Phase Transformation Behavior of Al2O3 Scale Formed on Pt-Modified γ′-Ni3Al-Based Alloys With and Without Hf Addition

Abstract

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.