Microstructural changes in the sub-surface area of Ni-base superalloys as a consequence of oxide scale failure

In the present study, the isothermal and cyclic high-temperature corrosion behavior of three single-crystalline Ni-base superalloys were characterized applying a thermogravimetric method as well as thorough microstructural examinations. Under cyclic exposure conditions, all alloys exhibited oxide cracking and spalling giving rise to internal oxidation and nitridation processes. Significant microstructural changes in the sub-surface area resulted which extended far into the bulk material. One important process is the Al and Ti consumption due to internal oxide and nitride precipitation (Al2O3, AIN, and TiN). This leads to a dissolution of the ordered gamma' phase deteriorating the usually outstanding creep resistance of these alloys. It is shown that the extent of gamma' dissolution can be predicted taking the oxidation and nitridation kinetics into account by applying a finite difference diffusion calculation in combination with thermodynamic considerations performed by means of the software packages ChemSage and ChemApp.