Oxidation behavior of Cr₂N, CrNbN, and CrTaN phase mixtures formed on nitrided Cr and laves-reinforced Cr alloys

A series of single-phase Cr(X), single phase Cr₂X Laves phase, and two-phase Cr(X) + Cr₂X 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 Cr ₂N while the Cr₂X phases nitrided to form a complex local mixture of Cr₂N/Cr and CrNbN/CrTaN, Cr₃Nb ₃N/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, Loves-strengthened Cr alloys, as well as possible applications as oxidation and wear-resistant coatings are discussed.