<i>In Situ</i> Investigation of Liquid-Liquid Phase Separation in Hypermonotectic Alloys

Abstract

Hypermonotectic alloys are distinguished by a temperature region for which the homogeneous melt decomposes into two liquid phases. In Al-based hypermonotectics, the minority phase is much higher in density than the matrix melt phase and consequently macro-segregation due to sedimentation is an inherent problem when casting these alloys. However, under the correct solidification conditions, it may be feasible to counteract sedimentation by thermocapillary forces that arise due to the thermosolutal dependence of the surface tension between the two liquid phases. The current investigation involved in-situ X-ray video microscopy studies during directional solidification of Al-Bi samples of various compositions employing a Bridgman furnace. It was found that large undercoolings were required to initiate the L  L1 + L2 reaction and L2 nucleation occurred heterogeneously on the monotectic front. L2 droplets were then set in collective size-dependent motion by hydrodynamic forces coupled to external fields (i.e. temperature gradient and gravity) and internal fluctuations. The resulting flow fields are superimposed onto the short range coagulation mechanisms, such as diffusion coupling, and are all crucial in determining the final size distribution and dispersion of the Bi-rich phase in the cast material.