Liquid decomposition, droplet coagulation and droplet-interface interactions in hypermonotectic Al-Bi alloys

摘要

Liquid decomposition in slightly hypermonotectic Al-Bi alloys has been investigated by synchrotron radiography during directional solidification. The L→L ₁+L ₂ reaction was found to be almost completely suppressed, appearing in close proximity to or simultaneous with the monotectic reaction, corresponding to a L(Bi)-supersaturation of 0.3. The liquid supersaturation also undercooled the monotectic reaction, where gravity, thermo-solutal Marangoni and Stokes drag set up a force-balance that control a strongly size dependent L ₂ droplet motion. The size dependant droplet motion gives rise to complex meso- and micro scale hydrodynamics that together with short-range non-hydrodynamic and hydrodynamic L ₂-L ₂ interactions have been identified as the main mechanisms behind droplet coagulation. The relatively substantial coagulation dynamics result in a bimodal droplet size distribution at the advancing solidification front. By varying the experimental parameters it was possible to study the droplet-α-Al-liquid interface interactions with both planar and curved interface morphologies. These interactions will be decisive for the final Bi particle distribution in the α-Al matrix. In contrast to what is most commonly assumed for pushing/engulfment of solid particles it was found that the larger droplets tended to be pushed by the front while smaller droplets are more frequently engulfed or entrapped. This result could be ascribed to effects on the droplets caused by the presence of solutal gradients ahead of the solidification front as well as dropletdroplet interactions.