In situ X-ray radioscopy studies of microstructure and freckle formation in solidifying alloys

In-situ observations of the solidification process of Ga-25wt%In alloy within a Hele-Shaw cell were obtained by means of X-ray radioscopy. This work is devoted to an experimental study of the initiation and development of segregation channels inside the mushy zone of a solidifying metal alloy under the influence of thermosolutal convection. We also focused on the analysis of dendrite formation and remelting, flow pattern and microstructure features, for instance, dendrite orientation, primary and secondary spacing. A series of directional bottom-up solidification experiments were carried out at a cooling rate of 0.01 K/s accompanied by vertical temperature gradients in the range between 0.5 and 2 K/mm. Because of the conical shape of the solidification cell there is no uniform cooling of the sample and an additional lateral temperature difference arises. The real time studies of solidification of Ga-In alloys deliver simultaneous data of both the structure of the solid fraction in the mushy zone and the flow pattern in the vicinity of the solidification front.
Different scenarios were observed concerning the formation of segregation channels in the mushy zone which can be related to variations of the vertical temperature gradient. A temperature gradient up to 1 K/mm results in the formation of various segregation channels, but a sustainable development of stable chimneys was observed only in 4 of 10 experiments. Stable chimneys occur mainly at positions with initial growth defects or grain boundaries, however, not every initial segregation channel evolves into a stable chimney. In the case of higher temperature gradients (up to 2 K/mm) we can identify a converging flow ahead of the mushy zone coming from the side walls and leading to a concentration of the rising plumes in the central part of the solidification cell. The continuous strengthening of the central plume causes a distinct solute accumulation in the mushy zone behind followed by a remelting of the solid fraction and the occurrence of a sustaining channel. In all solidification experiments performed under such conditions at least one stable chimney can be detected. This mechanism of chimney formation is different as compared to the case where the development of the segregation channel is linked with any initial growth defect. Our results reveal a clear impact of the flow field on the microstructure and the formation of chimneys.
In summary, the initiation and development of the chimneys and the probability of their surviving depend sensitively on the spatial and temporal properties of the flow field. Variations of the vertical temperature gradient along the solidification cell induce modifications of the melt flow pattern, which lead to different segregation structures. In situ characterizations of dendrite growth and chimney formation can give access to key parameters for modelling of freckle formation and its verification.