Equiaxed dendritic solidification and grain refiner potency characterised through in situ X-radiography

Results of near-isothermal equiaxed solidification experiments on the Al-Cu system performed in situ with laboratory-based radiography are presented. The ability to align the X-ray beam axis both perpendicular and parallel to the gravity vector provides limited control over buoyant grain motion normally associated with terrestrial solidification. The latter alignment case provides solidification conditions where the impact of gravity during solidification is strongly reduced. Exploiting this experimental configuration, direct observations of the effect of buoyancy on grain motion, nucleation density, and average grain size are presented. Detailed measurements of solid area fraction evolution were extracted from image sequences, demonstrating alloy-characteristic temperature dependence, allowing for comparison with theoretical predictions. Finally, measurements of nucleation density and dendrite growth velocities, as a function of applied cooling rates and alloy composition, were made and are discussed here with reference to relevant theoretical models. This work represents the first such study where solidification conditions without significant effects from gravity were achieved, for both solid and liquid phases, in ground-based experiments.