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Dendrites are common microstructures that are formed during industrial casting or welding. The processes involved in the formation of different dendrite morphologies and the orientation selection during dendritic growth are rather complex and still far from being fully understood [1-3]. In this work, in situ synchrotron X-ray radiography and diffraction methods have been combined to study the evolution of dendritic microstructures during the solidification of Ga - In alloys. The in situ directional solidification experiments were performed at the ID19 and BM20 beamlines (ESRF, France) at a high spatial resolution of < 1 µm.
Solidification processes are affected by natural convection as soon as instable density stratification arises in the melt. Melt flow induces various effects on the dendrite and grain morphology primarily caused by the convective transport of the solute. Usually, the morphologies of these dendrites differ from those developing under purely diffusive condition. Our observations show a facilitation of the growth of primary trunks or lateral branches, a suppression of side branching, dendrite remelting and fragmentation [4]. The final microstructure reveals dendrites with random and complicated morphologies.
The flow-induced variations of the local solute concentration may result in the changes of dendrite crystal orientations. According to theoretical predictions, the dendrite growth directions should not follow necessarily low indexed crystallographic directions [1]. Therefore coupling of in situ X-ray imaging with X-ray diffraction provides additionally information of the crystallographic orientation of the growing dendrites. Our measurements show that majority of the Indium dendrites grow along the <110> orientation, typically observed in body-centered metals. The analysis of the diffraction patterns obtained from the complex dendritic structures shows that a further improvement towards a 3D imaging experiment is needed. These first results demonstrate that the combination of these X-ray techniques can provide new data about the solidification processes and help to validate microstructural solidification models.
This work is financially supported by the Helmholtz Association “LIMTECH”.