Impurity Segregation to Scale/Alloy Interfaces and its Effect on Interfacial Properties

Abstract

This paper summarizes experimental results on the chemical changes at Al 2 O 3 /alloy interfaces as a function of oxidation time, temperature and cooling rates. Surfaces of FeCrAl, FeAl and Fe 3 Al alloys after removal of the scale in ultra high vacuum were analyzed using an Auger microprobe. Sulfur was the only segregant on FeAl and Fe 3 Al, but C and Cr were also present on FeCrAl, where all of the C and some of the Cr segregated during cooling. The S content on FeCrAl built up quickly with oxidation time, reaching a saturation level with rates in accordance with sulfur diffusivity in the alloy. The final concentration on FeAl and Fe 3 Al was only half that of saturation. On Fe 3 Al, S strongly segregated upon initial oxidation; it then desegregated from the interface before slowly building up again to a constant level. The strong initial segregation, however, was not found on FeAl. These results indicate that segregation to oxide/metal interfaces closely resembles that to free surfaces and alloy grain boundaries, where interface microstructure and co-segregation effects dictate the final concentration. A unique feature on growing oxide/metal interfaces is that this concentration can change with time according to changes in interfacial structures as a result of scale growth. Preliminary results on scale spallation behaviors did not indicate a strong relationship between the amount of interfacial sulfur and the interface fracture resistance, whose strength was more heavily related to interfacial pore density.