ROLE OF RARE-EARTH ELEMENTS ON HIGH-TEMPERATURE OXIDATION BEHAVIOR OFFE-CR, NI-CR AND NI-CR-AL ALLOYS

The influence of rare-earth elements (REEs) on oxidation behavior of F e-Cr-Ni, Fe-Cr and Ni-Cr alloys has been studied and the role of REEs on oxidation behavior of chromia forming alloys discussed. The REEs we re (a) applied superficially to Fe-18Cr-8Ni (AISI 304) and Fe-25Cr-20N i (AISI 310) alloys, and (b) added as alloying additions or as oxide d ispersions to Fe-20Cr and Ni-20Cr alloys. Superficial application of r are-earth oxides (REOs) and concentrates of Y and Ce improved the isot hermal and cyclic oxidation resistance of the low Cr alloys, but not t hat of the high Cr alloy. The oxides of Y, Ce, Nd, La, Sm and Gd influ enced oxidation resistance in decreasing order. The addition of 0.1% C e and Y as well as 1% CeO2 or Y2O3 as dispersoids to Fe-Cr and Ni-Cr a lloys improved their oxidation resistance and the alloy with Al and Y was found to be the most resistant. In the presence of Y or Ce (as all oying addition or as oxide on the surface) the oxide formed on the sur face was convoluted and fine grained chromia. The increase in oxidatio n resistance of chromia forming alloys in the presence of REEs is due to changes in the cation diffusion process. This is brought about by s egregation of REEs to the grain boundaries of the scale, and consequen t variations in ionic defect concentrations.