Anti-oxidation characteristics of Cr-coating on surface of Ti-45Al-8.5Nb alloy by plasma surface metallurgy technique

TiAl-based alloys have received extensive attention recently due to their excellent properties. However, the weak oxidation resistance at temperatures higher than 800 ^°C can limit their further high-temperature structural applications. To improve the oxidation resistance of a high-Nb-content γ -TiAl alloy (Ti-45Al-8.5Nb, in units of at.%), a chromium (Cr) coating is prepared by using the plasma surface alloying technique, separately, at 800 ^°C and 1000 ^°C. The x-ray diffraction (XRD) patterns reveal that an oxide surface layer consisting of Cr₂O₃, Al₂O₃, and TiO₂ is produced on the Cr-coated Nb containing γ -TiAl substrates during the initial oxidation. However, the Cr₂O₃ is dominated in the oxide surface layer after being isothermally oxidized for 300 h. The oxidation kinetic curves are composed of a parabolic law stage ( ≤ 90 h) and a biquadratic law stage ( ≥ 90 h), fit by weight-gain curves. Due to diffusion in the fabrication process and oxidation process, the Cr-coated specimens have an adhesion force after being isothermally oxidized, specifically 69 N for a specimen after oxidation for 300 h. These results demonstrate that the Cr coating enhances the oxidation resistance and adhesion of a Ti-45Al-8.5Nb alloy, which may provide a new feasible scheme for designing oxidation protection layers.