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

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.