High Temperature Oxidation of UHP-Based Fe20Cr5Al Alloys

Abstract

UHP-base alloys have been used to study the effects of sulfur and yttrium on the oxidation resistance of ferritic Fe20Cr5Al alloys. Doping with sulfur and/or yttrium gave a series of alloys with sulfur contents between 0.2 to 50 ppm sulfur, with or without yttrium (0.1 wt% maximum). The a-alumina scale and the metal±oxide interface structures were investigated after oxidation between 1060 and 1350 C (1 to 100 h), using several techniques: SEM, TEM, STEM and Scanning AES. It was found that sulfur contents lower than 1 ppm or doping with yttrium gave flat and adherent scales. The equiaxed structure observed on Y-undoped specimens was transformed into a columnar structure when yttrium was present in the alloy. Sulfur and carbon were measured in the spalled regions of the surface where emergent grain boundaries of the substrate were present. Bending tests in the Auger device at room temperature confirmed this result: in the oxide areas cracked by bending, sulfur and carbon were found at the emergence of grain boundaries of the metal but neither sulfur nor carbon were observed in the cracked zones away from the grain boundaries of the metal. Yttrium was measured at the metal±oxide interface and in the grain boundaries of the oxide. No sulfur was found in the grain boundaries of the oxide. When yttrium was found at the metal±oxide interface, no sulfur could be detected. Chromium enrichment was observed at the metal±oxide interface in parallel with sulfur segregation but in yttrium containing alloys Cr enrichment disappeared at the same time as sulfur.

Sulfur Segregation at Al2O3/γ-Νi + γ′-Ni3Al Interfaces: Effects of Pt, Cr and Hf Additions

“…The effect of Pt on S segregation in b-NiAl has also been investigated, and Pt was found to reduce S segregation to the free surface [18][19][20] and even eliminate S at the interface [18]. Hf, a reactive element, is expected to reduce, if not eliminate, S segregation to the interface, as has been found for Zr in Fe 3 Al [21] and Y in FeCrAl [22][23][24]. Hf has been shown to segregate to the Al 2 O 3 /NiAl interface (K. L. More, private communication, unpublished results), but whether it stops S segregation has yet to be determined.…”

Effect of Trace Amounts of Carbon and Nitrogen on the High Temperature Oxidation Resistance of High Purity FeCrAl Alloys

“…In an earlier work 5) it has been found that carbon together with sulphur tend to segregate to the scale/metal interface, hence deteriorating the scale adhesion. In the present work the impurities of C and N were found to form carbonitride precipitates by reaction with the minor additions of Ti and Zr, or, in the absence of the latter, with the alloy Cr.…”

“…The second important RE-effect is attributed to prevention of the deleterious sulphur segregation to the scale/metal interface. 5) Although plenty of scientific literature exists on the REeffect, some of the data extremely important for industrial manufacturing of the FeCrAl alloys are still not available. In particular, little is known about the interaction of the RE with common alloy impurities other than sulphur.…”

The model thin film alumina catalyst support suitable for catalysis-oriented surface science studies

“…It was evident a priori that the conventional procedure established for the formation of anti-corrosion coating [27,28] would not meet the conditions necessary for STM. Indeed, XPS spectra of the alumina layer obtained after the 5 h FCA foil annealing on air in muffle operated at 1470 K exhibit the signals from Al and O mainly and C as surface contamination.…”