Structure and Stability of Quasicrystalline Al-Mn Alloys

The quasicrystalline (qc.) phase with the icosahedral symmetry in melt-spun Al_100−x Mn_x (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 Al_78.35Mn_21.65 is proposed.