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

Abstract

Crystal structure characteristics of coexisting  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 βand 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 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.

Effect of Mn and cooling rates on α-, β- and δ-Al–Fe–Si intermetallic phase formation in a secondary Al–Si alloy

“…A fraction of the plate-shaped particles exhibits stripes in dark and light gray contrast upon SEM imaging with BSE, which can be attributed to the simultaneous occurrence of the β and the δ phase in the same plate-shaped particle according to [49]. The light gray stripes occur irregularly distributed, mostly at the edges and/or sometimes in the center of the plates.…”

“…In a ternary Al7Si0.9Fe alloy, the β phase formed at cooling rates less than 2.7 K/s and the δ phase formed between 2.7 and 4.4 K/s [45]. However, the microstructural appearance of the β and/or δ phase in plate-shaped particles had not been clearly shown until a related study [49]. There, the simultaneous presence of the β and the δ phase in plate-shaped particles is scrutinized.…”

“…a and b are the in-plane translation basis vectors of the double layer. An elaborate description of the crystal structures of the β and the δ phase can be found in a related study [49]. Additionally, severe intergrowth of the β and the δ phase within a plate-shaped particle was encountered [49].…”

“…An elaborate description of the crystal structures of the β and the δ phase can be found in a related study [49]. Additionally, severe intergrowth of the β and the δ phase within a plate-shaped particle was encountered [49]. Consequently, correct identification of the phases present within the plate-shaped particles requires careful choice of analytical method, including access to crystallographic information data.…”

Corrosion-Resistant Steel–MgO Composites as Refractory Materials for Molten Aluminum Alloys

“…It is obvious that pure steel is unable to withstand even brief contact with molten aluminum [ 1 , 2 , 9 ]. Liquid aluminum alloy dissolves iron and forms complex phases in the ternary Al-Si-Fe system [ 3 , 24 , 25 ]. The dissolution of steel is continuously progressing as a function of time and does not omit other steel elements such as chromium or nickel [ 4 , 26 , 27 , 28 , 29 ].…”