[en] Unidirectionally solidified composites of a rare-earth sesquioxide + CeO₂ + (Mo or W) and of CeO₂ + (Mo or W) were grown using the direct-rf-coupling internal-molten-zone technique. The dramatic fiber improvement on addition of CeO₂ to the sesquioxide systems suggests that it provides extra loosely bound oxygen to increase the solubility of the metal in the molten oxide. The composites have extensive areas of ordered eutectic growth containing from 8 to 50 x 10⁶ metal fibers/cm²; the fibers are 0.1 to 0.6 μm in diameter. Fiber density increased almost linearly as the growth rate was increased from 0.5 to approximately 5.0 cm/h. Slow growth favors the platelet metal morphology over the fiber morphology. Platelets are typically observed in areas where high impurity concentrations would be expected, e.g., at the top of the solidified zone and at grain boundaries. (U.S.)