Evolution of dendritic morphology in a solidifying Ga-In alloy studied by in-situ synchrotron radiography

The local dynamics of dendritic tips and side arms during the growth and coarsening stages are studied by X-ray synchrotron radiography and by numerical simulation. The direct investigation of dendritic side arm evolution appears to be rather complex and impose high requirements with respect to the spatial and temporal resolution and sensitivity of the detector. The synchrotron imaging experiments were performed at the ID19 beamline (ESRF, France) at a spatial resolution of < 1 µm. A flat sample of a Ga-In alloy is solidified from top to down applying a vertical temperature gradient. The present measurements provide real-time in-situ data on three phenomena that are of major importance in dendrite morphology evolution: side arm coarsening, side arm-splitting and dendrite tip evolution. The combination of numerical modeling and synchrotron experiments has allowed to improve the understanding of coarsening of dendritic side arms and to obtain material information that is relevant for quantitative modeling. Another interesting effect can be observed during in situ solidification experiments: a transition from a four-fold symmetry to a hyperbranched dendritic morphology. This morphological transition originating from the splitting of dendrite side arms has been detected.