3-D microstructural model of freckle formation validated using in situ experiments

A 3-D model of freckle (solute channel) formation at a microstructural level was coupled with in situ X-ray radiography to investigate the mechanisms of freckle initiation and growth. The model predictions for solute partitioning, diffusion and convection were validated via in situ X-ray radiographic measurements in Ga-25 wt.% In alloy, showing good agreement. Other key features, such as freckle channel width and critical Rayleigh number, also correlated well. The validated model was used to investigate freckle formation under a range of solidification conditions. Two distinct stages of freckle onset were observed, identified via the dendrite tip growth and solute profiles. The first stage corresponds to lower flow velocities with large fluctuations; in the second stage the velocities stabilize, with established recirculating flows forming solute channels. The influence of imperfections in dendritic morphology, such as grain boundaries and primary spacing variations, on the critical Rayleigh number was studied. It was found that that these features initiate freckles. Non-intuitively, converging grain boundaries were observed to have the greatest propensity for freckle formation. The resulting new insights on solute plume formation impact a range of phenomena from single-crystal superalloys to magma flows.