In situ observation of solidification patterns in diffusive conditions

Abstract

International audienceWe present a review of recent in situ experimentation studies on solidification front patterns and microstructures in alloys. Front-tracking diagnostics and real-time observation methods using high-resolution optical or X-ray imaging devices currently apply to model transparent systems as well as metallic alloys in thin and bulk samples. On a theoretical basis that spans the physics of nonequilibrium pattern formation and materials science, in combination with time-resolved numerical simulations, conclusive results of both fundamental-and applied-science interest have been obtained on major problems relative to multiscale microstructure selection, morphological transitions, and crystallographic effects during single-and multi-phase solidification. We will mainly focus on the dynamics of cellular, dendritic, and eutectic growth patterns in diffusive-growth conditions, that is, in the absence of convection in the liquid. This can be achieved in (semi-)thin samples, or, for bulk solidification, in the reduced-gravity environment of orbiting facilities. A selection of emerging work on, e.g., faceted growth and adaptive control of solidification patterns will furthermore be reported. We conclude by pointing out open questions and ne

Comparative study of directional solidification of Al-7 wt% Si alloys in Space and on Earth: Effects of gravity on dendrite growth and Columnar-to-equiaxed transition

“…Within this context, solidification experiment under microgravity conditions can provide unique benchmark data by effectively suppressing the gravity-driven phenomena and we can expect to achieve nearly diffusive conditions [36][37][38]. Within the ESA project CETSOL (Columnar to Equiaxed Transition in SOLidification processes), directional solidification experiments under both gravity (1g) and microgravity conditions (µg) [39,40] have been carried out on selected aluminum-based alloys.…”

Effect of solidification conditions and surface pores on the microstructure and columnar-to-equiaxed transition in solidification under microgravity

“…Microgravity experimentation is a perfect tool to deepen the study of CET, as it provides unique benchmark data by suppressing most of the gravity-driven phenomena during solidification, such as natural convection, as well as sedimentation or buoyancy. In microgravity environment, we can expect to achieve nearly diffusive conditions for transport [18][19][20]. Nevertheless, even in gravity-free conditions, fluid flow, generated by the sample shrinkage induced by the difference of solid and liquid densities, still exists [21,22].…”

Free dendritic tip growth velocities measured in Al-Ge

“…Thus, gravity-induced fluid flow is minimized and the diffusion field is comparable to microgravity conditions, at least for the isothermal case. This is an important aspect for the comparability of the experiments to diffusionlimited growth models [40].…”