ICOSAHEDRAL AND DECAGONAL QUASI-CRYSTALS IN THE AL-PD-MG SYSTEM

A Frank-Kasper (FK) type icosahedral phase, a Mackay icosahedron (MI) type icosahedral phase and a decagonal phase have been found to form i n the Al-Pd-Mg system. This is the first example in which the three ty pes of quasicrystalline phases are formed in the same alloy system. Th e formation ranges in the melt-quenched state are 5-20 at.% Pd and 20- 45 at.% Mg, about 30 at.% Pd and 15-20 at.% Mg, and about 20 at.% Pd a nd 0-10 at.% Mg, for the FK-type icosahedral phase, MI-type icosahedra l phase and decagonal phase, respectively. In particular, the FK-type icosahedral phase with the compositions around Al43Pd14Mg43 has been s hown to be thermodynamically stable. Large grains of the order of 0.1 mm in size have been grown through slow solidification for the stable FK-type icosahedral phase. The stability of various FK-type icosahedra l phases has been discussed in view of atomic packing efficiency and e lectron band structure energy.