The role of high-temperature water vapor on oxidation behavior for CoNiCrAlHf alloys at 1100 °C

Abstract

The external atmosphere can significantly affect the microstructure within the oxide scale of alloys during high temperature oxidation. In this paper, we investigate the isothermal oxidation of CoNiCrAlHf alloys in dry air and water vapor (Ar-25%H2O and air-25%H2O). The different test atmospheres led to very different results. HfO2 peg formed at the Al2O3/alloy interface duo to in-situ oxidation. The presence of water vapor significantly increases the oxygen potential gradient and the diffusive flux of Hf, causing the linear distribution of HfO2. In addition, water vapor has effectively retard the phase transformation of metastable-Al2O3 to α-Al2O3. In high temperature water vapor, the CoNiCrAlHf alloys form a duplex-layer, consisting of external δ-Al2O3 and internal γ-Al2O3. The distinction can be attributed to the promotion of defect concentration due to the adsorbed water molecules. The formation of Al vacancies would promote the outward diffusion of Al and the inward diffusion of O, which greatly facilitates the formation of γ -Al2O3.