In situ study of solidification patterns of a ternary Ga-In-Bi alloy in presence of melt convection

In situ real-time observations, which are important in studies of binary alloy systems for verification of theoretical hypotheses, are scarce for ternary and multi-component alloys. This work is devoted to in situ visualization of solidification patterns observed during bottom-up solidification of a Ga-In-Bi alloy in a Hele-Shaw cell under buoyancy-driven convection. The investigations are based on a combination of micro-focus X-ray radiography and advanced image processing techniques. A model ternary system is selected from the family of low-temperature metallic alloys with working temperatures below 100 °C. The phase diagram of the ternary Ga-In-Bi system indicates the appearance of an indium-based primary solid phase and only a minor amount of intermetallic phase for concentrations chosen within this study.
Our observations show complex and strongly disoriented solidification patterns with curved primary and secondary branches. By studying the solidification dynamics we establish that some grains exhibit a morphology that is rather similar to the "seaweed" pattern. The appearance of seaweed grains is usually related to the low anisotropy of the surface energy of metal crystals that can be relatively easy modified by the presence of another element. We focus on the role of melt flow in transition from dendritic arrays to seaweed structures in this ternary system. Morphological analysis and comparison of dendritic and seaweed patterns are performed using a new X-ray image processing approach.