The quasicrystalline (qc.) phase with the icosahedral symmetry in melt-spun Al100−x Mnx (x=14, 17, 20) has been investigated by X-ray diffraction, TEM and DSC. Nearly single phase quasicrystal is obtainable for Al80Mn20 when the quenching rate is sufficiently fast, while slower quenching rate produces an alternative metastable phase called T phase. On heating, the qc. phase in Al80Mn20 changes to T phase at around 300°C and then at around 600°C to the stable hexagonal Al4Mn, whereas the qc. phase in Al86Mn14 crystallizes directly to the stable orthorhombic Al6Mn. Heat evolution of the latter reaction is 0.40 kcal/mol, while those of the qc. phase to T phase and T phase to the hex.-Al4Mn in Al80Mn20 are 0.12 and 0.17 kcal/mol, respectively. Atomic model of the qc. phase with a stoichiometric composition of Al78.35Mn21.65 is proposed.
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