In-situ synchrotron studies of dendritic growth in solidifying Ga – In alloys

X-ray absorption contrast techniques are an important diagnostic tool to investigate solidification processes in metallic alloys. This work is devoted to an in situ visualization of the dendritic growth during the bottom-up solidification of a Ga-25wt%In alloy under natural convection. The coupling of X-ray imaging with X-ray diffraction techniques provides additionally information of the crystallographic orientation of the growing dendrites.
A main advantage of X-ray radiography is the possibility to study simultaneously solidification phenomena on different length scales delivering information on both: the dendrite structure and the flow patterns especially in the vicinity of solidification front. Melt flow, mainly the convective transport of the solute, induces various effects on the dendrite and grain morphology [5]. All these phenomena depend sensitively on the local conditions like the dendrite arm spacing and orientation, the detachment of side branches, the local direction and intensity of the flow it self [3-5]. A more detailed analysis of these particular processes requires a much better spatial resolution. Using synchrotron radiation the spatial resolution of the radiography experiment was improved by more than a factor of 10 reaching a value of below 1µm. Moreover, using synchrotron radiation, the time to record a tomogram was considerably reduced making it possible to visualize a stable dendrite structure in a melt flow.
An other not sufficiently investigated phenomenon is the orientation selection in dendrite evolution. It was demonstrated that the primary dendrite growth directions can vary continuously between different crystallographic directions as a function of the composition–dependent anisotropy parameters [6]. Therefore a challenging part of this experiment was the combination of two in situ techniques: X-ray radiography and synchrotron X-ray diffraction. X-ray diffraction measurements can help to reconstruct crystallographic orientations of growing dendrites.
The first radiography / diffraction experiments with solidifying Ga-25wt%In alloy were performed at BM20 and ID19 at a spatial resolution of < 0.5 µm. The radiography/ diffraction experiments performed at the beamline BM20 were carried out at an energy of 28.5keV, where as at ID 19 the radiography and tomographic experiments were performed at energies at about 65keV using a filtered undulator spectrum without any additional monochromator.
An existing solidification setup and the solidification cell were improved for synchrotron experiments guaranteeing a stable(~0.1°C) temperature gradient to control the convection inside the cell. The nominal composition of the Ga–25wt%In alloy was prepared from 99.99% Ga and 99.99% In. The alloy was melted and filled into the Plexiglas hele-shaw cell with an area of ~230 x 230 mm2 and inner cell thickness of 150μm. The tomography experiment was carried out using a capillary with an inner diameter of 400µm cell.