Oxidation behaviour of chromium-based Cr(Fe)-NiAl bcc-superalloys between 900 and 1200 °C

Recently explored chromium-based ‘bcc-superalloys’ strengthened by ordered-bcc nickel-aluminide precipitates in the Cr(-Fe)-NiAl system have shown remarkable coarsening resistance and high temperature yield strength. However, little is known of their performance and mechanisms under high temperature oxidative environments. In this study, two arc-melted and heat-treated Cr(-10Fe)-5Ni-5Al (at.%) ‘chromium-superalloys’ were investigated after exposures in synthetic air at 900/1000/1200 °C for up to 100 hours using thermogravimetric analysis. Mass change data at 900 and 1000 °C fitted parabolic protective oxide behaviour, however, a significant amount of volatilisation of CrO 3 also occurred at 1200 °C - requiring a modified para-linear approach. Both alloys showed improved oxidation resistance over pure chromium representing a reduction in mass gain of ~71.5% for Cr-5Ni-5Al and ~44% for Cr-5Ni-5Al-10Fe across the temperatures investigated. Post-exposure analysis revealed a dominant external scale of Cr 2 O 3 with discrete particles of (Ni,Fe)Cr 2 O 4 spinel, interestingly with a thin Al 2 O 3 subscale that appeared to have reduced the outward diffusion of chromium, slowing chromia scale growth. Two distinct regions of Al 2 O 3 and AlN internal degradation were also observed, resulting from the dissolution of B2-NiAl. Diffusion depths and the influence of grain boundaries as diffusion highways for oxygen were investigated, and preliminary diffusion coefficients were determined. Iron was found to only have a noticeable effect at 1200 °C, where it resulted in an increased mass change & chromia scale growth - while still outperforming pure chromium. The chromium-based bcc-superalloy design approach has been demonstrated to offer substantial benefits for high temperature oxidation resistance over pure chromium.