Ionic segregation on grain boundaries in thermally grown alumina scales

This study first examined segregation behaviour in the alumina scale formed after 100 h at 1100°C on bare and MCrAlYHfSi-coated single-crystal superalloys with ~10 ppma La and Y. For the bare superalloy, Hf and Ti were detected on the grain boundaries of the inner columnar alumina layer. Increasing the oxidation temperature to 1200°C for 2 h did not change the segregation behaviour. With the bond coating, both Y and Hf were segregated to the grain boundaries as expected. However, there was evidence of Ti-rich oxide particles near the gas interface suggesting that Ti diffused from the superalloy through the coating. To further understand these segregation observations with multiple dopants, other alumina-forming systems were examined. Alumina scale grain boundary co-segregation of Ti with Y is common for FeCrAl alloys. Co-segregation of Hf and Ti was observed in the scale formed on co-doped NiAl. No La segregation was detected in the scale formed on NiCrAl with only a 19 ppma La addition, however, the scale was adherent.