Solute transport coupled microstructure evolution of immiscible beryllium-aluminum alloy

The morphology of primary α-Be dendrites during the solidification process directly determines the porosity defect and thus the mechanical properties of beryllium-aluminum alloys. However, the features of the three-dimensional microstructure remain unclear, and how the structure evolves under the complex multiple physical fields still requires to be explored. Here, X-ray microtomography characterisations were conducted to uncover the three-dimensional dendritic structure of the hexagonal close-packed (HCP) α-Be dendrite with 12 preference growth directions. A thermal-fluid-solute-microstructure fully-coupled model was developed to describe dendritic morphology evolutions under the solute transport driven by the natural convection. The results reveal that the Al element significantly enriched at the solid-liquid interface drove an intense downward natural convection, accelerating outward solute transport from the roots of the dendritic branches. Furthermore, due to the in-phase arrangement of growth directions on both sides of {0001}, the unique symmetrical structure of α-Be dendrites induces solute distribution and thus the asymmetric melt flow. Such asymmetry inhibits the secondary dendrite arm growth in some directions, ultimately resulting in the hard-to-identify asymmetric three-dimensional dendritic structure. Accelerated solute transport also leads to the increase in the secondary dendrite arm spacing and the decrease in the interfacial area density. As the temperature approaching to 1431 K (the temperature at which the liquidus slope approaches to zero), a rapid increase in solid fraction occurs and thereby induces frequent grain coalescences, causing an additional increase in the melt flow velocity by compressing the space and enhancing the solute enrichment.