Oxidation–Nitridation Mechanism in Eutectic Cr-Silicide Alloy and Its Mitigation by Germanium Alloying

Abstract

The oxidation and nitridation of the eutectic Cr–Cr3Si alloys during exposures in air was investigated with focus on the detailed mechanism of the attack. Exposures in synthetic air (N2–21%O2), and nitrogen (N2–5%H2) atmospheres revealed inward diffusion of nitrogen as the key mechanism for the nitridation process which was markedly higher in nitrogen compared to that in air. Post-exposure investigations proved selective nitridation of the Cr-solid solution phase in both synthetic air and N2–5%H2 atmosphere. During oxidation in air, a continuous A15 silicide layer (Cr3Si) formed at the oxide–alloy interface inhibiting the internal nitridation. This layer formed exclusively in oxidizing atmosphere as a result of the primary oxidation and outward diffusion of chromium, and dissolution of the Cr-solid solution phase. The barrier character of the A15 phase against nitridation was evident based on microstructural and chemical investigations as well as on thermodynamic considerations. Long-term oxidation results indicated that overtime the protective A15 layer was damaged by the formation porosities and internal oxidation during exposure which led to internal selective nitridation. Additions of germanium (Ge) was found beneficial by supporting the formation of the self-protecting A15 layer with long-term protective behavior against internal nitridation for more than 1000 h.