Characterization of quasicrystalline primary intermetallic particles in Mg-8wt% Zn-4 wt% Al casting alloy

The quasicrystal structure of primary intermetallic particles found in the as-cast microstructure of a permanent mould-cast Mg-8 wt% Zn-4 wt% Al alloy has been characterized using transmission electron microscopy. Most primar y intermetallic particles have an average composition of Mg9+/-2Zn4+/-1Al3/-1 and a primitive icosahedral structure, with a quasilattice parameter of 0.515 nm and a point group of m (3) over bar(5) over bar. The metastable q uasicrystalline phase persists for extended periods at elevated temperature s; while the volume fraction decreases with thermal exposure at 325 degrees C, the decrease is small up to 120 h and a fraction persists after 720 h. T here is no evidence of transformation of or within the quasicrystalline pha se during this period. Instead, the metastable quasicrystalline phase is gr adually replaced by the equilibrium Phi phase (Mg-21(Al, Zn)(17); space gro up, Pbcm). The relationship between the quasicrystalline phase, its cubic a pproximant T phase (Mg-32(Al, Zn)(49); space group, Im (3) over bar) and th e Phi phase is discussed.