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

Conventional Cr-containing Ni-based alloys or alloy coatings often suffer from corrosion failure in marine environments, where NaCl compromises the stability of Cr 2 O 3 and the degradation of γ' to γ. To address this challenge, we designed Cr-free, γ' phase-dominated NiAlY X ( 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 ) 5 Y + 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 Al 2 O 3 layer promoted by Si and the modification of oxidation behavior of the (Ni, Al, Si) 5 Y phase by stabilizing Y 2 SiO 5 and Y 2 Si 2 O 7 . 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 Al 2 O 3 layer, resulting in internal oxidation zones characterized by γ + Al 2 O 3 + Ta 2 O 5 /Nb 2 O 5 /TiO 2 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) 5 Y 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.