Microstructure formation of Al₂O₃-HfO₂ eutectic

The microstructure formation of Al₂O₃-HfO₂ eutectic prepared by unidirectional solidification and rapid solidification was examined. The microstructure of the Al₂O₃-HfO₂ eutectic samples showed lamellar structure and the relationship between the inter-lamellar spacing λ and solidification rate R can be expressed by λ²R=constant. A small amount of tetragonal HfO₂ phase exists in the sample prepared by rapid solidification. Only monoclinic HfO₂ phase can be found in the eutectic samples prepared by unidirectional solidification. A tensile stress was induced in Al₂O₃ phase during the cooling step due to the transformation of HfO₂ from tetragonal to monoclinic phase. The tensile stress of the Al₂O₃ phase in the eutectic structure was estimated to 420 MPa.