Synchrotron X-ray Imaging and Numerical Modelling of Dendritic Sidebranch Evolution during Coarsening

We study the local dynamics of dendritic side arms during coarsening by combining in-situ radiography observations with numerical and analytical models. A flat sample of a Ga-In alloy is partially solidified and then held isothermally in a vertical temperature gradient. The evolving dendritic microstructure is visualized by synchrotron X-ray imaging at the BM20 beamline (ESRF, France). The resulting 2D images provide a high resolution in space and time at low noise levels, enabling accurate dynamical measurements. Throughout the initial growth stage there is evidence of solutal natural convection, which however vanishes towards the subsequent coarsening processes. During the coarsening stage, the time evolution of essential geometrical features of side branches was captured by automated image processing. This data is used to quantify the dynamics of three basic evolution scenarios for side branches: retraction, pinch-off and coalescence. We exploit the universal dynamics of sidearm necks during pinch-off to determine the product of liquid diffusivity and capillarity length, 𝐷𝑑0, as a parameter that is crucial in the calibration of quantitative reference models. By employing an idealized phase-field model for the evolution of a single side branch, we are able to predict the behaviour of selected side branches from the experiment in a consistent way.