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

Abstract

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.