IN-SITU FORMATION OF METAL-CERAMIC MICROSTRUCTURES BY PARTIAL REDUCTION REACTIONS

The formation of metal-ceramic multi-phase microstructures by partial reduction of ternary or higher ceramic oxides has been experimentally investigated in the model system Fe-Mn-O and in the more practical system Al-Ni-O by changing the oxygen partial pressure at constant temperature and total pressure. In the Fe-Mn-O system the addition of impurities such as Al2O3, CaO, Cr2O3 and ZrO2 to the initial, polycrystalline oxide solid solution (Fe1-xMnx)O strongly influences the location of the metal precipitation during reduction. The experimental observations are discussed based on solubility limits and segregation of impurities. In the Al-Ni-O system, depending on the reduction temperature, two different morphologies of nearly pure Ni particles, equiaxed and rod-like, form within an oxide matrix. Equiaxed Ni particles (0.02 to 0.5 mu m in diameter) embedded in alpha-Al2O3 were formed at 1350 degrees C, while rod-like Ni particles (similar to 5 mu m in length and 0.1 mu m in diameter) in a metastable ''defect spinel'' phase, containing much less Ni than NiAl2O4, were formed at 1100 degrees C. Electron microscopy studies were performed for microstructural characterization, phase identification and chemical analysis.