Aγ'-phase Volume-fraction at Effect on the Hot Corrosion Resistance of Ni-based Superalloys Varying the Compositions Along aγ-γ' Tie Line

Abstract

The effect of the γ'-phase volume-fraction on the corrosion resistance was investigated of nickel-based superalloys in a Na₂SO₄-25wt%NaCL salt. Both Ni-Al-Cr-Ti-Ta-W system and Ni-Al-Cr-Ti-Ta-W-Mo system were used in this study and their compositions lay on a constant γ-γ' tie line. The corrosion resistance was evaluated by means of the coated test, the immersion test and the thermobalance test, and the corrosion products were identified by the EPMA analysis and the conventional X-ray diffraction method. In addition, morphological changes of the γ and the γ' phases at the interface between the salt and the base alloy were observed by scanning electron microscopy (SEM). It was found that the corrosion resistance did not change with the γ' volume-fractions in the alloys as far as they were positioned on the same γ-γ' tie-line. Furthermore, some kinds of oxides and sulfides were formed as the corrosion products in every alloy. The SEM observation of the corrosion interface indicated that a single phase region having a composition similar to that of the γ phase was produced underneath the corrosion products. This region is considered to be formed as the result of fading away the γ'-former elements such as Al and Ti from the surface.