Formation, microstructure, chemical long-range order, and stability of quasicrystals in Al-Pd-Mn alloys

The icosahedral (i-) phases are found over a wide composition range in melt-quenched Al-Pd-Mn alloys. The i-phase has a simple icosahedral lattice in the low Pd content region (Pd <10 at. %) and transforms to a face-centered icosahedral with increasing Pd content (Pd >10 at. %). A decagonal phase with a periodicity of 1.6 nm exists at a low Mn content region. On the basis of the change in diffraction peak intensity, it was presumed that the chemical ordering between A1 and Pd exists in a face-centered icosahedral lattice in which Pd and Mn occupy one superlattice site and A1 occupies the other one. A dendritic growth morphology in a mixed structure containing i and fcc A1 phases changes to a planar growth morphology in a fully single i-phase. An interpretation of dependence of morphology on composition is given in terms of the interface stability criterion using a metastable i-Al phase diagram.