Multiscale model of metal alloy oxidation at grain boundaries

Abstract

High temperature intergranular oxidation and corrosion of metal alloys is one of the primary causes of materials degradation in nuclear systems. In order to gain insights into grain boundary oxidation processes, a mesoscale metal alloy oxidation model is established by combining quantum Density Functional Theory (DFT) and mesoscopic Poisson-Nernst-Planck/classical DFT with predictions focused on Ni alloyed with either Cr or Al. Analysis of species and fluxes at steady-state conditions indicates that the oxidation process involves vacancy-mediated transport of Ni and the minor alloying element to the oxidation front and the formation of stable metal oxides. The simulations further demonstrate that the mechanism of oxidation for Ni-5Cr and Ni-4Al is qualitatively different. Intergranular oxidation of Ni-5Cr involves the selective oxidation of the minor element and not matrix Ni, due to slower diffusion of Ni relative to Cr in the alloy and due to the significantly smaller energy gain upon the formation of nickel oxide compared to that of Cr2O3. This essentially one-component oxidation process results in continuous oxide formation and a monotonic Cr vacancy distribution ahead of the oxidation front, peaking at alloy/oxide interface. In contrast, Ni and Al are both oxidized in Ni-4Al forming a mixed spinel NiAl2O4. Different diffusivities of Ni and Al give rise to a complex elemental distribution in the vicinity of the oxidation front. Slower diffusing Ni accumulates in the oxide and metal within 3 nm of the interface, while Al penetrates deeper into the oxide phase. Ni and Al are both depleted from the region 3-10 nm ahead of the oxidation front creating voids. The oxide microstructure is also different. Cr2O3 has a plate-like structure with 1.2-1.7 nm wide pores running along the grain boundary, while NiAl2O4 has 1.5 nm wide pores in the direction parallel to the grain boundary and 0.6 nm pores in the perpendicular direction providing an additional pathway for oxygen diffusion through the oxide. The proposed theoretical methodology provides a framework for modeling metal alloy oxidation processes from first principles and on the experimentally relevant length scales.

Basic Research Needs for Environmental Management

“…For example, combined quantum/classical density functional theory has been used to account for short-range chemical forces and many-body electrostatic and electrodynamic forces driving the dissolution process. Seamless coupling of quantum and classical density functional theory approaches within a tightly coupled embedding multiscale framework has been demonstrated by Sushko et al (2015) in modeling the oxidation of nickel alloys. These methods avoid the traditional shortfalls of both small cell size and difficult interpretation of kinetic data inherent in density functional theory and the challenge of defining force fields and representing true materials interactions inherent in molecular dynamics.…”

Vacancies and Vacancy-Mediated Self Diffusion in Cr2O3: A First-Principles Study

“…Material degradation in corrosion-resistant, high-Cr alloys involves various mechanisms including localized corrosion/oxidation and stress corrosion cracking. − Self-diffusion in Cr 2 O 3 plays an important role in the protective nature of the chromia coating − by influencing the growth of the chromia scales as well as by controlling the supply of oxygen to the oxide/metal interface. Diffusion in oxides is usually mediated by point defects such as vacancies or interstitials .…”

Visualizing the Nanoscale Oxygen and Cation Transport Mechanisms during the Early Stages of Oxidation of Fe–Cr–Ni Alloy Using In Situ Atom Probe Tomography

“…These include the free energies of Coulomb interactions, electrostatic correlations, hard sphere repulsion, and short-range interactions with the stationary (lattice) sites, which represent the equilibrium sites for matrix and minor elements in the crystal structure of the alloy and oxide. [47,48] A metal alloy/oxide interface is modeled in 3D as an array of interaction centers representing atomic positions of Fe, Cr, Ni, and O in the crystal lattices of the alloy (FCC), Fe 3 O 4 , and Cr 2 O 3 oxide phases. The crystallographic vectors of the Fe FCC lattice are aligned along the Cartesian vectors of the simulation cell.…”