Thermodynamic assessment of ternary NiCrAl alloys: from calculations to experiments

Oxidation of pure metals is a chemical reaction in which the metal loses one or more electrons, such that the atom of the metal changes from the neutral state to a positively charge ion and reacts with oxygen to form an oxide of the metal. The oxide can form as a protective and adherent scale that slows down further oxidation, or may spall off repeatedly, exposing fresh metal surface to react with oxygen. In multicomponent alloy systems such as typical high temperature NiCrAlY bond coats, more than one oxidation mechanism may take place at once and thus, complicates the study of oxidation. To better understand the competition between formations of oxides of different compositions, a thermodynamic assessment is carried out to study the oxidation mechanism of a Ni-27Cr-9Al (at-%) ternary alloy. Based on the assessment, it is demonstrated that the layering of different oxides can be determined, with the most stable oxide (alumina) existing directly above the metal/oxide interface and the least stable oxide (nickel oxide) locating on the outermost layer. A comparison between the calculated results and experimental observations shows excellent agreement.