Al 2 O 3 -ZrO 2 composites exhibit excellent mechanical and high-temperature properties. The solidification of various hypoeutectic compositions has been studied by means of aero-acoustic levitation. A high-speed camera recorded the crystallization, to the correlation of the video stills with the observed microstructures. Solidification takes place by formation of several nuclei and subsequent growth. Nuclei are formed in the supercooled melt, entailing to a fine-grained, simultaneously solidified structure. The remaining melt between the growing nuclei is heated due to recalescence leading to primary precipitation of zirconia, followed by eutectic solidification. A consistent behavior is presented to explain the observed microstructures. Additionally, samples between 40 and 50 mol% ZrO 2 exhibit lamellar areas, which exceed the initial zirconia composition. The observed microstructure strongly indicates the existence of a liquid miscibility gap.

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