FORMATION, MICROSTRUCTURE, CHEMICAL LONG-RANGE ORDER, AND STABILITY OF QUASI-CRYSTALS 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 Al and Pd exists in a face-centered icosahedral lattice in which Pd and Mn occupy one superlattice site and Al occupies the other one. A dendritic growth morphology in a mixed structure containing i- and fcc Al 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.