Yttrium-promoted selective oxidation of aluminium in the oxidation at 1100°C of an eutectic Ni-Al-Cr3C2 alloy

The effects of an yttrium addition of 0.2 wt% on the oxidation behaviour of an eutectic Ni-Al-Cr3C2 alloy of average composition Ni-12.3 Cr-6.9 Al-1.8 C (wt%) have been investigated at 1100 degrees C in 1 atm oxygen. Experiments have been carried out under isothermal and thermal cycling conditions on the as-cast alloy, in which Cr,C, was present as a finely dispersed eutectic phase, and on directionally-solidified specimens having an aligned structure, In all cases, Al2O3 formed almost entirely as an external scale. Although Cr2O3 formed in some regions of the surface in the early stages, compact subsurface Al2O3 layers generally developed beneath such locations, protecting the alloy from severe internal attack. This behaviour is quite different from that observed for similar yttrium-free alloys, where aluminium was oxidized internally beneath continuous external Cr2O3 scales. In the present case, while it was almost certainly formed during the transient stage, the growth rate of Cr2O3 was probably reduced due to the presence of the reactive element and the extent of chromium depletion in the alloy was thereby decreased. This yielded an increase in the concentration of chromium in the alloy at its interface with the transient Cr2O3 scale compared to the yttrium-free alloy, with a corresponding decrease in the oxygen activity at the same location. As a consequence, the inward flux of oxygen in the alloy was reduced, allowing a transition from internal to external oxidation of aluminium to become possible on longer periods of oxidation, The presence of carbides at the alloy/scale interface was highly detrimental to the adhesion of the oxide scale to the metal substrate, and the scales tended to crack and/or lift on cooling. However, no massive spalling occurred, even during thermal cycling for periods up to 124 h, while the Al2O3 scales tended to re-form rapidly after mechanical damage. (C) 1999 Elsevier Science Ltd. All rights reserved.