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-30Cr5-Al-0.5 Y. They were oxidized in air for 50 hr at 1000° C. The scale micro-structures 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 αAl₂O₃ 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.