Oxidation Behavior of Cr2N, CrNbN, and CrTaN Phase Mixtures Formed on Nitrided Cr and Laves-Reinforced Cr Alloys

Abstract

A series of single-phase Cr(X), single phase Cr 2X Laves phase, and two-phase Cr(X) + Cr2 X alloys (X = Nb or Ta) were thermally nitrided for 24 hr at 1100°C in N2--4H2 and then oxidized for 2 hr at 1100°C in air. The Cr(X) phase nitrided to form Cr2N while the Cr2X phases nitrided to form a complex local mixture of Cr2N/Cr and CrNbN/CrTaN, Cr3Nb3N/Cr3Ta3N phases depending on the depth in the nitrided zone. The Ta only slightly increased the isothermal oxidation rate of nitrided Cr(Ta) and Cr2Ta-reinforced Cr alloys, compared with nitrided, unalloyed Cr. Further, the nitrided two-phase alloys Cr--9.5Ta and Cr--20Ta exhibited improved Cr2O3 scale adherence relative to nitrided unalloyed Cr and Cr--1Ta. In contrast, Nb was detrimental to the oxidation resistance of the nitrided Cr(Nb) and Cr2Nb-reinforced Cr alloys, resulting in the formation of nonprotective Cr--Nb oxides rather than continuous Cr2O3. A phenomenological explanation for these effects based on phase chemistry and microstructural distribution is presented. Implications of these results for understanding the oxidation behavior of developmental high-temperature, Laves-strengthened Cr alloys, as well as possible applications as oxidation and wear-resistant coatings are discussed.