TEM Study of the Effect of Y on the Scale Microstructures of Cr2O3- and Al2O3-Forming Alloys

This study was undertaken to investigate and compare the effects of a yttrium addition on the oxide scale development of α-Cr2O3- and α-Al2O3-forming alloys under isothermal oxidation conditions. The alloys had a nominal composition (in wt.%) of Ni–30Cr, Ni–30Cr–0.5Y, Ni–30Cr–5Al, and Ni–30Cr– 5–Al–0.5Y. They were oxidized in air for 50 hr at 1000°C. The scale microstructures were characterized using cross-sectional transmission-electron microscopy combined with energy-dispersive X-ray spectroscopy. It was observed that the scale thickness decreases and the scale adherence increases due to the Y addition. The growth direction of α-Cr2O3 scale changes from predominately outward to inward while countercurrent diffusion within α-Al2O3 is replaced by inward diffusion due to Y modification. It is considered that the ability of Y to scavenge sulfur from the alloy and its segregation to the oxide grain boundaries primarily account for most of its beneficial effects.