Influence of thermosolutal convection on the solidification front during upwards solidification

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Abstract
For some years now, much effort has been devoted to the study of thermosolutal convection in the liquid phase during upwards solidification of a binary alloy, which is coupled to the dynamics of the solid-liquid interface. While the theoretical analysis is well developed, there is a need for experimental evidence. Experiments in cylinders have thus been carried out on lead – 30 wt % thallium alloys in order to obtain significant information about the convective patterns in the melt adjacent to the solidification front, from a knowledge of the macroscopic shape of the phase boundary. This shape is determined as a function of the lateral confinement θ (the ratio of the crucible diameter to the unstable wavelength at the threshold for an infinite medium) from a series of contour lines for the solid in the two-phase region of the quenched samples. When θ is small, the pattern always has a central axisymmetric core and an outer annulus which is at first complex or structureless and then presents a mixture of festoons and solid sectors. For θ very close to unity, a hexagon, which is the basic element of a laterally infinite array, dominates the morphology. At higher θ, a hexagon can no longer remain naturally centred and is replaced by two main cells which contact the wall by again making a completely festooned ring. The fluid flow in the liquid just ahead of the solid-liquid interface is then inferred. Analogy with Bénard-Marangoni patterns suggests a qualitative analysis of the convective structures. The present observations are finally compared to previous ones on similar alloys grown in crucibles with a smaller diameter.