β- and δ-Al-Fe-Si intermetallic phase, their intergrowth and polytype formation

Abstract

Crystal structure characteristics of coexisting β and δ phase were investigated in a purely intermetallic Al-Fe-Si alloy and in plate-shaped intermetallic particles formed in a model secondary Al7.1Si1.2Fe0.3Mn alloy. The β- and the δ-Al-Fe-Si intermetallic phase are thermodynamic and crystallographic distinct phases despite some structural similarities. Nevertheless, both phases can appear severely intergrown within single plate-shaped particles after solidification of Fe-containing secondary Al-Si alloys under non-equilibrium conditions which is attributed to the good fit along the (001) planes of both phases. Thereby, few layers of the δ-like stacking sequences within the β phase can be considered as a typical planar defect of the β phase. The β phase exhibits polytype structures for which structural models have been formulated based on ordering of the constituting double layers being shifted by a/2 or b/2 displacements leading to 4 different double layer positions A, B, C, and D. An AB polytype with idealized Acam symmetry actually shows a monoclinic distortion towards A12/a1 symmetry. An ABCD polytype with idealized I41/acd symmetry has been derived, too. Next to the ordered polytypes, stackings with non-periodic sequences of A, B, C and D double layers exist.