Phase selection in the containerless solidification of undercooled CaO · 6Al 2O 3 melts

Abstract

The CaO · 6Al 2O 3 melts were solidified on an aero-acoustic levitator under a containerless processing condition at various undercoolings. A high-speed video was operated to monitor the recalescence behavior, from which the growth velocity as a function of melt undercooling was determined. The microstructures were observed and the crystalline phases were identified using the X-ray diffraction technique, indicting that the Al 2O 3 was solidified when the melt temperature was higher than the peritectic temperature, T p. When the melt was undercooled below T p, the CaO · 6Al 2O 3 (CA 6) peritectic phase was crystallized directly from the undercooled melts. With respect to the direct formation of the peritectic phase, further analysis from the viewpoints of competitive nucleation indicated that the minimum free energy principle may be applied to elucidate the nucleation of CA 6 phase. In terms of the competitive growth behavior, the interface attachment kinetics for Al 2O 3 and CA 6 phases are calculated by using the classical BCT model indicating that although the Al 2O 3 phase doped by CaO has about four times larger interface kinetic coefficient than that of the CA 6 peritectic phase, the growth kinetics of Al 2O 3 in the melt with the CaO · 6Al 2O 3 chemical composition is not sufficiently high to replace the CA 6 phase as the primary phase. Therefore, once CA 6 is nucleated, it can develop into a macro crystal as the primary phase. The competitive nucleation and growth behavior in the CA 6 system is different from those in other well-studied peritectic alloys and the present investigation on the phase formation will be an essential supplement to the phase selection theory.