METASTABLE ALUMINA STRUCTURES IN MELT-EXTRACTED ALUMINA-25 WT-PERCENT ZIRCONIA AND ALUMINA-42 WT-PERCENT ZIRCONIA CERAMICS

Microstructures are examined of rapidly solidified hypoeutectic Al2O3-25 wt% ZrO2 and eutectic Al2O3-42 wt% ZrO2 ceramic alloys by using transmission electron microscopy and scanning transmission electron microscopy. Structures observed in the hypoeutectic alloy were dendritic. Three different types of dendrite morphologies were observed. These are believed to be the stable alpha- and metastable gamma- and delta- modifications of alumina. The cores of the gamma-alumina dendrites were somewhat richer in ZrO2 than those of alpha-alumina dendrites; delta-alumina dendrites were substantially enriched in ZrO2. The lamellar structure of the eutectic in the Al2O3-42 wt% ZrO2 alloy became increasingly finer with increasing cooling rate and at the highest cooling rates was replaced by a fully amorphous structure (except in some instances a ZrO2-rich needlelike phase identifiable as delta-alumina was found in the amorphous matrix). An interpretation is given of the results obtained, based on assumed metastable free energy curves and dendrite growth theory.