Quasicrystalline phases and their approximants in Al-Mn-Zn alloys

Two Al-Mn-Zn alloys in melt-spun condition have been studied by transmission electron microscopy. The $A1_{24}Mn_5Zn$ alloy was found to be fully icosahedral, while the $Al_{12}Mn_{2.9}Zn$ alloy gave rise to decagonal quasicrystal. The decagonal phase grew in clusters with an orientation relationship between the grains suggesting nucleation on an icosahedral seed. On annealing at $500-6OO^0C$, the quasicrystalline phases transformed to a body centered orthorhombic phase L (a = 1.24, b = 1.26 and c = 3.05 nm) with a high density of planar defects. This phase transforms to an ordered and defect free monoclinic phase M, a superlattice structure of L (a = c = 1.77, b = 3.05 nm and $/beta = 89. 1^0)$. The L phase is shown to be a rational approximant of the icosahedral phase. The interrelationship among quasicrystalline phases and their rational approximants in Al-Mn-Zn system are highlighted