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 Al 2 O 3 -based materials. A direct current through the two Mo crucibles effectively heats specimen and reference up to 2 400 K. The small heat capacity of the crucible and the specimen enabled a cooling rate up to the order of 10 2 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 aAl 2 O 3 and the Al 2 O 3 -YAP-ZrO 2 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, a-Al 2 O 3 phase started to solidify as a primary phase. Furthermore, the divorced eutectic solidification was recognized at a cooling rate of 360 K/s. KEY WORDS: differential thermal analysis (DTA); solidification; ceramics; phase transformation; morphological transition. techniques.Recently, eutectic ceramic composites produced by unidirectional solidification have been taken into attention as a candidate for high temperature materials, [11][12][13][14][15][16] since the Al 2 O 3 -YAG (Y 3 Al 5 O 12 , yttrium-aluminum-garnet) eutectic structure that consists of Al 2 O 3 single crystal and YAG single crystal exhibits desirable for high temperature materials. Besides the high temperature properties, the Al 2 O 3 -Y 2 O 3 system has unique properties in solidification. According to a phase diagram reported by Caslavsky and Viechnicki,17) there are two eutectic reactions in the Al 2 O 3 -rich portion. One is the equilibrium eutectic reaction of Al 2 O 3 -YAG at 2 099 K and the other is the metastable eutectic reaction of Al 2 O 3 -YAP (YAlO 3 , yttrium-aluminumperovskite) at 1 975 K. Solidification in the metastable eutectic path normally occurs when the melt is once heated up to temperatures above 2 273 K. [17][18][19] Heating the melt above 2 273 K seemed to inhibit YAG nucleation in the liquid. The experimental results suggested the existence of an irreversible change at temperatures above melting point. However, there is still ambiguity regarding the irreversibility of the change because of the difficulties of thermodynamic measurement. Thus, it is expected that the DTA is used for understanding the unique solidification behavior. However, there was no conventional apparatus to measure the heat release from the specimen at the high temperatures up to 2 400 K. The requirement for the thermal analysis at the high temperatures motivated to develop a DTA technique.In addition, it has been found that heating the metastable eutectic up to temperatures above the metastable eutectic temperature but the equilibrium eutectic temperature produced the undercooled melt for the Al 2 O 3 -Y 2 O 3 system 20) and the Al 2 O 3 -Y 2 O 3 -ZrO 2 system.21) The undercool...

Internal Defects of Continuous Casting Slabs Caused by Asymmetric Unbalanced Steel Flow in Mold

“…[1][2][3] Theoretical models not to take macro steel flow into account, have also been published, 4) and a new research has demonstrated that the thermal conductivity of particles is important for entrapment probability. 5,6) Al 2 O 3 particles are usually observed on the surface of bubbles in cast steel, where they seem to coagulate. This suggests that the presence of bubbles accelerates coagulation and thus encourages the formation of larger Al 2 O 3 clusters.…”

Thermal Analysis

“…The utility of the method is illustrated by analysis of heat treatment of thin colloidally processed xerogel plates. Yasuda et al (10) present an optical DTA apparatus capable of examining the solidification behavior of the Al 2 O 3 -based materials. A direct current through the two Mo crucibles effectively heats specimen and reference up to 2400 K. Two-color pyrometers are used for evaluating the temperature changes.…”