There are two eutectic systems in the Al 2 O 3 -rich portion of the Al 2 O 3 -Y 2 O 3 system: one is the Al 2 O 3 -YAG equilibrium eutectic system and the other is the Al 2 O 3 -YAP metastable eutectic system. Heating the Al 2 O 3 -YAP metastable eutectic structure up to temperatures above the metastable eutectic temperature but below the equilibrium eutectic temperature produced the undercooled melt. Solidification in the equilibrium path immediately followed the undercooled melt formation. The solidification in the equilibrium path along with the melting of the metastable eutectic structure resulted in a fine and uniform equilibrium eutectic structure with lamellar spacing of less than 1 mm. The equilibrium eutectic structure was not affected by the metastable eutectic structure used for the undercooled melt formation. Shape casting and joining of alumina rods were demonstrated using the undercooled melt produced by the melting of the Al 2 O 3 -YAP metastable eutectic structure. The fine eutectic structure was obtained throughout the castings. Coupling of the solidification and the melting achieved a higher growth rate than conventional solidification from the undercooled melt. The skewed coupled growth zone due to the kinetic effect contributed to formation of the fine eutectic structure at the metastable eutectic composition. q

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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...

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When the Al 2 O 3 -YAP metastable eutectic system was heated above the metastable eutectic temperature but below the equilibrium eutectic temperature, undercooled melt was formed. Solidification in the equilibrium path accompanied the melting of the metastable eutectic system, resulting in a fine and uniform eutectic structure. A novel solidification process using undercooled melt formation was developed. In this paper, the influences of the initial Al 2 O 3 -YAP eutectic particle size, forming pressure and holding time at the maximum temperature on the microstructure and/or the mechanical property of the Al 2 O 3 -YAG compact were examined to optimize the forming conditions. The Al 2 O 3 -YAG compact produced from the Al 2 O 3 -YAP eutectic particles with diameters less than 20 mm exhibited a higher flexural strength and the smaller porosity as compared to that produced from Al 2 O 3 -YAP eutectic particles with diameters ranging from 20 mm to 45 mm. The volume fraction of the porosity was approximately 4% under a forming pressure of 10 MPa and a holding time of 60 s. The flexural strength of the Al 2 O 3 -YAG compact decreased with the increase in the holding time due to the increase in the lamellar spacing. For Al 2 O 3 -YAP eutectic particles with diameters less than 20 mm, a low forming pressure of 10 MPa and a short holding time of 60 s were sufficient to synthesize a dense Al 2 O 3 -YAG compact. Thus, the solidification technique that uses undercooled melt produced by melting the Al 2 O 3 -YAP metastable eutectic system has good advantages for shaping.