Early-Stage Oxidation Behavior of Pt-Modified γ'-Ni3Al-Based Alloys with and without Hf Addition

The early-stage oxidation behavior in air of Pt-modified gamma'-Ni3Al-based alloys of composition (in at.%) Ni-22Al-30Pt with and without 0.5Hf was investigated in terms of oxidation kinetics, scale evolution and Al2O3 phase transformation. Oxidation exposures included heating to and short-term holds at 1,150 A degrees C. Hafnium addition did not appear to affect microstructural evolution and growth rate of the oxide scales during heating to 1,150 A degrees C; however, it was found that Hf delayed the metastable-to-alpha-Al2O3 phase transformation, thus allowing continued fast growth of oxide scale. After the transient oxidation stage of up to about 10 min (including heating time), Ni-rich metallic particles precipitated in the lower part of the metastable Al2O3 layer, due to a decrease in the oxygen potential resulting from scale evolution. The present results indicated that the period of oxide phase transformation was followed by the establishment of steady-state oxidation kinetics. However, the steady-state kinetics were different for the two alloy systems. Specifically, after complete phase transformation to alpha-Al2O3, rapid growth of oxide grains occurred on the Hf-free alloy; whereas, the oxide grain size remained small for the Hf-containing alloy. Such a difference of transformation and subsequent grain-growth behavior greatly affected oxide thickening kinetics.