Crystal structure refinement of the Γ- and Γ1-phase compounds in the Fe-Zn system and orientation relationships among α-Fe, Γ and Γ1 phases in the coating layer of galvannealed steel

Abstract

The crystal structures of the Γ- and Γ1-phase compounds in the Fe-Zn system have been refined by single-crystal synchrotron X-ray diffraction. The Γ-phase compound consists of four Fe-centred (Zn,Fe)12 icosahedra connected with one another by face-sharing with the vertex sites of the shared triangle face being occupied by both Fe and Zn atoms. The Fe occupancy at the vertex sites increases with increasing Fe composition of the compound. The stoichiometry of the Γ phase is described as Fe4Zn9 (Zn-30.77 at.%Fe), realized when all the vertex sites of the shared triangle face are occupied exclusively by Fe atoms. The Γ1-phase compound consists of Zn12 and (Zn,Fe)12 icosahedra whose central site is exclusively occupied by Fe atoms, which is different from the previously reported structural model where some centres of these icosahedra are occupied by both Fe and Zn atoms. The crystal structures of all Fe-Zn intermetallic compounds can thus be best understood by considering the packing of the common structural unit; Fe-centred Zn12 and/or Fe-centred (Zn,Fe)12 icosahedron. Although the crystal structures of the α-Fe, Γ and Γ1 phases with cubic symmetries are related with each other in terms of the arrangement of structural vacancies, there is no orientation relationship either between the Γ and Γ1 phases or between the α-Fe and Γ phases in the coating layer of galvannealed steel sheet.