Influence of surface roughness on the oxidation behavior of a Ni-4.0Cr-5.7Al single crystal superalloy

The high-temperature oxidation dynamics and mechanisms of a Ni-based single crystal superalloy with four kinds of surface roughnesses were investigated by virtue of XRD, OM, SEM and EDS at 1000 degrees C. In the initial oxidation stage, outer (Ni, Co)O was mainly produced on the surfaces of the samples with Ra = 90 nm and 19 nm. Correspondingly, outer Cr2O3 and transient theta-Al2O3 were mainly formed on the surfaces with Ra = 509 nm and 182 nm. After 180 min oxidation, the values of instantaneous parabolic mass gain coefficients (k(p)) of the samples with all surface roughnesses were gradually consistent with the data of the growth parabolic coefficient of alpha-Al2O3. The oxidation mechanisms of Ni-based superalloy with different surface roughnesses were discussed by a model. The external diffusion flux of Al (D-Al) increases with the increases of surface roughness. Thus, the required Al concentration decreases with the increases of surface roughness when the selective oxidation of Al occurrs to form a protective single alpha-Al2O3 film. (C) 2018 Elsevier B.V. All rights reserved.