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 Mg₅(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 Mg₂₁(Zn,Al)₁₇ that is consistent with the composition obtained experimentally. Computer simulations of electron diffraction patterns provide very good agreement with experimental observations.