Superior oxidation resistance of ultrafine Ni-Zr-(Al) eutectic composites in the temperature range of 500-900 °C

The oxidation behavior of (Ni0.92Zr0.08)(100)Al--x(x) (0 <= x <= 4 at.%) ultrafine eutectic (gamma-Ni + Ni5Zr) composites (UECs) with average lamellae thickness (lambda(w)) in the range of 150-275 nm and the constituent individual phases, have been investigated systematically in the temperature range of 500-900 degrees C up to 120 h of exposure. The specific mass gain (Delta m) is low and varies between 0.91 mg/cm(2) (x = 1) and 1.67 mg/cm(2) (x = 4) at 500 degrees C, and between 10.36 mg/cm(2) (x = 0) and 11.20 mg/cm(2) (x = 2) at 900 degrees C. The mass gain has increased with temperature due to the enhanced diffusivity of the ions in presence of large fraction of the lamellae interface in the eutectic matrix. Furthermore, the mass gain has increased upon Al addition due to the refinement of the lambda(w) from 275 nm (x = 0) to 150 nm (x = 3). The oxide scale is comprised of mainly NiO, which forms at the scale/air interface due to the oxidation of Ni5Zr and the reaction between outwardly diffused Ni-ions with oxygen. The oxidation kinetics follow nearly parabolic at 500 degrees C exhibiting the rate exponent n = 0.45-0.56 and sub-parabolic (n = 0.21-0.37) in the temperature range 600-900 degrees C, causing decrease of Delta w with exposure time, which is lower than that of eutectic superalloys. The mechanism of oxidation of the UECs, and the mass gain are explained considering the phase fraction and Delta w of individual eutectic phases using an analytical model. (C) 2020 Elsevier B.V. All rights reserved.

Automated summary

The oxidation behavior of (Ni0.92Zr0.08)(100)Al--x(x) ultrafine eutectic composites was systematically investigated at temperatures ranging from 500 to 900 degrees C. The addition of Al was found to enhance the oxidation resistance of the composites by refining the lamellae thickness and decreasing the mass gain.