Eutectics, oxide scales and solubility limits: Understanding divergent oxidation mechanisms in NiAlYX (X = Si, Ta, Nb, and Ti) alloys for marine applications

Abstract

Conventional Cr-containing Ni-based alloys or alloy coatings often suffer from corrosion failure in marine environments, where NaCl compromises the stability of Cr₂O₃ and the degradation of γ' to γ. To address this challenge, we designed Cr-free, γ' phase-dominated NiAlYX (X = Si, Ta, Nb, and Ti) alloys via the CALPHAD (CALculation of PHAse Diagrams) approach and revealed contrasting roles of dopants by coupling ab initio calculations, thermodynamic calculations, and experimental analyses. As dopants increase, NiAlYTa/Nb/Ti alloys develop oxidation-susceptible (Ni, Al, X)₅Y + β + γ' eutectic microstructures. In contrast, NiAlYSi alloy lacks these eutectics due to a high fraction of γ' precipitates. After oxidation, the NiAlYSi alloy exhibited excellent oxidation resistance, attributed to the formation of a continuous Al₂O₃ layer promoted by Si and the modification of oxidation behavior of the (Ni, Al, Si)₅Y phase by stabilizing Y₂SiO₅ and Y₂Si₂O₇. These resulted in the generation of a thinner NiO outer layer and inhibited internal phase oxidation. In contrast, NiAlYTa/Nb/Ti alloys failed to promote the formation of a continuous Al₂O₃ layer, resulting in internal oxidation zones characterized by γ + Al₂O₃ + Ta₂O₅/Nb₂O₅/TiO₂ and thickening of the outer NiO layer, as synergistically validated by experimental observations, ab initio analysis, and potential-composition phase diagrams. The negligible solid solubility of Ta/Nb/Ti in (Ni, Al, X)₅Y phases renders them ineffective in altering the oxidation behavior. This work reveals Si as a promising Cr-free substitute in marine-grade Ni-based alloys, while emphasizing the importance of controlling eutectic structures and dopant solubility for high-temperature oxidation resistance.