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

Rapidly solidified microstructures of four different alloys in the Mg32(AlxZn1-x) series, Mg32Al49, Mg32Al24.5Zn24.5, Mg32Al17Zn32 and Mg32Al7Zn42 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 first alloy, while icosahedral phases showing considerable distortion of the icosahedral symmetry were found to coexist with different crystalline phases in the second and the third alloys. Certain features, notably sectorial division and particulate contrast, were found to be common to the icosahedral phases in the second and the third alloys and the derivative of the icosahedral phase in the first alloy. The extent of departure from ideal icosahedral symmetry appears to be a function of the [Al]/[Zn] ratio. The crystalline phase in the third alloy was identified as MgZn2 (C14), which is a well known tetrahedrally close-packed phase. This phase comprises the bulk of the fourth zinc-rich alloy. A propensity for faulting was observed in the MgZn2 phase in both these alloys. These and several other finer microstructural features are discussed in the light of the experimental evidence and relevant analyses of earlier reports on this system.