Oxidation kinetics of microcrystalline Ni–11.5Cr–4.5Co–0.5Al superalloy sheet fabricated by electron beam physical vapor deposition at 900 °C

Abstract

Isothermal oxidation of microcrystalline Ni–11.5Cr–4.5Co–0.5Al (wt%) superalloy sheet fabricated by electron beam physical vapor deposition (EB -PVD) at 900 °C in air was investigated. Mass gain curves were obtained on testing the alloy in dry air. And characterization of the surface morphology and cross-sections was performed on samples isothermally oxidized for different exposure times. Surfaces and cross-sections of the oxidized specimens were studied by scanning electron microscopy (SEM). Phase identification of the oxide scale was performed by glancing angle X-ray diffraction (GAXRD) and energy dispersive X-ray microanalysis (EDX). The experiment results indicated that the oxidation kinetics of microcrystalline Ni–11.5Cr–4.5Co–0.5Al superalloy sheet follows a cubical power law at initial oxidation stage and fourth power law for long oxidation terms. The oxide scales consisted mainly of a mixed oxide at initial oxidation stage and a double-layer scale with an outer NiO oxide layer, and inner mixed oxides exposed for long terms at 900 °C. The fine oxide grains could be formed on microcrystalline superalloy surface. Taking the short circuit diffusion through grain boundaries into account, an oxidation kinetic with cubical power law in mixed oxide and fourth power law in single-phase oxide were derived. And the excellent fitting of the present model to the experimental data was obtained. The results suggested that the growing oxide layer of microcrystalline Ni–11.5Cr–4.5Co–0.5Al superalloy sheet, which controlled the ionic transportation, contained mainly mixed oxide at initial oxidation stage and the scale/substrate interface single-phase oxide for long terms.