A comparative study of the behaviour of forsterite melts under atmospheric and sub-atmospheric conditions

Abstract

The study is focussed towards understanding the difference in behaviour of forsterite droplets subjected to sub-atmospheric conditions. Melt droplets, 1.5-2.5 mm in diameter, are made to crystallize under levitated conditions. The spherule is initially superheated by about 250oC above its liquids temperature, for both atmospheric and sub-atmospheric conditions, and then, subsequently cooled at different cooling rates. Crystallization of molten droplets was observed from their hypercooled states in all the cases. It was found that the level of undercooling was more (~150K-200K) for sub-atmospheric cases. In addition, the degree of recalescence was also found to be higher. However, the varying cooling rates did not produce any considerable effect on the level of undercooling or, even, the degree of recalescence. For better insight into the dynamics of crystallization, in situ visualization of the recalescence process was made possible by use of high-speed camera. A clear difference in the growth mechanisms was observed. The role of cooling rate in affecting crystallization was seen as the difference in the growth of the crystal-front; from a clear rim front, in high cooling rates, to a faded irregularly shaped crystal front, in low cooling rates. Two proper crystal fronts were observed in all the cases, which correspond to surface and volumetric crystallizations. It was observed that the molten droplets, under sub-atmospheric conditions, undergo high rate of volumetric crystallization, which is marked by the sightings of surface textures during the recalescence process. High-speed camera images of recalescence provided direct observational proof of the delay in volumetric crystallization, compared to surface crystallization.