MICROSTRUCTURAL STUDIES ON RAPIDLY SOLIDIFIED MG32(ALXZN1-X)49 POTENTIALLY QUASI-CRYSTAL FORMING ALLOYS

Rapidly solidified microstructures of four different alloys in the Mg3 2(AlxZn1-x) series, Mg32Al49, Mg32Al24.5Zn24.5, Mg32Al17Zn32 and Mg32A l7Zn42 were examined using transmission electron microscopy. A single- phase microstructure comprising grains of a low symmetry quasiperiodic derivative of the icosahedral phase was found to be present in the fi rst alloy, while icosahedral phases showing considerable distortion of the icosahedral symmetry were found to coexist with different crystal line phases in the second and the third alloys. Certain features, nota bly sectorial division and particulate contrast, were found to be comm on to the icosahedral phases in the second and the third alloys and th e derivative of the icosahedral phase in the first alloy. The extent o f departure from ideal icosahedral symmetry appears to be a function o f the [Al]/[Zn] ratio. The crystalline phase in the third alloy was id entified as MgZn2 (C14), which is a well known tetrahedrally close-pac ked phase. This phase comprises the bulk of the fourth zinc-rich alloy . A propensity for faulting was observed in the MgZn2 phase in both th ese alloys. These and several other finer microstructural features are discussed in the light of the experimental evidence and relevant anal yses of earlier reports on this system.