Oxidation behavior of co-doped NiCrAl alloys in dry and wet air

Cast NiCrAl alloys with additions of Y, La, Hf and Ti were evaluated at 1100 °C in wet (10 and 50% H2O) and dry air (0% H2O) in order to optimize such dopants for superalloy bond coatings. The results suggest that the typical Y addition in most coatings could be replaced by La. Also, scale adhesion in cyclic testing was improved with the co-addition of Hf with La or Y. Ti was added to investigate its incorporation in coatings on superalloys containing significant Ti additions. Particularly with co-doped alloys, the addition of Ti had little effect on scale adhesion but did reduce the depth of internal oxidation. Water vapor increased spallation, especially for the least adherent alloys, such as Y,Ti-doped NiCrAl. For the co-doped compositions with Hf, water vapor had a limited effect on scale adhesion and on the alumina growth rate in isothermal exposures. In addition to specimen mass change, the extent of internal oxidation and the depletion of β phase in the substrate were evaluated. Analytical transmission electron microscopy showed that Y, La and Hf co-segregated to the alumina scale grain boundaries and formed dopant-rich oxide precipitates in the scale.