Crystallographic features of the primary Al3Zr phase in as-cast Al-1.36 wt% Zr alloy

The habit and morphology of the primary Al3Zr crystals formed during the solidification of Al-1.36 wt% Zr alloy were analyzed. It was observed that the crystals have a tabular habit. The longitudinal cuts of the smaller and larger sized crystals appear, respectively, in octagonal and rectangular forms. The primary crystals also appear in the dendritic form. A crystallographic study indicates that the smaller sized crystals are bound by {0 0 1}, {1 0 1} and {1 1 1} faceted planes, whereas the larger sized ones by {0 0 1} and {1 0 1} faceted planes. One compound twin with two variants is found in the dendritic crystals and the complete twinning elements are determined. The tabular habit of the primary crystals is attributed to the low roughness of {0 0 1} planes. The disappearance of {1 1 1} planes in the larger sized crystals result from their high roughness. The twin formation is realized by lattice shear plus local atom stacking fault accompanied by local atomic reshuffling. The dendritic crystals are the result of the formation of the compound twin.