Phase Transformation Behavior of Al2O3 Scale Formed on Pt-Modified γ′-Ni3Al-Based Alloys With and Without Hf Addition
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The allotropic phase transformation behavior of Al2O3 scale formed on Ni–22Al–30Pt (in at.%) with and without 0.5Hf was investigated during short-term (i.e., 3 min dwell) cyclic oxidation at 1,150 °C in air. Hafnium addition did not appear to affect the oxidation rate in the early oxidation cycles, but it did delay the phase transformation from the metastable θ-Al2O3 structure to the stable α-Al2O3. Small dimples, which corresponded to α-Al2O3 grains, started to form on the Hf-free alloy after only three oxidation cycles; whereas, no apparent morphological change of the oxide scale surface was observed on the Hf-modified alloy. The transformation to α-Al2O3 was found to initiate at scale/alloy interface on the Hf-free alloy, but it initiated at gas/scale interface on the Hf-modified alloy. Depth profiling using glow discharge optical emission spectroscopy revealed that Hf enriched at the scale/alloy interface due to Hf rejection associated with the formation of an Al-depleted γ-layer, which has a low Hf solubility. Higher positive strain energy due to Hf solution in the metastable Al2O3 was inferred to be the main contributor to the delayed the transformation.
KeywordsHf effect Al2O3 phase transformation Grain growth Short-term kinetics
Partial support for this work was provided by the Office of Naval Research, contract number N000140911127, with Dr. David Shifler being the Program Manager.
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