Abstract
The microstructures of niobium‐based alumina composites prepared by pressureless sintering of compacts of attrition milled Al2O3, Nb, and Al powder mixtures were studied. The addition of a small amount of Al is assumed to assist in rapid sintering. X‐ray diffraction analyses show that Al2O3, Nb, NbO, and the intermetallics AlNb2 and AlNb3 are present in the composites. Electron microscopy studies confirm the existence of these phases and reveal dense, fine‐grained (≤500 nm) composites. Al2O3 and Nb grains form the matrix. NbO occurs as grains and additionally as small particles within Al2O3 grains and at Al2O3/Al2O3 grain boundaries. The intermetallic AlNb2 and AlNb3 phases do not exceed 300 nm in size if they occur at grain boundaries, and possess even smaller dimensions when occluded within Al2O3 grains or located at Al2O3 triple junctions. While the niobium intermetallics are expected to form during the heating cycle before reaching the sintering temperature, the NbO is assumed to form during the cooling cycle due to precipitation of oxygen dissolved in the niobium.
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