Structurally complex metallic coatings in the Al-Cu system and their orientation relationships with an icosahedral quasicrystal

Quasicrystals have been identified as alloys possessing unusually low surface energy. This results in poor adhesion properties of quasicrystalline coatings when deposited on metallic substrates, hindering the development of these new materials for technological applications. Here we investigate the possible use of complex Al-Cu metallic phases as interface layers to accommodate the structural and electronic mismatch between a quasicrystalline coating and a metallic substrate and improve adhesion. First, we show that all stable low-temperature phases of the Al-Cu system can be grown as thin films using DC magnetron sputtering. Among the various possible phases, we select the γ-brass γ-Al4Cu9 as a promising candidate for the interface layer. Then the γ-Al4Cu9 phase is grown on the fivefold surface of an icosahedral (i-) Al-Pd-Mn quasicrystal. The interface is investigated by transmission electron microscopy and shows a clear texturing of the film. The grains exhibit rotational epitaxy with the substrate. We find that the interface is mainly composed of a ß-phase of unknown chemical composition and sometimes exhibits γ grains in direct contact with the quasicrystalline substrate. Occasionally, we observe a fourth phase at the ß/γ interface, identified as ß1, possessing a lattice parameter aß1 equal to 2aß and 2/3aγ.