Application of an optical DTA for morphological transition in the Al2O3-YAP-ZrO2 metastable eutectic system

Abstract

This paper presents an optical DTA apparatus which is capable to examine the solidification behavior of the Al2O3-based materials. A direct current through the two Mo crucibles effectively heats specimen and reference up to 2400 K. The small heat capacity of the crucible and the specimen enabled a cooling rate up to the order of 102 K/s. Since accuracy of temperature measurement by two two-color pyrometers was not affected by emissivity change to phase transformation, exothermic and endothermic heats in the specimen was efficiently detected at high temperatures. The developed apparatus was used to examine DTA for α-Al2O3 and the Al2O3-YAP-ZrO2 metastable eutectic system. The morphological transition from the entangled eutectic structure to the lamellar eutectic structure was observed at a cooling rate around 10 K/s. At a cooling rate more than 70 K/s, α-Al2O3 phase started to solidify as a primary phase. Furthermore, the divorced eutectic solidification was recognized at a cooling rate of 360 K/s.