Oxidation Behavior of TiAl₃/Al Composite Coating on Orthorhombic-Ti₂AlNb Based Alloy at Different Temperatures

Abstract

This paper reports the oxidation behavior of TiAl₃/Al composite coating deposited by cold spray. The substrate alloy was orthorhombic-Ti-22Al-26Nb (at.%). The oxidation kinetics of the coating was tested at 650, 800, and 950 °C, respectively. The parabolic rate constant for the coating oxidized at 650 °C was k _p = 7.2 × 10⁻² mg·cm⁻²·h^−1/2 for the tested 1200 h. For the coating oxidized at 800 °C, the oxidation kinetics could be separated into two stages with k _p value of 39.8 × 10⁻² mg·cm⁻²·h^−1/2 for the initial 910 h and 17.7 × 10⁻² mg·cm⁻²·h^−1/2 for the stage thereafter. For the coating oxidized at 950 °C, the oxidation kinetics can be separated into three stages with k _p of 136.9 × 10⁻² mg·cm⁻²·h^−1/2 in the first 100 h, followed by 26.9 × 10⁻² mg·cm⁻²·h^−1/2 from 100 to 310 h, and 11.8 × 10⁻² mg·cm⁻²·h^−1/2 from 310 to 1098 h. XRD, SEM, and EPMA were used to study the microstructure of the coating. The results indicated that the oxidation took place throughout the entire coating instead of only at the surface. The aluminum phase in the composite coating was soon oxidized to Al₂O₃ in all tested cases. The aluminum in TiAl₃ phase was depleted gradually and oxidized to Al₂O₃ along with the degradation of TiAl₃ to TiAl₂ and TiAl as the temperature increased and time proceeded. AlTi₂N was also a typical oxidation product at temperature higher than 800 °C. The experimental results also indicated that the protection of the coating was attributed greatly to the interlayer formed between the coating and the substrate.