Evidence for endothermic quasicrystalline-crystalline phase transitions in Al<SUB>6</SUB>CuMg<SUB>4</SUB>

Since the discovery of quasicrystalline phases with icosahedral symmetry in an Al-Mn system by Shechtman et al., a variety of other quasicrystals have been reported. Recently a new class of quasicrystalline alloys has been predicted and produced (two prominent members of this class are Al6CuLi3 and Al6CuMg4) which has resulted in the production of large quasicrystals. Large (~ millimetres) quasicrystals of Al6CuLi3 have already been reported. We have used electron microscopy, differential scanning calorimetry (DSC) and X-ray diffraction to investigate the quasi-crystalline-crystalline phase transition in the rapidly solidified Al6CuMg4 alloy system. We present the first evidence that an endothermic quasicrystalline-crystalline transition occurs in this sytem at ~340 °C. This is in contrast with the exothermic crystallization of quasicrystalline Al86Mn14 (ref. 4) and suggests that certain quasicrystalline phases may be stable (as recently observed in the Al6CuLi3 system). This study also reveals that a close structural relation exists between crystalline and quasicrystalline phases of Al6CuMg4