Investigation of Initial Transient Oxidation of Fe–xCr–6at.%Al Alloys Using Synchrotron Radiation During Heating to 1000 °C in Air

Abstract

The initial oxidation behavior of Fe–6Al alloys with different Cr contents (0–24 at.%) during heating to 1000 °C at 50 °C/min, followed by isothermal oxidation for 1 h, was investigated by means of synchrotron radiation in order to clarify the well-known beneficial effect of Cr on external Al2O3 scale formation. An external Al2O3 scale formed on the Cr-containing alloys but did not form on Fe–6Al. A corundum-type transient oxide scale, (Fe,Cr)2O3, formed on all alloys at about 500 °C, and an Al or (Al,Cr)-rich amorphous oxide layer was confirmed to form below the transient scale. This amorphous inner oxide layer was found to provide oxidation resistance up to at least 500 °C for the binary Fe–Al and about 900 °C for the ternary Fe–Cr–Al alloys. Internal precipitates of Al2O3 formed in the alloys when temperatures reached about 900 °C. The volume fraction of these internal precipitates increased with increasing alloy Cr content, which led to be inference that the outward diffusion flux of Al was enhanced by Cr addition.