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 Al 2 O 3 scale formed on Ni-22Al-30Pt (in at.%) with and without 0.5Hf was investigated during short-term (i.e., 3min dwell) cyclic oxidation at 1150ºC 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-Al 2 O 3 structure to the stable α-Al 2 O 3 . Small dimples, which corresponded to α-Al 2 O 3 grains, started to form on the Hf-free alloy after only 3 oxidation cycles; whereas, no apparent morphological change of the oxide scale surface was observed on the Hf-modified alloy. The transformation to α-Al 2 O 3 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 Al 2 O 3 was inferred to be the main contributor to the delayed the transformation.

Charge optimized many-body (COMB) potential for dynamical simulation of Ni–Al phases

“…It is known that a protective coating of Al 2 O 3 forms on the Ni-base superalloys at high temperatures [9]. This interface is thus critical to the performance of superalloys and addition of alloying elements in the alloy may modify this adhesion.…”

“…However, EAM potentials are best suited for metallic systems. A potential formalism with dynamically varying charge would be required to capture the influence of Al 2 O 3 film formation on Ni-based superalloys following the introduction of oxygen at high temperatures [9], or to model thermal barrier coatings on the Ni-based superalloy gas-turbines [10].…”

Pt effect on early stage oxidation behaviour of Pt-diffused γ-Ni/γ’-Ni3Al coatings

“…According to Hayashi and Gleeson [31], α-Al2O3 nucleates uniformly along the coating/oxide interface, leading to a duplex layered structure of the oxide scale, as shown schematically in Conversely, for coatings with higher Pt content, this transformation is retarded because of the higher γ' phase to γ phase fraction ratio near the coating/oxide interface (Fig. 16 c).…”