〈110〉 dendrite growth in aluminum feathery grains

Abstract

Automatic indexing of electron backscattered diffraction patterns, scanning electron microscopy, and optical microscopy observations have been carried out on aluminum-magnesium-silicon, aluminum-copper, 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 〈110〉 primary dendrite trunks (e.g., [01$\bar 1$]) split in their centers by a coherent (111) twin plane. The average spacing of the dendrite trunks in the twin plane (about 10 to 20 µm) is typically one order of magnitude smaller than that separating successive rows of trunks (or twin planes). The [01$\bar 1$] orientation of these trunks is close to the thermal gradient direction (typically within 15 deg)—a feature probably resulting from a growth competition mechanism similar to that occurring during normal 〈100〉 columnar dendrite growth. On both sides of these trunks, secondary dendrite arms also grow along 〈110〉 directions. Their impingement creates wavy noncoherent twin boundaries between the coherent twin planes. In the twin plane, evidence is shown that 〈110〉 branching mechanisms lead to the propagation of the twinned regions, to the regular arrangement of the primary dendrite trunks along a [$\bar 2$11] direction, and to coherent planar twin boundaries. From these observations, it is concluded that the feathery grains are probably the result of a change from a normal 〈100〉 to a 〈110〉 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 〈110〉 growth directions of the side arms. Finally, the proposed mechanisms of feathery grain growth are further supported by the observation of 〈110〉 dendrite growth morphologies in thin aluminum-zinc coatings.