The gradual transformation of a mushy zone during alloy solidification, from a continuous liquid film network to a fully coherent solid, has been simulated using a granular model. Based on a Voronoi tessellation of a random set of nucleation centers, solidification within each polyhedron is computed considering back-diffusion and coalescence. In the network of connected liquid films, a pressure drop calculation is performed assuming a Poiseuille flow in each channel, Kirchhoff's conservation of flow at nodal points and flow Losses compensating solidification shrinkage (KPL model). In addition to intergranular liquid pressure maps, the model shows the progressive formation of grains clusters, the localisation of the flow at very high solid fraction, and thus natural transitions of the mushy zone.

Automatic indexing of electron backscattered diffraction patterns, scanning electron microscopy, and optical microscopy observations have been carried out on aluminum-magnesium-silicon, aluminumcopper, and aluminum-silicon alloys directionally solidified or semicontinuously cast using the direct chill casting process. From these combined observations, it is shown that the feathery grains are made of ͗110͘ primary dendrite trunks (e.g., [01 ]) split in their centers by a coherent (111) twin 1 plane. The average spacing of the dendrite trunks in the twin plane (about 10 to 20 m) is typically one order of magnitude smaller than that separating successive rows of trunks (or twin planes). The [01 ] orientation of these trunks is close to the thermal gradient direction (typically within 15 deg)-1 a feature probably resulting from a growth competition mechanism similar to that occurring during normal ͗100͘ columnar dendrite growth. On both sides of these trunks, secondary dendrite arms also grow along ͗110͘ directions. Their impingement creates wavy noncoherent twin boundaries between the coherent twin planes. In the twin plane, evidence is shown that ͗110͘ branching mechanisms lead to the propagation of the twinned regions, to the regular arrangement of the primary dendrite trunks along a [ 11] direction, and to coherent planar twin boundaries. From these observations, it 2 is concluded that the feathery grains are probably the result of a change from a normal ͗100͘ to a ͗110͘ surface tension/attachment kinetics anisotropy growth mode. This change might be induced by the added solute elements, by the local solidification conditions (thermal gradient, growth rate, and melt convection), and possibly by the help of the twin plane itself. Convection in the melt could also play a role in the symmetrization of the ͗110͘ growth directions of the side arms. Finally, the proposed mechanisms of feathery grain growth are further supported by the observation of ͗110͘ dendrite growth morphologies in thin aluminum-zinc coatings.

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The equiaxed solidification of Al-20 wt.% Zn alloys revealed an unexpectedly large number of fine grains which are in a twin, or neartwin, relationship with their nearest neighbors when minute amounts of Cr (1000 ppm) are added to the melt. Several occurrences of neighboring grains sharing a nearly common h1 1 0i direction with a fivefold symmetry multi-twinning relationship have been found. These findings are a very strong indication that the primary face-centered cubic Al phase forms on either icosahedron quasicrystals or nuclei of the parent stable Al 45 Cr 7 phase, which exhibits several fivefold symmetry building blocks in its large monoclinic unit cell. They are further supported by thermodynamic calculations and by grains sometimes exhibiting orientations compatible with the socalled interlocked icosahedron. These results are important, not only because they provide an explanation of the nucleation of twinned dendrites in Al alloys, a topic that has remained unclear over the past 60 years despite several recent investigations, but also because they identify a so far neglected nucleation mechanism in aluminum alloys, which could also apply to other metallic systems.

The phenomena responsible for the formation of macrosegregations and grain structures during solidification are closely related. We present a model study of macrosegregation formation in an industrial sized (350 mm thick) direct chill (DC) cast aluminum alloy slab. The modeling of these phenomena in DC casting is a challenging problem mainly due to the size of the products, the variety of the phenomena to be accounted for, and the nonlinearities involved. We used a volume‐averaged two‐phase multiscale model that describes nucleation on grain refiner particles and grain growth, fully coupled with macroscopic transport: fluid flow driven by natural convection and shrinkage, transport of free‐floating equiaxed grains, heat transfer, and solute transport. The individual and combined roles of shrinkage, natural convection, and grain motion on the sump profile and macrosegregation formation are analyzed. The formation and evolution of grains are discussed. We show that it is important to account for all the named transport mechanisms to be able to explain the macrosegregation pattern observed experimentally in DC cast ingots.