Effect of Substrate Roughness on Oxidation Resistance of an Aluminized Ni-Base Superalloy

In the present work, it is shown that the surface preparation method used on two Ni-based superalloys prior to aluminizing chemical vapor deposition (CVD) is one of the most important factors determining the oxidation resistance of aluminized Ni-based superalloys. It was found that grit blasting the substrate surface negatively affects the oxidation resistance of the aluminized coatings. For grit blasted and aluminized IN 625, a thicker outer NiAl coating was formed compared to that of IN 738. In contrast, no effect on NiAl coating thickness was found for grit blasted and aluminized IN 738. However, a thicker interdiffusion zone (IDZ) was observed. It was shown that the systems with grit-blasted surfaces reveal worse oxidation resistance during thermal shock tests, namely, a higher mass loss was observed for both grit blasted and aluminized alloys, as compared to ground and aluminized alloys. A possible reason for this effect of remaining alumina particles originating from surface grit blasting on the diffusion processes and stress distribution at the coating/substrate is proposed.Comment: Accepted manuscript Metals 201

The Analysis of the Residual Stress Evolution during Cycling Oxidation of the Ni-base Superalloys at High Temperature

The lifetime of the elements made of Ni-base superalloys can be strongly increased by introducing compressive stresses. Such stresses increase the resistance of cracks nucleation and formation during cyclic loads. Therefore, it is important to know how the stresses in the cold rolled Ni-based superalloys evolve during the service. Ni-base superalloys are dedicated to the usage at elevated temperature. However, exposing the Ni-based superalloys to high temperature results in their oxidation. So far, not a single work on the studies of the residual stress evolution in the Ni-based superalloys during cycling oxidation at high temperature was performed. Thus in the present study the residual stress in the materials in the as-received conditions and the changes in the residual stresses during cycling oxidation of IN 625 and IN 718 at 1273 K in air was investigated and described. The obtained results showed differences in the residual stresses level measured for investigated alloys. It was also found that thermal cycling of studied alloys influences the residual stresses. However even after the end of the test, the measured residual stresses were still compressive. Slightly different oxidation resistance was found for the studied alloys, namely, IN 718 was found to be more prone to oxide scale spallation. The latter was correlated with different alloy chemical composition, which results in formation of δ - phase in IN 718. The dissolution of δ - phase during high temperature exposure resulted in formation of sub-scale enriched with Nb and Ti in the near oxide scale/substrate interface. The latter was claimed to have a negative effect on oxide scale adherence