Directional Solidification of Transparent Eutectic Alloys

When an alloy at eutectic concentration is solidified, two distinct phases, α and β, of different concentrations separate. In directional solidification, with thin (50 m) samples of the transparent eutectic mixture CBr4-8.4 % C2Cl6, one obtains a periodic structure with lamellae perpendicular to the solidification interface1 (left-hand side of Fig 1). From the point of view of the fundamental physicist, this is an interesting case of pattern formation2. From a more practical point of view, eutectic structures are met during the solidification of a number of metal alloys and it is useful to be able to study their dynamics on a transparent model. The eutectic pattern appears without a threshold, however low the pulling velocity (there is no analogue to the Mullins and Sekerka threshold in cellular growth of dilute alloys). Figure 1 shows one onset mechanism. In this case the sample had been pulled, creating the pattern to the left. Pulling was then stopped and a narrow single phased band developed at the interface, due to the sample being slightly off-eutectic. When pulling was resumed, the other phase nucleated locally, at the tips of previous lamella. One can also observe propagation of the two phase pattern, not yet well organized, along the interface as reported by Seetharaman and Trivedi3.