Relationships between the liquidus temperature and the formation of quasicrystalline phases in rapidly solidified Al-Cu-Mn alloys

Abstract

The alloys Al 75Cu 10Mn 15 (T 1), Al 57Cu 32.5Mn 10.5 (T 2) and Al 65Cu 20Mn 15 were prepared by rapid solidification from the melt. The flakes thus obtained were investigated by means of differential thermal analysis (DTA), X-ray diffraction (XRD) and transmission electron microscopy (TEM) in the as-quenched state and after annealing. The occurrence of icosahedral (i) and decagonal (d) quasicrystalline phases was observed in the as-quenched alloys. The i and d phases are both metastable and transform on heating into crystalline phases. The nature of the as-formed crystalline phases has been identified and found to be in agreement with those expected from the equilibrium phase diagram. For a heating rate of 10°C min −1, the d phase begins to be decomposed above 600°C and the i phase above 400°C; the d phase is therefore more stable than the icosahedral phase. The liquidus temperatures of each alloy were determined by DTA on heating. By assuming that the undercooling interval is the same for each alloy during the quench, it is shown that the formation of the i or d phase is related to the respective liquidus temperatures of each alloy.