Chromium raises vacancy concentration and promotes grain-boundary Al segregation in Ni–Cr–Al

Chromium additions are known to promote the formation of a protective Al oxide, alumina, in Ni-Cr-Al alloys, a phenomenon known as the third-element effect (TEE). Using atomistic simulations, we show that Cr lowers vacancy formation energies while leaving migration barriers largely unchanged. This reduction in formation energy leads to a strong amplification of equilibrium vacancy concentrations: compared to Ni–4Al, the concentration in Ni–15Cr–4Al is 11 times higher at 1000 ∘C and more than 54 times higher at 480 ∘C. When combined with tracer mobilities, this produces effective diffusion coefficients up to an order of magnitude greater in the ternary alloy. In parallel, Cr promotes aluminum enrichment at grain boundaries, raising Al levels to about one and a half times the bulk concentration across the boundaries studied. These metal-phase mechanisms provide a quantitative route by which Cr facilitates rapid Al delivery to oxidation fronts, complementing oxygen-gettering models and helping explain the TEE, at both high and intermediate homologous temperatures.