Containerless solidification of highly undercooled mullite melts: Crystal growth behavior and microstructure formation
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INTRODUCTION
SOLIDIFICATION of highly undercooled melts is of great significance, provided that in-situ diagnosis of experimental details, including accurate nucleation temperature, the configuration of the solid/liquid (S/L) interface, advancement of the S/L interface, etc., are available, which are essential for a comprehensive understanding of solidification behavior from the viewpoints of kinetics and thermodynamics. Due to the poor electronic conductivity of oxides, electromagnetic levitation facilities are helpless in processing oxides. Fortunately, an aero-acoustic levitation (AAL) setup has been successfully constructed, which can offer a unique path for solidifying oxides in a containerless condition when laser-beam heating is incorporated.[1] In comparison with achievements acquired in metallic systems, the understanding of the solidification behavior of undercooled oxides to date has been in an embryonic stage. Weber et al.[2,3] utilized an AAL setup to levitate and undercool Al2O3[2] and Y-Ba-Cu-O[3,4] oxides under different atmospheric conditions, in order to investigate the effect of oxygen pressure on the microstructure formation. The most promising superconducting candidates of NdBa2Cu3O7-␦[5,6,7] and YBa2Cu3O7-␦[8] were investigated from the viewpoints of microstructural control,[5] coupled growth behavior,[6] competitive phase selection,[7] and the direct growth of tetragonal phase from undercooled melts.[8] Rapid solidification of undercooled Y3Al5O12 garnet beyond the MINGJUN LI, Postdoctoral Research Fellow, is with the Institute of Space and Astronautical Science, Kanagawa 229-8510, Japan, Contact e-mail: [email protected] KOSUKE NAGASHIO, Ph.D Student, Institute of Space and Astronautical Science, is with the Department of Materials Science, The University of Tokyo, Tokyo, 113-8656, Japan. KAZUHIKO KURIBAYASHI, Professor, Institute of Space and Astronautical Science, is with CREST, Japan Science and Technology Corporation, Ibaraki 305-0047, Japan. Manuscript submitted November 20, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
hypercooling limit was analyzed,[9] and the resulting microstructure was observed.[10] Note that all the aforementioned oxides are stoichiometric compounds. Consequently, there is no contribution of solute undercooling to bulk undercooling in solidification. It is, therefore, of great interest to investigate the solidification of an oxide solid solution, where constitutional undercooling may be produced due to solute redistribution at the S/L interface. Mullite (3Al2O3⭈2SiO2) is a typical solid-solution oxide, which has increasing potential in electronic, optical, and high-temperature structural applications.[11] However, no undercooling experiment has been performed in mullite, and solidification was accomplished either in a refractory crucible (for example, in a Mo crucible[12]) or by the Czochralski[13] or floating-zone[14] techniques. In these experiments, the melt undercooling is extremely small and negligible. A lot of metallic solid solutions, which
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