A Study of High Temperature Reactions in Oxide-Dispersion-Strengthened Molybdenum at Reduced Oxygen Partial Pressures
Mohammed, Jelila Sarah
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Rare-earth oxides used in oxide dispersion strengthening are known to provide excellent strength and deformability over ordinary dispersion strengthening. It has been suggested that this may be due to the development of molybdate compounds instead of pure dispersed oxide particles. These alloys are produced by dispersing particles of certain rare-earth oxides in a molybdenum matrix and forming the mixture into a composite ingot. During the high-temperature consolidation process, the oxides are converted into rare-earth molybdates. With subsequent processing, these molybdate phases undergo deformation to form high-surface-area ribbons that serve to inhibit dislocation movement, thus improving the mechanical properties of the molybdenum matrix. It is still unknown what specific compounds, phases, and crystal structures provide these metal-oxides with their high strength and deformability. Because the molybdate phases are formed at high temperatures and low oxygen partial pressures, little is also known of the high-temperature phase equilibria of the REO-Mo systems under these conditions. The primary goal of this study was to deifine phase equilibria on systems of Mo with rare-earth oxides. The project aimed to identify compounds, phases, and specific oxidation states of molybdenum at various processing conditions. Systems of LaO1.5-MoOx, YO1.5-MoOx, and ZrO2- MoOx were investigated at temperatures of 1000??nd 1200??and O2 partial pressures ranging from 10-4 Pa to 10-13 Pa. Samples were prepared using powder starting materials of Mo and rare-earth oxides were combined in stoichiometric ratios. The samples were then electrically heated in a ceramic tube furnace in which the oxygen partial pressure was controlled by means of a combined flow of H2 and CO2 gas. Characterization was performed using x-ray diffraction, with published powder diffraction files for phase identification.