Effect of Annealing on the Growth and the Microstructure of Thermally-grown Oxides on Foils of Alloy 617

Abstract

The effect of annealing on the growth and the microstructure of thermally-grown oxides (TGOs) of foils of alloy 617 were investigated. Oxidation tests were performed on similar to 100 mu m thick foils. Three types of foils, namely as-rolled (A-R), rolled-annealed at 950 degrees C (R-A:950) and rolled-annealed at 1100 degrees C (R-A:1100) were oxidized at 1000 degrees C. The surface and the cross-sectional microstructures, the inter-diffusion behavior and the chemical compositions of the TGOs were investigated by using scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX). The surface roughness of the TGOs, the voids in the oxide scale, and the cracks in the oxide nodules were suppressed for the R-A:1100 foils. According to EDX-mapping, the surface oxide was observed to be chromia (Cr2O3), and internal oxides were observed to be alumina (alpha-Al2O3). Decarburization was observed beneath the superficial oxide scales while the thickness of carbide layer increased towards the interior of the metal.