Oxidation Processes for Alloys in the Ni – Zr System. Part 2. Phase Composition of Scale Formed on Ni7Zr2

Abstract

We have used x-ray and metallographic layer-by-layer phase analysis to study the structure and composition of scale formed on the alloy Ni7Zr2 during its oxidation in air over a period of 1 h and 10 h in the temperature range 500-1200°C. In the scale we find NiO, the cubic and monoclinic modifications of ZrO2, and also Ni and Ni5Zr. The phase components are nonuniformly distributed over the thickness of the scale. The outer scale consists of the oxides NiO and ZrO2, while the composition of the inner scale includes Ni and Ni5Zr in addition to monoclinic ZrO2. Cubic ZrO2 is formed on the surface of the specimen in the initial stages of its oxidation at 500-700°C. For T ≥ 900°C, on the surface of the scale we find both modifications of ZrO2, while the nickel phase is itself a solid solution Ni(Zr). We note that the mechanisms for the formation of low-temperature (T ≤ 800°C) and high-temperature (T ≥ 900°C) scales are different. It is hypothesized that these differences are determined mainly by the fact that at high temperatures, diffusion of zirconium ions toward the outer boundary of the scale is superimposed on diffusion of oxygen toward the scale – alloy boundary.