Microstructure of Y3Al5O12 garnet solidified from the melt undercooled beyond the hypercooling limit

The microstructural evolution of Y3Al5O12 garnet (YAG) in a wide undercooling range beyond the hypercooling limit (DeltaT(hyp)) was investigated by containerless solidification processing. The dendrite to cellular-dendrite transition at high-growth velocity was observed at the undercooling beyond DeltaT(hyp). This transition may be explained by the hypothesis that it is difficult to form the well-developed secondary-dendritic arms from the hypercooled melt because of no remaining melt in the interdendritic regions. With a further increase in undercooling beyond DeltaT(hyp), a cellular microstructure disappeared, and copious amounts of small particles appeared at an undercooling of approximately 1000 K, which is near the glass-transition temperature where the viscosity is approximately 10(12) Pas. It is suggested that multiple nucleation occurred in the highly viscous undercooled melt because of the high nucleation rate. The grain size of YAG, which was analyzed as a function of undercooling, gradually decreased with increasing undercooling even beyond DeltaT(hyp), and no fragmentation of dendrites was observed.