J. Roger

Synthesis of Ti matrix composites reinforced with TiC particles: thermodynamic equilibrium and change in microstructure

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 1xa0h. Finally Ostwald ripening is responsible for the growth of particles for longer heat treatment times.

Tin flux synthesis of rare-earth metal silicide compounds RESi1.7 (RE = Dy, Ho) : a novel ordered structure derived from the AlB2 type

Abstract The binary rare-earth metal silicides RESi1.7 (RE = Dy, Ho) were prepared by tin flux synthesis between 800–1000 °C. Single crystals were obtained after electrochemical dissolving of molten ingots. Their crystal structures were determined from single-crystal X-ray data: orthorhombic symmetry, space group Imm2; unit cell parameters a = 19.144(1), b = 8.2562(3), c = 6.6571(3) Å and a = 19.063(1), b = 8.2120(2), c = 6.6313(2) Å for DySi1.7 and HoSi1.7, respectively. They represent a new ordered structure type, which is a derivative of the defect AlB2 type. The main structural feature is the occurrence of a vacancy ordering in the two-dimensional planar network of silicon atoms, leading to rows of edge-shared twelve-membered rings separated by distorted hexagonal rings. Structural comparison with the parent hexagonal silicide Yb3Si5 (Th3Pd5 type), the structure of which has been re-determined on single crystal, is also given.

Crystal structure of the ternary compoundγ-Al3FeSi

Abstract Ternary iron silicide aluminide γ-Al3FeSi crystals were grown from two Al—Fe—Si melts quenched in cold water. The crystal structures were determined from single-crystal X-ray data: trigonal symmetry, space group R-3 (n°148), unit cell parameters a = 10.2223(2) Å, c = 19.6791(4) Å (V = 1781 Å3) for the Si-poorer crystal and a = 10.1987(2) Å, c = 19.5320(3) Å (V = 1759 Å3) for the Si-richer one. The structure of γ-Al3FeSi may be described in terms of Al-cubes connected together by Al—Al pairs. The structure contains also Al cuboctaedra with one Fe—Al mixed atom in the center. The average chemical formula obtained from the refinements is Al3FeSi. This phase shows a partial disorder on the aluminium network because of the substitution of aluminum atoms by silicon. This substitution mechanism is at the origin of the large homogeneity range of this phase.

Identification of Complex Oxidation/Corrosion Behaviours of Boron Nitride Under High Temperature

To characterize the oxidation/corrosion behaviour of boron nitride (BN) at high temperature and to determine kinetics, thermo-gravimetric analyses were performed on BN flat coatings under dry (80% N2, 20% O2) and wet conditions (10% H2O, 70% N2, 20% O2), at temperatures ranging from 700 to 900xa0°C. The surface morphology was analysed by scanning electron microscopy. The aim of the study was to understand the complex behaviour of BN at high temperature under wet air. A model was then proposed to explain the recorded mass variations, by taking into account a competition between absorption and volatilization phenomena. Finally, an estimation of the amount of adsorbed species was extracted from these data.