Observation of melt flow effects on dendritic growth of Ga-In alloys by X-ray radioscopy

Capabilities of the X-ray attenuation contrast radioscopy were utilized to provide a real-time diagnostic technique for observations of dendritic growth and the melt flow during solidification of a Ga-30wt%In alloy. The solidification process was visualised using a microfocus X-ray tube. The X-ray facility provided shadow radiographs at spatial resolutions of about 10 µm and frame repetition rates up to 25 Hz. We used the optical flow approach to derive information about the velocity field ahead of the solidification front and in the mushy zone from the observed displacement of the brightness patterns appearing during the X-ray image sequence. Experiments have been carried out to solidify the Ga-In alloy unidirectionally either starting from the bottom or the top of the specimen. The first case is significantly affected by solutal convection. Buoyancy-driven flow patterns were found at length scales being larger than the length scales of the solidifying microstructure. A strong coupling between convection and dendritic growth became apparent via flow-induced modifications of the concentration profile in the liquid. Dendritic fragmentation can be observed during the solidification in the reverse top-down direction. Variations of the applied cooling rate evoked a transition from a columnar to an equiaxed dendritic growth (CET).