Oxidation Protection of γ-TiAl Alloys by Intermetallic Ti-Al-Cr-Zr Coatings

Abstract

The oxidation behavior of γ-TiAl specimens coated with an intermetallic Ti-49Al-34Cr-4Zr layer was investigated at 1000°C under cyclic conditions in laboratory air. The 11 μm thick coating was produced using a combined technique of high power impulse magnetron sputtering and unbalanced magnetron sputtering. The as-deposited coating exhibited a dense layered structure and excellent adhesion to the substrate. The Ti-Al-Cr-Zr coating possessed high oxidation resistance associated with the formation of a thin continuous alumina scale for exposure time periods exceeding 1000 cycles of 1 h dwell time at 1000°C. During the high temperature exposure, the coating being amorphous in the as-deposited condition became crystalline exhibiting different polytypes of Ti(Cr,Al) 2 Laves phases with Ti probably partially substituted by Zr and Nb. Due to alumina formation and interdiffusion the coating was depleted in aluminum and chromium as well as enriched in titanium. After 1000 cycles at 1000°C, the coating consisted of an outer layer of the hexagonal C14 Laves phase and an inner layer of a probably orthorhombic phase whose structure was not yet determined. In both layers, pores and fine precipitates rich in Zr and Y were found.