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The morphology of the crystalline phases prepared at different cooling rates, temperatures, and compositions of melts in the ZrO 2 -Al 2 O 3 system is investigated. It is established that both the quenched and slowly crystallized samples containing 35-70 wt % ZrO 2 have a submicron structure. Outside this concentration range, the ingots have a zonal structure: the peripheral region is formed by large baddeleyite crystals (at a ZrO 2 content higher than 70 wt %) or corundum crystals (at a ZrO 2 content lower than 30 wt %). This character of the crystallization confirms the presence of the phase separation (liquid immiscibility) region in melts of this system. A variant is proposed for the phase diagram of the system under investigation.

The morphology of the quenched and slowly crystallized samples in the ZrO 2 -Al 2 O 3 system is investigated in the composition range 25-70 wt % ZrO 2 . It is revealed that, irrespective of the cooling rate, the samples contain large baddeleyite (or corundum) crystals, eutectic mixtures, and characteristic regions of intergrown elongated baddeleyite and corundum grains with micron sizes. These regions have the same phase composition at any initial ratio between zirconium and aluminum oxides and at any cooling rates of the melt. A hypothesis is put forward that these regions are products of the decomposition of ZrO 2 · 2Al 2 O 3 associates.

Abstr act. Reasons and mechanism of cordierite-mullite refractory materials destruction in the ovens for glazing ceramic wares baking have been examined. The change of elemental and phase compositions of refractory surface layers under the influence of cyclic action of water vapour and volatile components of glazing coating has been established. The ways of increasing cordierite-mullite refractory materials longevity have been proposed.

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