The crystal structure of the equilibrium Φ phase in Mg–Zn–Al casting alloys

Abstract The crystal structure of the equilibrium intermetallic Φ phase formed in a Mg–Zn–Al casting alloy has been characterised using transmission electron microscopy. Electron diffraction patterns recorded from particles of the Φ phase in the casting alloy can be well indexed according to a primitive orthorhombic unit cell, with lattice parameters a =0.90 nm, b =1.70 nm, and c =1.97 nm. Examination of the whole pattern symmetry of principal zone axis diffraction patterns indicates a space group of Pbcm . A model for the decoration of the unit cell of the Φ phase is proposed, in which the Mg 5 (Zn,Al) 12 Friauf polyhedron is the key structural unit. The Zn and Al atoms are all in icosahedral coordination, but their icosahedral shells are distorted due to the presence of Mg atoms. A total of 84 Mg atoms and 68 Zn/Al atoms can be accommodated in the orthorhombic unit cell, resulting in a formula of Mg 21 (Zn,Al) 17 that is consistent with the composition obtained experimentally. Computer simulations of electron diffraction patterns provide very good agreement with experimental observations.