The role of yttrium on the oxide adherence of Fe-24Cr base alloys

A study on isothermal oxidation of Fe-24Cr base alloys containing 0.063 and 0.87 wt pct yttrium has been made to examine the role of yttrium on the improvement of oxide adherence. The isothermal oxidation resistance of the 0.063Y alloy was nearly equal to that of the 0.87Y alloy at 1000 °C and 1100 °C. The oxide scales of the former spalled partially during cooling to room temperature, while the adherence of the oxide on the latter was extremely strong. A morphological observation of the substrate-oxide interface by a scanning electron microscope revealed that cavities developed at the substrate-oxide interface of the 0.063Y alloy caused the partial spallation. No cavities were observed at the substrate-oxide interface of the 0.87Y alloy and voids centered on Y₂O₃ particles were observed in the substrate of the alloy. Those voids indicated that internal oxide particles had acted as preferential sinks for excess vacancies which would otherwise have condensed to form voids at the substrate-oxide interface. A model is proposed for a diffusional process based on excess vacancy sinks at the boundaries of internal oxide particles dispersed in the matrix of the Fe-24Cr-O.87Y alloy, and the effect of yttrium on the surface oxide adherence of the alloy.