Surface orientation dependent oxidation behavior of aluminized DS CM 247 LC nickel-base superalloy

A study has been carried out into the oxidation behavior of a directionally solidified superalloy. Three types of specimens were cut transversely (T), longitudinally (L) and at 45° angle (45°) in relation to the solidifying direction of the DS CM 247 LC rods. These specimens were aluminized by the pack cementation method and then were oxidized at 1000 °C for 200 h. The coating grows epitaxially on (0 0 1) surfaces of the directionally solidified superalloy. Epitaxy of the coating on the L specimens is also evident. All the coatings on the above specimens consist of a precipitation-rich layer, a precipitation-free layer and an interdiffusion layer. Lamellar alpha-Cr precipitates directly in the substrate underneath the interdiffusion layer during the aluminization process, and in the subsequent interdiffusion process during the oxidation tests. The formation of the θ-Al2O3 leads to a fast oxidation rate for the T, L and 45° specimens in the transient stage. The 45° specimens oxidized at the fastest rate, while the T specimens gave the smallest weight gain. After 20 h, further weight gains of all these specimens are very small, due to more stable α-Al2O3 growth and transformation from θ-Al2O3 to α-Al2O3. The slower oxidation rate in the case of the T specimens might be a result of epitaxial growth of θ-Al2O3 on the β-NiAl coating.