Interactive effects of interfacial energy anisotropy and solute transport on solidification patterns of Al-Cu alloys

Combined effects of the cooling rate, alloy composition, and crystal-melt (CM) interfacial anisotropy on solidification of Al-Cu alloys are studied by integrating molecular dynamics and phase-field simulations. Capillary fluctuation method is used to determine the CM interfacial energy properties by molecular dynamics simulations for alloys ranging from 3 to 11 at% Cu. While the average CM interfacial energy decreases with increasing Cu content, its anisotropy does not present a clear trend with composition change. Primary and secondary dendrite arm spacings as well as θ-phase fraction are calculated by phase-field simulations, and validated against experimental measurements and analytical solutions at cooling rates ranging from 1 to 1250 K/s. Results show that the θ-phase fraction decreases with increasing the cooling rate, and this reduction is more drastic in alloys with a higher Cu content. Also, the microstructure features are influenced by the growth dynamics, where seaweed structure formation results in a more homogenous distribution of θ-phase and a finer microstructure. The effects of temperature gradient, Cu concentration gradient, and interfacial energy properties on the dendritic growth morphology of Al-Cu alloys are summarized by a map of supercooling versus the CM interfacial anisotropy to predict pattern formation. The results show that, irrespective of Cu content and cooling rate, the seaweed structure formation is halted at CM interfacial anisotropies larger than 0.005. As the anisotropy decreases, different seaweed structures can form regarding the constitutional supercooling. At low anisotropies (Al-3 and Al-8.4 at% Cu) and low supercooling (Al-3 at% Cu) fractal or degenerate seaweed is dominant while at high supercooling (Al-8.4 at% Cu) compact seaweed forms. This difference in supercooling stems from different solute atom transport rates.