Selective oxidation and internal nitridation during high-temperature exposure of single-crystalline nickel-base superalloys

The process of internal nitridation of the three commercial single-crystalline nickel-base superalloys CMSX-2, CMSX-6, and SRR99 has been studied-in air:and oxygen-free nitrogen atmospheres at 800 degrees C to 1100 degrees C using thermogravimetric techniques supplemented by extensive microstructural examinations. non-protective oxide formation, particularly cracking and spalling at edges or curved surfaces, enables nitrogen to penetrate into the alloy leading to the precipitation of stable Ti and Al nitrides. The high-temperature corrosion behavior of the superalloys studied is strongly affected by compositional differences between dendritic and interdendritic areas due to segregation resulting in an inhomogeneous internal precipitation zone. Furthermore, the stability of the strengthening gamma' phase (Ni-3(AI, Ti, Ta)) in front of the growing internal-nitridation zone was observed to depend clearly on the alloy composition. Therefore, the near-surface area of the alloys can be weakened by gamma' depletion and by embrittlement resulting from internal-nitride: precipitation. The results obtained on the nickel-base superalloys are discussed, taking into account the results of a computer-based simulation of internal-corrosion processes. Furthermore, results on Ni-base model alloys of the system Ni-Cr-Al-Ti provided information on the role of the alloy composition. It was found that a higher Cr concentration seems to increase the nitrogen solubility and diffusion in Ni-base alloys.