AlCuFe(Cr) and AlCoFeCr coatings for improvement of elevated temperature oxidation resistance of a near-α titanium alloy

Abstract

In this work, an AlCuFe, AlCuFeCr and AlCoFeCr multicomponent coatings were deposited on a near-α titanium alloy, TIMETAL 834, by a non-reactive magnetron sputtering. A microstructure, phase composition, coatings adhesion to the substrate, micro-mechanical properties and oxidation resistance were investigated. The AlCuFe coating was quasicrystalline and consisted of icosahedral ψ-Al65Cu20Fe15 phase grains. The AlCuFeCr coating consisted mainly of decagonal d-AlCuFeCr phase and some Al6.1Cu1.2Ti2.7 phase grains. The AlCoFeCr coating microstructure was composed of AlCrFe2 nanocrystals embedded in an Al13Co4 matrix.It was established that all multicomponent coatings exhibit higher hardness and elastic moduli than the titanium alloy substrate. Among all investigated coatings, the AlCoFeCr one exhibits the best adhesion to the substrate. It was found that a very brittle, non-protective scale formed on the uncoated alloy during isothermal oxidation consisted of TiO2 (rutile) grains. The oxidation resistance of the TIMETAL 834 alloy at elevated temperature (750°C) was significantly improved by the coatings deposition. The smallest weight gain during oxidation test, 82% lower than in the case of uncoated sample, was found for the AlCoFeCr coated alloy. The microstructure investigation of the specimen after oxidation test was performed by SEM and TEM. It was found that the improvement of oxidation resistance of TIMETAL 834 by the coatings deposited in the present work is mainly due to an occurrence of the protective α-Al2O3 scale.