Oxidation and internal nitridation behaviors of a Ni-based superalloy Rene 65 during high-temperature homogenization

Homogenization heat treatment is a key process to remove the micro-segregation and re-dissolve the undesired phases for wrought superalloy. The oxidation behavior of the wrought superalloy during the high-temperature homogenization process, however, was rarely studied. The oxidation film evolution and growth kinetics of an as-cast superalloy Rene 65 during the homogenization were systematically studied. The oxide film consists of Cr2O3 external oxidation layer and dendritic TiO2 and Al2O3 internal oxidation layer. And the growth kinetics of the oxide film followed a parabolic law. Internal nitridation occurs during the oxidation process, and TiN is apparently formed at the tip of internal oxidation layer. The originally formed TiN can be transformed into TiO2 or retained with the progress of oxidation. Meanwhile, the TiN is newly formed in the deeper matrix at the new oxidation-layer tip. Thermodynamic analyses revealed that there is a competition between the oxidation and nitridation. Nitridation can occur when the partial pressure of nitrogen exceeds the threshold of nitridation and the critical partial pressure ratio of nitrogen and oxygen.