Cellular-to-dendritic transition during the directional solidification of binary alloys

Abstract

The transition from a cellular to dendritic microstructure during the directional solidification of alloys is examined through experiments in a transparent system of succinonitrile (SCN)-salol. In a cellular array, a strong coupling of solute fields exists between the neighboring cells, which leads not only to multiple solutions of primary spacing, but also includes multiple solutions of amplitude, tip radius, and shape of the cell. It is found that these multiple solutions of different microstructural features in a cellular array, obtained under fixed growth conditions and compositions, play a key role in the cell-dendrite transition (CDT). The CDT is controlled not only by the input parameters of alloy composition (C 0), growth rate (V), and thermal gradient (G), but also by microstructure parameters such as the local primary spacing. It is shown that the CDT is not sharp, but occurs over a range of growth conditions characterized by the minimum and maximum values of V/G. Within this transition range, a critical spacing is observed above which a cell transforms to a dendrite. This critical spacing is given by the geometric mean of the thermal, diffusion, and capillary lengths and is inversely proportional to composition in weight percent.