Intermediate temperature internal oxidation in fourth generation Ru-bearing single-crystal nickel-base superalloys

Fourth-generation, Ru-bearing, Ni-base superalloys offer superior microstructural stability over previous generations, affording greater density-corrected creep strength, but in common with their predecessors, have only borderline ability to form and maintain a protective oxide layer. This is particularly so at intermediate temperatures of around 750 degrees C at which temperature internal oxidation can occur. In this present study, the nature of this attack has been examined in experimental 3 and 5 mass % Ru alloys. Both these alloys exhibited dispersed regions of oxide-filled pits whose depth increased parabolically with isothermal exposure time at a similar rate. Pits appear to nucleate progressively straight from the initial competitive oxidation stage. In all cases, the pits were associated with a surface mound of nickel- and cobalt-rich oxides. The results of detailed metallographic examination of the internal oxidation products are provided.