FORMATION OF QUASI-CRYSTALS AND APPROXIMANT CRYSTALS BY MECHANICAL ALLOYING IN MG-AL-ZN ALLOY SYSTEM

The formation of quasicrystals and related approximant crystalline phases is studied in the Mg-Al-Zn alloy system by applying mechanical alloying technique. This technique is advantageous over the conventional melt-spinning because of the ease in sample preparation involving only the solid-state reaction. The quasicrystal is formed over a wide composition range. Those formed along the Bergman line corresponding to Mg39.5(Al1-xZnx)60.5 are found to be metastable and to transform into stable (1/1-1/1-1/1) Frank-Kasper phase by evolving an exothermic peak in the DSC run. Its stability is found to increase with decreasing Al content. On the other hand, those formed in the Mg-rich and 12-15 at%Al region off the Bergman line are found to transform into the (2/1-2/1-2/1) approximant phase at temperatures exceeding 400-degrees-C, which accompanies an endothermic peak in the DSC run. It is suggested that the quasicrystal in the latter can exist as a stable phase up to the peritectic temperature, across which it decomposes into the (2/1-2/ 1-2/ 1) approximant and liquid. It is also found that, during mechanical alloying, the C14 Laves phase MgZn2 always grows prior to the formation of the quasicrystal. We believe that the growth of MgZn2 plays the key role as the precursor for the formation of the quasicrystalline phase.