Evidence for endothermic quasicrystalline-crystalline phase transitions in Al₆CuMg₄

Abstract

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.