Bond Synergy Model for Bond Energies in Alloy Oxides

TL;DR

This paper introduces a bond-energy model for alloy oxides that incorporates bond synergy factors, enabling better understanding of passive film formation and stability in corrosion-resistant alloys through density-functional theory data.

Contribution

The work presents a transferable bond-energy model for alloy oxides based on pure-phase energies and synergy factors, applicable to multi-component alloys and providing insights into alloy stability.

Findings

Bond energies can be quantified using the model and align with known behaviors.

Bond synergy factors reveal how alloying elements strengthen or weaken cation-oxygen bonds.

The model helps understand charge neutrality and oxidation states in alloy oxides.

Abstract

In this work we introduce a metal-oxide bond-energy model for alloy oxides based on pure-phase bond energies and bond synergy factors that describe the effect of alloying on the bond energy between cations and oxygen, an important quantity to understand formation and stability of passive films. This model is parameterized for binary cation-alloy oxides using density-functional theory energies and is shown to be directly transferable to multi-component alloy oxides. We parameterized the model for alloy oxide energies with metal cations that form the basis of corrosion resistant alloys, including Fe, Ni, Cr, Mo, Mn, W, Co, and Ru. We find that isoelectronic solutes allow quantification of pure-phase bond energies in oxides and that the calculated bond energy values give sensible results compared to common experience, including the role of Cr as the passive-layer former in Fe-Ni-Cr alloys for corrosion applications. Additionally, the bond synergy factors give insights into the mutual strengthening and weakening effects of alloying on cation-oxygen bonds and can be related to enthalpy of mixing and charge neutrality constraints. We demonstrate how charge neutrality can be identified and achieved by the oxidation states that the different cations assume depending on alloy composition and the presence of defects.