Crystallization and precipitation structures of quasicrystalline phase in rapidly solidified Al-Mn-X ternary alloys

Tranmission electron microscopic observations were carried out to reveal the solidification and precipitation structures of rapidly solidified Al-Mn-X alloys (X=Si or Zr), with particular focus on the formation of the icosahedral quasicrystals. On increasing the manganese content, the distribution of the icosahedral quasicrystals in the rapidly solidified alloys changed from the type of cell-boundary segregation to that of one component in the eutectic and the primarily dendritic phase. The precipitation of the icosahedral quasicrystals from the supersaturated solid solutions of the present alloys was restricted in the surface layers and/or largeangle grain boundaries. The orientation relationships between the icosahedral quasicrystals and the aluminium matrix were such that three mutually perpendicular two-fold axes of the icosahedron lie along the [100], , and [011] axes of the fcc crystallographic directions. The 15-fold symmetry diffraction pattern frequently obtained from (111) and/or (233) matrix planes could be explained by considering axial rotation between the five-fold and three-fold symmetry axes of the icosahedron and cubic lattices. It was suggested that the hierarchy of the free energies of the icosahedral phase and the other crystalline phases was very close.