Papers by Author: Jean Claude Viala

Abstract: 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.

Abstract: In this work, the growth by Vapour-Liquid-Solid (VLS) mechanism of 3C-SiC on silicon substrate is reported. Firstly, a germanium layer is deposited on the substrate. Then the temperature of the sample is increased above Ge melting point in order to form a SiGe liquid phase by reaction with the substrate. Upon reaching the target temperature (1100-1300°C) the VLS growth starts with the injection of propane in the reactor. Both Raman spectrometry and X-Ray diffraction analyses evidenced the formation of 3C-SiC on every sample. However, this SiC deposit, a few micrometers thick, is always found to be polycrystalline though textured. In parallel, the presence of an epitaxial Si-Ge alloy, whose composition depends on the growth temperature, was systematically detected between Si and SiC.