The influence of gravity levels on the horizontal Bridgman crystal growth of an alloy

The solidification of a dilute alloy under Bridgman crystal growth conditions in a reduced-gravity environment is investigated. The simulation consists of fully transient calculations of the species concentration, temperature and flow field, as well as conduction in the ampoule. Results are obtained at gravity levels of 1, 10 and 50 μg for a Bi-1.0 at.% Sn alloy to determine the influence of gravity level on thermosolutal convection, and consequently, on the concentration in the solid. A primary convective cell driven by thermal gradients forms in the bulk of the domain, while a secondary convective cell driven by solutal gradients forms near the interface. The magnitude of the velocities in the secondary cell increases with time, causing increasing solute segregation at the interface. For a gravity level of 1 μg, convection-induced radial segregation in the solidified material is minimal and the process is diffusion-controlled. In contrast, at the highest gravity level of 50 μg, very large levels of segregation are observed and the high levels of convective transport in the melt lead to a breakdown in the phenomena normally associated with diffusion-controlled solidification.