and the Al-7Si alloy, the mean shear strength is significantly lower (48 MPa). This result clearly shows that chemical interaction at the interface (formation of a thin TiSi layer at the Ti side and a thick Al3Ti(Si) layer at the Al-7Si alloy side) improves the mechanical properties of the assembly. It is also shown that the failure sequence is characterized both by crack propagation from bottom to top and matrix yielding from top to bottom. Actually, interface damaging begins by crack initiation at the specimen bottom face (not at the top face, under the indenter) in a nearly pure mode I solicitation at a radial tensile stress of about 100 MPa.

The aim of this work is to elucidate the mechanism involved in the 3C–SiC formation during growth by a vapor–liquid–solid mechanism on 6H–SiC substrate. Polytype selection is shown to occur at the first stage of the experiments, before propane injection into the reactor. The contact of the seed with a Si–Ge melt during the initial heating ramp causes the formation of 3C–SiC islands on the seed surface, probably below 1200 °C. The proposed mechanism first involves a partial dissolution of the seed in a Ge-rich liquid which becomes C-supersaturated. Then the Si content of the liquid rapidly increases, which provokes the precipitation of the dissolved carbon in the form of 3C–SiC islands. When growth starts upon propane injection, these islands enlarge and coalesce to form a continuous 3C–SiC layer. If the growth temperature is too high (⩾1550 °C), the initial 3C–SiC islands are dissolved and homoepitaxial layers are obtained.

The functionality of structural light alloy castings can be improved by inserting into them, upon moulding, local iron base reinforcements. To acquire a better knowledge of such bimetallic assemblies, samples were prepared by immersing a mild steel bar (5 mm in diameter) in aluminium base Al-Si alloy melts held at 730 °C. After melt solidification, the bimetallic samples were cut into 5 mm thick slices and pushout testing was performed on these slices. Characterization of damaging corresponding to different load level before complete debonding allows the determination of the failure mode. Crack initiation occurs at the specimen bottom face in the intermetallic reaction layer, important damage occurs before complete debonding and no brittle failure is observed. The results highlight the necessity of analysing pushout tests with a more integrated approach taking into account shear stress distribution along the interface such as interfacial crack growth.

Wetting of vitreous carbon Cv by NiSi alloys is studied in high vacuum using the sessile drop and dispensed drop techniques. The role of reactions between NiSi and Cv (simple dissolution or SiC formation) on wetting, adhesion and mechanical behaviour of the interface is determined and discussed. The experimental results on reactive wetting are compared with the predictions of two different approaches proposed recently in order to explain the thermodynamics and kinetics of this type of wetting.

Chemical interaction of Ti3SiC2 substrate with silver–copper (Ag–Cu) melt is studied after several minutes of contact at 800 °C and 900 °C. Experimental evidence of Cu insertion into the crystallographic structure of Ti3SiC2 is given both by X-ray energy dispersive spectroscopy (composition is about Ti47Cu8Si13C32) and by electronic diffraction patterns obtained by transmission electronic microscopy for single grains. Moreover, the X-ray diffraction pattern shows that insertion of Cu induces a slight increase in the lattice parameters and a modification of peak intensities.

(SiC/Ti 3 SiC 2 ) n multilayered coatings composed of n layers alternately of continuous SiC and Ti 3 SiC 2 sub-layers were prepared on carbon substrates. The method consists in depositing SiC by classical chemical vapor deposition (CVD) and then producing a new Ti-containing sub-layer from a reaction between SiC and a H 2 /TiCl 4 gaseous mixture by reactive CVD (RCVD), and repeating this sequence n times. In accordance with thermodynamics, this RCVD method produces only pure TiC sub-layers when a low dilution ratio of TiCl 4 in H 2 ( R = [H 2 ]/[TiCl 4 ]) is used. With a high R value, the growth of pure Ti 3 SiC 2 thin continuous sub-layers from solid–gas reaction is observed for short deposition times. But for long deposition times, the simultaneous growth of additional Ti 5 Si 3 C x and TiC sub-layers occurs from solid state diffusion when the initial Ti 3 SiC 2 sub-layer thickness exceeds about 1 μm. In that case, the repeating of the sequence results in the further conversion of the additional phases into new Ti 3 SiC 2 sub-layers from solid state reactions involving TiSi 2 as an intermediate phase and SiC.

The wettability of carbon materials by molten silicon was investigated at 1430∘C under vacuum by using the dispensed drop variant of the sessile drop technique. The results are compared with data in the literature and used to contribute to a comprehensive understanding of wetting in the liquid-Si/solid-C system. Consequences on the dynamics of Liquid Silicon Infiltration (LSI) processes are discussed.

In reactive metal/ceramic systems the wetting rate of small, millimeter sized droplets on smooth ceramic surfaces is controlled by the slower of two successive phenomena that intervene in the reaction process: diffusive transport of reacting species to or from the triple line, and local reaction kinetics at the triple line. The first case, of diffusive wetting, was modelled by Mortensen et al. [Scripta mater. 36 (1997) 645] a few years ago. The purpose of this paper is to present progress accomplished during the last few years in modelling the second type of wetting controlled by the reaction process at or close to the triple line. The predictions of equations derived from a model describing the change in contact angle and spreading rate with time are compared with experimental results obtained for different silicon alloys on carbon substrates.

In reactive wetting, considerable acceleration of spreading rate is usually observed in the high contact angle (short spreading time) range in comparison to the low contact angle range. This study examines the possibility to explain this acceleration by an evaporation–condensation process which may occur under high vacuum in front of the triple line. For this purpose, a method is developed to evaluate the thickness of the reaction product in front of the triple line during spreading. This method is based on calculations of ballistic exchange in vacuum between the drop and the substrate surfaces combined with Auger spectroscopic analysis of the substrate surface. The method is applied to Cu–40 at% Si and Cu–1.5 at% Cr droplets on vitreous carbon systems.

International audience; The mechanisms of reaction-limited spreading of molten copper and nickel silicides on carbon substrates are studied by the dispensed drop variant of the sessile drop technique under high vacuum. Both std. wetting expts. and expts. which sep. the effect of time and instantaneous contact angle on spreading rate are carried out. Equations are derived for the time-dependent variation of contact angle and spreading rate.

The wetting behaviour of a Cu–40 at.% Si alloy, non-reactive with SiC, on oxidized (0001) faces of α-SiC single crystals is studied at temperatures close to the melting point of copper by the “dispensed drop” technique under high vacuum. The experiments focused on wetting kinetics to determine the mechanisms controlling the rate of spreading under conditions allowing for “in situ” deoxidation of silicon carbide. It is shown that spreading occurs in three stages: (i) an initial stage consisting of a rapid spreading of the alloy on the oxidized SiC up to a contact angle which is close to that on vitreous silica, (ii) a second stage with a zero spreading rate during which the alloy goes through the oxide layer by dissolution near the triple line allowing an intimate contact between the alloy and the SiC surface to be established, and (iii) a final stage during which the alloy spreads with a constant rate up to a contact angle equal to that of clean SiC. These results are interpreted on the basis of a dissolution–diffusion–evaporation process occurring in the vicinity of the solid–liquid–vapour triple line.

The nature of liquid-solid phase equilibria in the Al-rich corner of the Al-Si-Ti system are determined by drawing three isothermal sections at 620, 680 and 727 °C. The solubility of Ti in Al-Si liquids is determined for four different compositions (0, 9, 13 and 18 at.%Si) at temperature below 800 °C. Combination of the two sets of experimental results leads to an attempt of liquidus projection. The primary crystallization surface of Al3Ti is found to extend up to 9.5 at.%Si in the liquid phase at 620 °C and 11 at.%Si at 727 °C. The solubility of Ti is found to be not significantly dependent on the Si content of the liquid. From DSC measurements and deduction on microstructure, the last invariant reaction of the solidification path is found to be quasi-peritectic: $${\text{L}} + \uptau_{1} - {\text{Ti}}_{7} {\text{Al}}_{5} {\text{Si}}_{12} \Leftrightarrow {\text{Al}} + {\text{Si}} .$$

International audience; The solubility of group IV transition metals Ti, Zr, and Hf in liquid Al was measured by the settling technique coupled with ICP-AES analysis after dissolution in hydrochloric acid. The kinetic aspect of the settling technique was studied in order to show that, after a cooling step, thermodynamic equilibrium between the liquid and solid phases could be achieved after only 1 h. Finally, it was verified that solubility values obtained after a cooling or a heating step were fully consistent. The present results demonstrate that the immersion-and-settling technique allows reliable solubility values to be determined. The results confirm the values previously reported in the literature and the good description for the three binaries by the existing CALPHAD optimizations concerning the liquidus below 800 C.

International audience

International audience; Ti +15%TiCp composites were prepared by three different powder metallurgy techniques: free sintering, HIPing and direct powder extrusion. The microstructures and textures were characterized by EBSD while the Ti/TiC reaction, which takes place during heat treatment, was characterized using X-ray diffraction and Rietveld analysis. The mechanical properties were obtained by the mean of tensile tests. TiC particles effectively reinforce the strength of the Ti matrix by at least 40% indicating the Ti/TiC reaction did not deteriorate the mechanical properties. The correlation between the microstructural features and the mechanical properties is discussed and the processes are compared. Direct powder extrusion was shown to be the most suitable process to obtain ductile composites (epsilon = 4.6%).

International audience; The phase transformation kinetics on cooling and resulting microstructures of steel-based matrix composites (MMC) reinforced with TiC particles by powder metallurgy were studied. In addition, the phase transformation kinetics of the MMC were compared to those of the same steel without TiC and consolidated in the same conditions. The presence of TiC particles strongly favors the diffusive transformations in the steel matrix of the MMC. Different complementary techniques (XRD, SEM, TEM/EDX, atom probe tomography, in situ synchrotron XRD) were performed to analyze the chemical reactivity between TiC particles and the steel powders occurring during consolidation process and further heat treatments. Composition changes in the TiC as well as in the matrix were characterized. The chemical composition after treatment in the TiC particles tends toward the thermodynamic calculations with ThermoCalc. The effect of changes in chemical composition and the role of TiC particles acting as new favorable nucleation sites are discussed in regards to the obtained results.

The Ag–Cu–Ti system is important for brazing applications, particularly for ceramic joining. This system is characterized by numerous intermetallics in the Cu–Ti binary and the existence of a miscibility gap in the liquid phase. For applications, knowledge of the phase equilibria, invariant reactions in the temperature range of interest and thermodynamic activity values (mainly of Ti) are important. Thermodynamic model parameters for all the stable phases in the Ag–Cu, Cu–Ti and Ag–Ti systems, previously obtained using the Calphad method and available in the literature are used. A new thermodynamic description for the ternary interaction parameter of the liquid is obtained from experimental informations. Ti2Cu and Ti2Ag which have the same crystallographic structure were modelled as a single phase. The same was done for TiCu and TiAg. Finally, solid solubility of Ag in the Ti–Cu intermetallics is taken into account. The parameters obtained in this assessment are later used for the calculation of selected sections that can be useful for research and applications in the field of joining with Ti-activated Ag–Cu braze.

cited By 12; International audience; A previous paper reported on the mechanical behaviour of insert-moulded Ti/Al-7Si bimetallic test pieces as studied by a classical push-out test as well as a variant: the circular bending test. When a chemical bond was formed at the Ti/Al-7Si interface, promising results were obtained in terms of joint strength and damage mechanism (Dezellus et al., 2008 [1]). As a continuation, the aim of the present work was to examine the influence on this mechanical behaviour of a T6 heat treatment (re-heating for 10. h at 540 °C, quenching in cold water and ageing for 6. h at 170 °C) applied to the as-moulded Ti/Al-7Si test pieces. For that purpose, push-out and circular bending tests were performed on heat-treated samples, and the results were correlated with a characterization of the morphology, the constitution and composition of both transverse sections through the metal/metal reaction zone and fracture surfaces, as revealed after removal of the Ti insert. As expected, applying the T6 heat treatment to chemically bonded Ti/Al-7Si bimetallic assemblies resulted in an improvement of the mechanical properties of the Al-7Si matrix itself. Moreover, a significant increase of the load level for the onset of joint damage in push-out mode was observed. Concerning the damage mechanism, the presence of angular Si particles in the eutectic region of the Al-7Si matrix near the interface had a weakening effect. After T6 solution heat-treatment, the shape of the Si particles changed from angular to globular. Moreover, due to the formation of Si-rich compounds at the Al-7Si/Ti interface, Si diffuses from the alloy towards the Ti rod and the size and number of Si particles became significantly decreased near the insert/alloy interface. These two features explained the favourable influence of the T6 heat-treatment on the mechanical properties of the Ti/Al-7Si assemblies. © 2010 Elsevier B.V.

International audience; The aim of this work was to characterize the chemical changes during solid state solution heat treatment of a metallurgically bonded steel/Al-Si interface. For this purpose, low carbon steel plates covered with the A-S7G03 aluminium alloy (7wt%Si, 0.3wt%Mg analogous to A356) were prepared by dip coating, water-quenching to room temperature and reheating in the solid state at 480-560°C for 3 to 160 hours. Upon reheating at 535 °C, a reaction layer was observed to grow at the interface between steel and the iron-saturated Al-Si alloy. As long as an intimate contact could be maintained, the total thickness, x, of the reaction layer increased with time, t, according to a nearly parabolic growth law x2 = K.t-b. At 535°C, the value of the growth constant was K = 4.045 x 10-14 m2s-1. This constant was found to be thermally activated [K = K0exp (-Q/RT)] with K0 = 4.37 x 10-4m2s-1 and Q =153kJ.mole-1. The whole chemical interaction process was controlled by solid state volume diffusion and the reaction layer sequence corresponded to a diffusion path in the Al-Fe-Si phase diagram. A striking feature of the reaction process is the unbalanced diffusion of aluminium atoms through the reaction zone which rapidly results in the formation of Kirkendall voids. As these voids coalesce, solid state diffusion becomes more and more difficult and the steel/alloy bond gets weakened. Oxidation appears to be an aggravating factor, where applicable.

International audience; The influence of nitrogen impurity on the stabilization of 3C-SiC polytype has been studied during vapour-liquid-solid (VLS) growth on 6H-SiC(0001) seed with Si-Ge melt. By changing the partial pressure of N2 during growth, it was found that the proportion of 3C-SiC inside the grown material increases with N2 partial pressure. 6H inclusions are only found for high purity (low N2 content) conditions. The possible interactions proposed to explain this effect are divided in two effects: i) lattice parameter modification and ii) surface induced lateral enlargement variation. A combination of both effects is suspected.

International audience; Many pure metals and alloys do not wet covalent or ionocovalent ceramics and adhere weakly on them .This is the case for the low or moderate melting point metals Pb,Sn,In ,Ag,Au,Cu on the various forms of carbon(graphite ,diamond, vitreous carbon ..), on stable oxides (alumina, silica ,magnesia...) as well as on silicon and boron nitrides. A strong improvement in wetting can be obtained by certain alloying elements able to modify in situ the metal/ceramic interface by reaction with the ceramic .Reactivity is probably the major specific characteristic of wetting at high temperatures. The presentation will focus on the progress made during the last five years in the understanding of driving force and spreading kinetics of reactive wetting. A special attention will be paid to discuss the contribution on reactive wetting of the two following processes, adsorption at the initial metal/ceramic interface and formation of a 3D compound at the interface. The presentation will focus on the progress made during the last five years in the understanding of driving force and spreading kinetics of reactive wetting. A special attention will be paid to discuss the contribution of adsorption at the initial metal/ceramic interface and formation of a 3D interfacial compound.

International audience; Reaction zones formed at 790 °C between solid titanium and liquid Ag-Cu eutectic alloys (pure and Ti-saturated) have been characterized. When pure Ag-Cu eutectic alloy with 40 at.% Cu is used, the interface reaction layer sequence is: alpha-Ti / Ti2Cu / TiCu / Ti3Cu4 / TiCu4 / L. Because of the fast dissolution rate of Ti in the alloy, the reaction zone remains very thin (3-6 µm) whatever the reaction time. When the Ag-Cu eutectic alloy is saturated in titanium, dissolution no longer proceeds and a thicker reaction zone with a more complex layer sequence grows as the reaction time increases. Four elementary chemical interaction processes have been identified in addition to Ti dissolution in the liquid alloy. These are growth of reaction layers on Ti by solid state diffusion, nucleation and growth from the liquid of TiCu4, isothermal solidification of silver and, finally, chemical conversion of the Cu-Ti compounds by reaction-diffusion in the solid state. A mechanism combining these processes is proposed to account for the constitution of Ti/ Ag-Cu/ Ti joints brazed at 780-800 °C.

International audience; Titanium has been successfully joined to aluminium nitride AlN at a temperature as low as 795 ◦C, using Ag–Cu Cusil® commercial braze alloy. While reactive wetting and spreading proceeds at the AlN/braze alloy interface, chemical interactions develop at the titanium side rendering possible isothermal solidification of the joint. The determining factor in the solidification process is the fast formation of TiCu4 crystals by heterogeneous nucleation and growth in the liquid phase. As a consequence, the braze alloy is depleted in Cu and solid Ag precipitates. After annealing, the re-melting temperature of the resulting joint can be increased up to about 910 ◦C which is nearly 130 ◦C higher than the melting point of the starting braze alloy.

The functionality of structural light alloy castings can be considerably improved by inserting into them upon moulding iron base local reinforcements. A necessary condition to obtain tight joints with good load transfer is that a sound metallurgical bond is formed at the insert/alloy interface. However, many physical–chemical factors play an important part in the mechanical behaviour of the joint. To acquire a better insight into this issue, bimetallic samples were prepared by immersing a mild steel bar (5 mm in diameter) in magnesium base Mg–Al alloy melts held at 650–750 °C in a graphite crucible (mean diameter 19 mm). Different processing conditions were used to vary the degree of chemical interaction at the insert/alloy interface. After melt solidification, the bimetallic samples were cut into slices (2–10 mm thick) and push out testing was performed on these slices. Attempts have been made to relate the main features of the load–displacement curves thus obtained with the nature and extent of the transition zones formed at the insert/alloy interface.

Using the “dispensed drop” variant of the sessile drop technique, spreading kinetics of dilute Cu–Cr alloys on smooth vitreous carbon substrates are measured under helium microleak conditions. In this system, it is known that the drop spreading rate is controlled by diffusion of the reactive atom species (Cr) from the bulk liquid to the triple line, where wetting is induced by formation of an interfacial layer of chromium carbide. Microstructural characterization of rapidly cooled drops shows that growth of the interfacial reaction product layer continues behind the moving solid–liquid–vapor triple line. The spreading velocity is modeled by finite-difference numerical analysis of diffusion near the triple line in the presence of continued interfacial reaction, simplifying the growth rate as being constant and using realistic parameter values. We show that continued interfacial reaction explains the dependence of the triple line spreading rate on the instantaneous wetting angle that is observed in this system.

International audience; Sessile drop expts. using pure Cu and Au on different C substrates under high vacuum showed that these metals wet monocryst. graphite better than vitreous C with the corresponding differences in the work of adhesion being 50-60%. These results were interpretted satisfactorily by assuming that the predominant interactions at the surface are van der Waals interactions due to dispersion forces. For nonreactive metal/C systems, wetting and adhesion of a metal is expected to vary only slightly around its m.p. This is also expected to be true for any metal/ceramic system in which adhesion is ensured mainly by van der Waals interactions.

Sessile drop experiments on metal/ceramic systems have shown that an important improvement in wetting may be produced by introduction of alloying elements which form, by reaction at the metal/ceramic interface, a continuous layer of solid that is better wetted by the metal than the original substrate. We present results from a finite-difference analysis of the kinetics of reactive wetting under conditions where the rate of spreading is controlled by the rate of diffusion of the reactive solute element.