• Littles, Louise Strutzenberg
• Trivedi, Rohit
• Karma, Alain
• Bergeon, Nathalie
• Medjkoune, Mehdi
• Mota, Fatima L.

Abstract

The study of solidification microstructure formation is of utmost importance for the design and processing of materials, as solid-liquid interface patterns largely govern the mechanical and physical properties of the resulting solid. Microstructure pattern selection occurs under dynamic conditions of growth in which the initial morphological instability evolves nonlinearly and undergoes a reorganization process. This dynamic and nonlinear nature renders in situ observation of the solid-liquid interface an invaluable tool for gaining knowledge on the time-evolution of the interface pattern. Transparent organic analogs, which solidify like metallic alloys, lend themselves to this type of observation, and have become the materials of choice for the direct study of interface dynamics. Extensive groundbased studies of both metallic and organic bulk samples have established that gravity causes significant and disruptive convection during solidification, which alters the microstructure formation. A reduced-gravity environment is therefore mandatory to render the convective flow effects negligible in bulk samples. Experiments were carried out in the Direct Solidification Insert (DSI) of the Device for the study of Critical Liquids and Crystallization (DECLIC) installed onboard the ISS. The DSI offered the unique opportunity to observe in situ and characterize the entire development of the microstructure in extended 3D patterns under diffusive growth conditions, using bulk samples of transparent organic alloys. Between 2010 and 2011, the first space campaign explored the entire range of microstructures resulting in unprecedented observations related to cellular patterns. A second campaign (DSI-R), performed between 2017 and 2018, expanded the benchmark database, particularly in the dendritic regime. In DSI-R, the alloy solute concentration was increased, leading to the formation of well-developed dendritic patterns. Interpretation of the images produced by these experiments necessitates a robust identification of each dendrite position and size during the whole solidification. We have developed several image analysis methods to achieve this goal reliably despite varying contrast and noise levels. Here, we present a typical solidification experiment and analyze the dynamics of the dendritic pattern formation to illustrate the application of the image analysis methods.

Topics

• impedance spectroscopy
• microstructure
• experiment
• crystallization