HIGH-TEMPERATURE OXIDATION OF UHP-BASED FE20CR5AL ALLOYS

UHP-base alloys have been used to study the effects of sulfur and yttr ium on the oxidation resistance of ferritic Fe20Cr5Al alloys. Doping w ith 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% maximu m). The a-alumina scale and the metal-oxide interface structures were investigated after oxidation between 1060 and 1350 degrees 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 ga ve flat and adherent scales. The equiaxed structure observed on Y-undo ped specimens was transformed into a columnar structure when yttrium w as present in the alloy. Sulfur and carbon were measured in the spalle d regions of the surface where emergent grain boundaries of the substr ate were present. Bending tests in the Auger device at room temperatur e confirmed this result: in the oxide areas cracked by bending, sulfur and carbon were found at the emergence of grain boundaries of the met al but neither sulfur nor cal bon were observed in the cracked zones a way from the grain boundaries of the metal. Yttrium was measured at th e metal-oxide interface and in the grain boundaries of the oxide. No s ulfur was found in the grain boundaries of the oxide. When yttrium was found at the metal-oxide interface, no sulfur could be detected. Chro mium 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.