Solidification furnace for in situ observation of bulk transparent systems and image analysis methods.

This paper aims to describe the experimental framework of the Directional Solidification Insert, installed onboard the International Space Station, dedicated to the in situ and real-time characterization of the dynamic selection of the solid-liquid interface morphology in bulk samples of transparent materials under diffusive growth conditions. The in situ observation of the solid-liquid interface is an invaluable tool for gaining knowledge on the time evolution of the interface pattern because the initial morphological instability evolves nonlinearly and undergoes a reorganization process. The result of each experiment, characterized by the sample concentration, a thermal gradient, and a pulling rate, is a large number of images.

Microgravity studies of solidification patterns in model transparent alloys onboard the International Space Station.

We review recent in situ solidification experiments using nonfaceted model transparent alloys in science-in-microgravity facilities onboard the International Space Station (ISS), namely the Transparent Alloys (TA) apparatus and the Directional Solidification Insert of the DEvice for the study of Critical Liquids and Crystallization (DECLIC-DSI). These directional-solidification devices use innovative optical videomicroscopy imaging techniques to observe the spatiotemporal dynamics of solidification patterns in real time in large samples. In contrast to laboratory conditions on ground, microgravity guarantees the absence or a reduction of convective motion in the liquid, thus ensuring a purely diffusion-controlled growth of the crystalline solid(s).

Cell invasion during competitive growth of polycrystalline solidification patterns.

Spatially extended cellular and dendritic array structures forming during solidification processes such as casting, welding, or additive manufacturing are generally polycrystalline. Both the array structure within each grain and the larger scale grain structure determine the performance of many structural alloys. How those two structures coevolve during solidification remains poorly understood.

Analysis of gravity effects during binary alloy directional solidification by comparison of microgravity and Earth experiments with in situ observation.

Under terrestrial conditions, solidification processes are influenced to a large degree by the gravity effects such as natural convection or buoyancy force, which can dramatically modify the final characteristics of the grown solid. In the last decades, the coupling of in situ observation of growth from the melt, that enables the study of microstructure formation dynamics, and microgravity experimentation, that allows to approach diffusive conditions, has been implemented for both transparent and metallic materials. The results of these investigations enable to test the validity of advanced solidification theories, to validate or develop numerical models and sometimes to reveal unexpected phenomena.

Oscillatory-nonoscillatory transitions for inclined cellular patterns in three-dimensional directional solidification.

The oscillatory behavior of cellular patterns produced by directional solidification of a transparent alloy under microgravity conditions was recently observed to depend on the misorientation of the main crystal axis with respect to the direction of the imposed thermal gradient [Pereda et al., Phys. Rev.

Experimental characterization and theoretical analysis of cell tip oscillations in directional solidification.

Experiments performed in DECLIC-DSI on board the International Space Station evidenced oscillatory modes during the directional solidification of a bulk sample of succinonitrile-based transparent alloy. The interferometric data acquired during a reference experiment, V_{p}=1 μm/s and G=19 K/cm, allowed us to reconstruct the cell shape and thus measure the cell tip position, radius, and growth velocity evolution, in order to quantify the dynamics of the oscillating cells. This study completes our previous reports [Bergeon et al.

Experimental observation of oscillatory cellular patterns in three-dimensional directional solidification.

We present a detailed analysis of oscillatory modes during three-dimensional cellular growth in a diffusive transport regime. We ground our analysis primarily on in situ observations of directional solidification experiments of a transparent succinonitrile 0.24wt% camphor alloy performed in microgravity conditions onboard the International Space Station.

Oscillatory cellular patterns in three-dimensional directional solidification.

We present a phase-field study of oscillatory breathing modes observed during the solidification of three-dimensional cellular arrays in microgravity. Directional solidification experiments conducted onboard the International Space Station have allowed us to observe spatially extended homogeneous arrays of cells and dendrites while minimizing the amount of gravity-induced convection in the liquid. In situ observations of transparent alloys have revealed the existence, over a narrow range of control parameters, of oscillations in cellular arrays with a period ranging from about 25 to 125 min.

Spatiotemporal dynamics of oscillatory cellular patterns in three-dimensional directional solidification.

We report results of directional solidification experiments conducted on board the International Space Station and quantitative phase-field modeling of those experiments. The experiments image for the first time in situ the spatially extended dynamics of three-dimensional cellular array patterns formed under microgravity conditions where fluid flow is suppressed. Experiments and phase-field simulations reveal the existence of oscillatory breathing modes with time periods of several 10's of minutes.

Cellular pattern dynamics on a concave interface in three-dimensional alloy solidification.

Three-dimensional interface patterns are common in condensed matter, whose dynamical behavior is still deserving clarification. The dynamics of cellular patterns formed at the concave solid-liquid interface during directional solidification in a cylinder of a transparent alloy is studied by means of bright-field live imaging. For each pulling velocity, in situ observation shows that the asymptotic cellular pattern, which establishes with time, is characterized by the continuous birth of a large number of cells at a circular source of morphological instability on the periphery, the sustained collective gliding of the whole cellular array down the interface slope, and the elimination of coarse cells at the central sink.

Cumulative mechanical moments and microstructure deformation induced by growth shape in columnar solidification.

The dynamical interaction between columnar interface microstructure and self-stress, resulting in unforeseen mechanical deformation phenomena, is brought to light by means of in situ and real-time synchrotron x-ray topography during directional solidification of dilute aluminum alloys. Beyond long-known local mechanical stresses, global mechanical constraints are found to be active. In particular, column rotation results from deformation caused by the mechanical moments associated with the very growth shape, namely, the cumulative torque acting together with the cumulative bending moment under gravity.

Free growth and instability morphologies in directional melting of alloys.

The dynamics of melting morphologies, namely, liquid droplets in the bulk solid and liquid dendrites due to morphological instability of the phase boundary, is observed in situ and in real time during directional melting of transparent succinonitrile-acetone alloys in a cylinder. Specific patterns are associated to grain boundaries. A model based on free growth but with time-dependent superheating is proposed for the lateral growth of the liquid inclusions.

Localized microstructures induced by fluid flow in directional solidification.

The dynamical process of microstructure localization by multiscale interaction between instabilities is uncovered in directional solidification of transparent alloy. As predicted by Chen and Davis, morphological instability of the interface is observed at inward flow-stagnation regions of the cellular convective field. Depending on the driving force of fluid flow, focus-type and honeycomb-type localized patterns form in the initial transient of solidification, that then evolves with time.

In situ observation and interferometric characterization of solid-liquid interface morphology in directionally growing transparent model systems.

The performance of a new directional solidification device dedicated to the characterization of solid-liquid interface morphology by means of optical methods is presented in this paper. In contradiction to usual solidification studies on transparent materials carried out on thin films, which eliminates the complex coupling between solidification and convection, this device enables in situ and real time studies on bulk transparent materials. The alloy is contained in a cylindrical crucible and observation is performed in two perpendicular directions: the growth one and the transverse one.