Bond-Order Bond Energy Model for Alloys

We introduce a novel way to parameterize alloy energies in the form of a bond-order bond energy model. There, a bond order function models the transition between competing phases and switches their respective bond energies on and off. For the case of the Ni-Cr-Mo alloys investigated here, which assume face- or body-centered cubic structures, we propose a sigmoidal switching function fitted to the c/a ratio of "Bain-like" cells. With that, the model does an excellent job in describing the DFT-calculated alloy energies. We also show that the average atomic charge density can vary considerably as a function of composition, which can significantly modify alloy bond energies from simple expectation. The fitted bond energies can answer questions that relate bond energies to processes such as those involved in field ion emission or corrosion, but can also be used to determine quantitative, composition-dependent chemical potentials. With those, we perform vacancy calculations for binary NiCr alloys for configurations that have been configurationally energy-minimized with Monte Carlo simulations. We show that the resulting regime of negative formation energies is a sign for thermodynamic instability of the underlying crystal and coincides with the two-phase concentration range in the phase diagram where neither fcc nor bcc is stable at any temperature, resulting in a holistic picture that unites defect and phase stability through the fully quantitative link between stoichiometry and chemical potentials enabled by the proposed bond energy model.