The effect of platinum additions on the oxidation of directionally-solidified Ni-Cr-Al-Y-Cr3C2 alloys at 1,100 and 1,200°C

Abstract

The effects of 2wt% platinum addition on the oxidation behaviour of directionally-solidified, yttrium-containing Ni-Cr-Al-Cr3C2 alloys (Ni-12.3Cr-6.9Al-1.8C-0.2Y in wt%) at 1,100° and 1,200°C in oxygen have been investigated under isothermal and thermal cycling conditions. Owing to the presence of yttrium, aluminium oxidized externally rather than internally at both temperatures. At 1,100°C, the addition of platinum apparently favoured the formation of a continuous Al2O3 scale over primary carbides intersecting the alloy surface. The scale remained substantially adherent after isothermal oxidation, but lifted and cracked on thermal cycling, though it did not spall significantly. Overdoping with yttrium (0.7 wt%Y) had a detrimental effect on the scale adherence, probably because it altered the homogeneity of the platinum distribution by forming Pt-Y intermetallics. At 1,200°C, an external Al2O3 scale formed initially, but spalled extensively after short times under both isothermal and thermal cycling conditions. During cyclic oxidation, re-formation of an aluminium scale was possible until a critical level of aluminium depletion in the alloy was reached, with the less protective oxides forming during the subsequent oxidation cycles. The inherent lack of ductility of the Al2O3 scale over the carbide particles, coupled with the CrO3 vaporization at high temperatures, could account for the observations.