On the role of initial conditions in the selection of eutectic onset mechanisms in directional growth

The early-stage dynamics and onset mechanisms for eutectic solidification are investigated experimentally using slab-geometry slides of succinonitrile–(D)camphor (SCN–DC) transparent organic eutectic material. By specifically focusing separately on the pre-growth or holding period and the growth or pulling period, the critical roles of each in the establishment of initial conditions and the competition between eutectic initiation mechanisms, leading to the development of a steady-state eutectic front, are examined. It is found that a single-phase layer forms and increases in thickness monotonically with time during the holding period with a corresponding increase in the interface temperature. Because the thickness of this layer is observed to influence subsequent eutectic initiation mechanisms, it is concluded that the pre-existing structure, holding period duration, single-phase identity and thickness, and specimen slide geometry should all be reported as standard practice, along with the pulling velocity and thermal gradient, for a complete description of a gradient-zone directional solidification experiment.