Effects of a NiFe co-deposited layer on α-Al2O3 formation by oxidation of a β-NiAl alloy

Abstract

Effects of Ni metal, Fe metal or NiFe alloy deposits on the oxidation behavior and oxidation products of a β-NiAl alloy at 1000°C in air were studied. The surface morphologies of the deposited layers and oxide scales were examined by using field emission scanning electron microscope (FE-SEM) and transmission electron microscopy (TEM). The chemical compositions of the deposited layers were determined before and after oxidation by using energy dispersive X-ray (EDX). The deposited layer and the resultant oxidation products were identified by using X-ray diffractometer (XRD). The chemical composition of the deposited layers was found to affect the morphology of the deposits and the final oxidation products. By oxidation at 1000°C in air, θ-Al2O3 was detected for bare, Ni-coated and Ni-rich coated (Ni17.7at.%Fe) samples, but was not detected for Fe-coated and Fe-rich coated (Ni72 at.%Fe) samples at all oxidation times. The oxidation mass gain after 100h on Fe-coated sample was the highest among the coated samples. TEM Cross-sectional images revealed that the grain size of α-Al2O3 on both Fe coated and Fe-rich NiFe coated samples were the smallest among the coated samples. XRD and EDX point analysis confirmed the formation of different multilayered oxide scales on oxidized samples, NiO/NiAl2O4/Al2O3 on Ni coated, complex spinel Ni(Fe)Al2O4/Al2O3 on Ni-rich NiFe coated, Fe2O3/(Fe, Ni, Al)2O3/α-Al2O3 on Fe-rich NiFe coated and Fe2O3/α-Al2O3 on Fe-coated samples, respectively. Introducing Fe or Fe-rich NiFe coating layer prior to the oxidation of a NiAl alloy resulted in suppression of θ-Al2O3 formation and finer grains of α-Al2O3, which accelerated the growth rate of α-Al2O3 scale. Up to 72at.% of Fe in NiFe coating was found to be beneficial as an oxidation pretreatment for NiAl alloys to obtain a better high temperature oxidation resistance.