Orthorhombic approximant phases and their relation to quasicrystals in mechanically alloyed Al-Cu-Fe-Si alloys

The possibility of synthesis of quasicrystalline and associated approximant phases during mechanical alloying and subsequent heat treatment of Al 65 Cu 20 Fe 10 Si 5 alloy powders has been investigated. It is shown that, in addition to the stable ordered quasicrystal and 1/1 cubic approximant, two more phases having orthorhombic symmetry coexist. The a and c parameters of both these phases are similar to the lattice parameters of the cubic approximant, whereas their b parameters are, respectively, three and four times larger than the c parameters. It is suggested that these phases evolve on account of local symmetry breaking of the cubic approximant phase, possibly due to ordering.