Microstructure development and properties of the AlCuFe quasicrystalline coating on near-α titanium alloy

Abstract A protective quasicrystalline AlFeCu coating was deposited on TIMETAL 834 substrate by nonreactive magnetron sputtering in order to improve resistance of the alloy to oxidation. Microstructure characterisation of the substrate and the coating was performed by analytical scanning- and transmission electron microscopy as well as X-ray diffractometry. Depending on annealing temperature and time, the deposited coating (2.7 μm thick) has a different microstructure. The coating in Specimen 1 (annealed 600 °C/4 h in vacuum) consisted of two zones: outer, composed of Al 5 Fe 2 and Al 2 Cu 3 phases and inner, in which only quasicrystalline ψ phase was present. The coating in Specimen 2 (annealed 600 °C/4 h + 700 °C/2 h in vacuum) was fully quasicrystalline and consisted of icosahedral ψ phase. Both coatings exhibit higher microhardness than the substrate material. It was established that the applied surface treatment essentially improves oxidation resistance of the alloy tested at 750 °C during 250 h in static air. Sample weight gain was 60% lower than in the case of uncoated sample. Oxide scale spallation occurred for uncoated alloy while the coated one did not show any spallation. It was found that the very brittle scale formed during oxidation on the uncoated alloy was consisting of TiO 2 , while that on the coated one consisted mainly of α -Al 2 O 3 .