F. Bosselet

Interface chemistry in aluminium alloy castings reinforced with iron base inserts

Abstract Bimetallic automotive components consisting of an Al–Si light alloy reinforced with a cast iron insert have been manufactured by gravity die moulding. Special precautions have been taken to ensure a uniform wetting of the insert by the liquid light alloy. Under these conditions, three intermetallic compounds are formed at the insert/alloy interface: η (Al 5 Fe 2 ), τ5 (Al 7.4 Fe 2 Si) and τ6 (Al 4.5 FeSi). Upon subsequent heat-treatment, τ2 (Al 5 Fe 2 Si 2 ) and τ10 (Al 12 Fe 5 Si 3 ) also appear. Growth of these compounds is discussed in terms of thermodynamics, kinetics and reaction mechanism in the Al–Fe and Al–Fe–Si systems. The effect of these chemical changes on the mechanical properties of the insert/alloy joint will be examined.

Mechanism and kinetics of the chemical interaction between liquid aluminium and silicon-carbide single crystals

Previous investigations of phase equilibria in the ternary system Al-C-Si have shown that silicon carbide is attacked by pure aluminium at temperatures higher or equal to 923±3 K and up to about 1600 K, according to the chemical reaction: 4Al+3SiC ↔ Al4C3+3Si In the present work, a study has been carried out to obtain more detailed information on the mechanism and kinetics of this reaction. For that purpose, 6H silicon carbide platelets with broad Si (0 0 0 1) and C (0 0 0 ¯1) faces were isothermally heated at 1000 K in a large excess of liquid aluminium. Characterization of the resulting samples by Auger electron spectroscopy (AES) and scanning electron microscopy (SEM) revealed that the reaction proceeds in both faces via a dissolution-precipitation mechanism. However, the polarity of the substrate surface strikingly influences the rate at which silicon carbide decomposes: dissolution starts much more rapidly on the Si face than on the C face, but, while a barrier layer of aluminium carbide is formed on the Si face protecting it against further attack, the major part of the C face remains directly exposed to liquid aluminium and thus may continue to dissolve at a low but constant rate up to complete decomposition of the α-SiC crystal.

Characterization of guest molecules adsorbed on zeolites of known structure. Part III-Localization of the p-XYLI and p-XYLII species sorbed in a high coverage B.ZSM-5p-XYLENE complex

By combining full profile X-ray powder diffraction structure refinements, extraction of the integrated intensities and difference-Fourier synthesis, it has been possible to characterize and localize the two XYL I and XYL II species of p-xylene organic guest molecules adsorbed in a high coverage B.ZSM-5p-xylene complex. The investigations have been performed on the calcined H1.40Al0.02B1.38Si94.60O192 boralite phase which has been saturated with liquid p-xylene at room temperature. Interpretation of a difference-Fourier map reveals the presence of fixed sites at the channel intersections and in the sinusoidal channels. These sites correspond respectively to the XYL I and XYL II species. Most surprisingly, the distribution of these two species in the zeolitic framework corresponds to relatively high xylene/xylene or xylene/framework interactions in the high-coverage B.ZSM-5nXYL complex (n upto ≈8 p-xylene molecules/unit-cell).

The MFI(zsm-5)/sorbate systems. Comparison between structural, theoretical and calorimetric results. Part II — The MFI/benzene system

Abstract Comparing experimental X-ray diffraction data and calorimetrically determined differential adsortion heats, with computer simulated atom-atom interactions in the MFI/benzene system by using the 6-exp (Buckingham) and 6–12 (Lennard-Jones) theoretical models, shows that at lower fillings the benzene molecules reside at the channel-intersections (4cl sites). At saturation (eight benzene/unit-cell) the sorbate molecules form infinite polymeric chains in the straight channels. The estimated diffusional activation energy for benzene in the straight channel is 22.8 kJ/mole. Reliable interaction energies might only be obtained if the starting structural data for the investigated topology/sorbate systems correspond to accurately interpreted experimental results.

Solid states solubility of aluminum in the δ-Ni2Si nickel silicide

Abstract Powder mixtures of Al, Ni and Si with compositions located on a line joining the nickel silicide δ-Ni2Si to the previously reported ternary compounds Ni5.4Si2Al1 were cold-pressed, flash melted by electron bombardment, annealed after 300 h at 1000°C under 1 atm of purified argon and characterized by optical metallography, electron microprobe analysis and X-ray diffraction. Analysis of 529 unique X-ray reflections collected on a crystals of pure δ-Ni2Si and Rietveld refinement of its powder diffraction spectrum entirely confirmed the structural data previously reported for this stoichiometric compound (space group Pbnm, Z=4) and allowed a precise refinement of its cell parameters: a=7.0664(4) A; b = 5.0088(3) A ; c = 3.7321(2) A . Moreover, it has been shown that the structure of δ-Ni2Si can accommodate Al atoms up to a content of 20.9 ± 0.5at.%, giving rise a ternary solid solutions with a wide homogeneity range. Full profile refinement of X-ray powder spectra has indicated that the formation of this solid solution proceeds mainly by substitution of Si atoms by Al atoms, while Ni vacancies may appear. For increasing amounts of Al atoms in the δ-Ni2Si framework, the cell parameters a, c and the cell volume icrease while parameter b decreases. On the basis of these results, the ternary compounds Ni5.4Si2Al1 (or Ni1.8Si0.67Al0.33) appears as a particular composition in the homogeneity range of a solid solutions that can be designated by the general formula δ-Ni2−xSi1−yAly.

Synthesis, and structure of T2-Al2MgC2

Abstract The T 1 and T 2 crystalline varieties of the ternary carbide Al 2 MgC 2 were synthesized by reacting graphite particles with Mg-Al melts at 930–1020 K. Below 1000 K, the T 1 variety was predominant but little amounts of T 2 were also present. At 1000 K and above, only the T 2 variety was obtained. Assuming a hexagonal close packing of the metal atoms, the crystal structure of T 2 -Al 2 MgC 2 was determined by Rietveld refinemenent from X-ray powder diffraction data.

Etude de la solution solide Pb2O(S1−xWxO4) et structure cristalline de la variete alpha du monooxodiplomb (II) tetraoxotungstate (VI) Pb2O(WO4) par diffraction X sur poudres

Abstract The Pb 2 O(S 1−x W x O 4 ) solid solution has been investigated by X-ray diffraction in the 0 3 mixtures at different temperatures, reveals the existence of two polymorphs: alpha-Pb 2 O(WO 4 ) is obtained for 823 2 O(WO 4 ) for T>1073K. For 833 2 O(WO 4 ) species is characterized for the first time, and crystal structure investigations by X-ray diffraction on polycristalline samples show that this phase is isostructural with its Pb 2 O(AO 4 ) homologues for A = S, Cr and Mo (1).

Chemical reactivity of iron base substrates with liquid Mg–Zr alloys

Abstract The chemical interaction between pure iron, mild steel or cast iron substrates and liquid magnesium containing 0.10–0.18 at% of zirconium has been investigated at 1000 K. As long as the Mg–Zr melt is pure and fully saturated in zirconium (0.18 at% Zr), a continuous reaction layer of zirconium carbide ZrC x is formed at the substrate surface. When active impurities such as Si or Mn are present in the Zr-saturated melt, thin segmented ZrC x layers with globular Fe 2 Zr crystals stuck to their outer side are found undulating in the liquid at 10–50 μm from the substrate surface. These observations are interpreted in terms of a competition between a direct interaction in which Fe–Zr compounds grow in the melt by dissolution-crystallization and an indirect reaction in which a protective ZrC x surface layer develops by solid state diffusion of carbon initially present in the bulk of the substrates.

Structural Changes of Binary/Ternary Spinel Oxides During Ethanol Anaerobic Decomposition

Several M‐modified iron oxides of the spinel family have proven to be effective electron and O2− vectors for the production of hydrogen in the chemical‐loop reforming of bio‐alcohols. The present work is specifically focused on investigation of ethanol anaerobic decomposition over spinel oxides, which results in significant structural changes of the oxygen carrier material itself and corresponds to a first step of the chemical‐loop reforming process. In particular, a series of binary/ternary M‐modified ferrospinels were prepared by a co‐precipitation method and tested in terms of both redox properties and intrinsic catalytic activity in addition to a complex ex situ study that encompasses the solid‐state chemistry investigations of the fresh and reduced oxygen carrier materials. It was found that Co/Cu incorporation facilitates total/partial oxidation of ethanol, giving rise to high yields of H2, COx, and H2O; whereas Mn incorporation predominantly favored dehydrogenation and condensation reactions, leading to the formation of acetaldehyde and acetone. In addition, the incorporation of Mn contributed to significantly reduce the amount of coke formed; however, it caused a lower intrinsic reducibility, which was explained by the formation of a thermodynamically stable and hardly reducible layer of MnxFeyO solid solution.