THE DELTA-AL4CU9 PHASE AS AN APPROXIMANT OF QUASI-CRYSTALS

This paper analyses the structural properties of the delta-Al4Cu9 phas e as an approximant to quasicrystals. The discussion is developed alon g two directions. One is the valence electron concentration (VEC) corr elation; both the delta-Al4Cu9 and Al-Cu-Fe icosahedral phases are Hum e-Rothery intermetallics with similar values of VEC per unit volume. T he other is atomic structural resemblance analysis; delta-Al4Cu9 can b e viewed either as six-layer stacking of flat and puckered layers alon g one [110] direction or as parallel pentagonal columns aligned along the same direction. It is also pointed out that vacancy sites play two important roles: to regulate a relatively constant VEC per unit volum e and to facilitate the formation of basic pentagonal configurations.