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

Crystal structure characteristics of coexisting beta and delta 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 beta- and the delta-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 delta-like stacking sequences within the beta phase can be considered as a typical planar defect of the beta phase. The beta 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 I4(1)/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. (C) 2018 Elsevier B.V. All rights reserved.