Interplay between magnetism, thermodynamics and diffusion in bcc Fe-Mn alloys: from first principles to finite temperatures

In 3D alloys, magnetic properties of solutes can be extremely sensitive to local chemical and structural environments. Therefore, the properties of these solutes are often at the origin of discrepancies between various data from both theoretical and experimental investigations. This is for instance the case of Mn solutes in bcc iron. Also, alloys presenting a magnetic competition between solute-solute and solute-host atom interactions often exhibit complex magnetic structures. Moreover, previous studies have shown that magnetism in Fe-based alloys has a crucial impact on various thermodynamic and kinetic properties. The objective of this work is to identify and understand the coupling between the magnetic and energetic properties of bcc Fe-Mn alloys at ground-state but also at finite temperature, and to evaluate the effects of magnetism on thermodynamic and kinetic properties. In order to properly understand the fundamental properties of these alloys, the first part of this work is dedicated to the study of the effects of local chemical environment on the magnetic state of Mn solutes in bcc Fe-Mn by means of Density Functional Theory. Namely, configurations containing Mn solutes, being isolated or forming a cluster, and in the presence of vacancies or interstitial impurities are investigated. The lowest-energy magnetic configuration of each system is determined, and general trends are identified. Based on the obtained results, we also provide possible explanations for the existing discrepancies between theoretical and experimental studies. The ab-initio data produced are then used to parameterize an effective interaction model in order to study the properties of Fe-Mn alloys at finite temperature. The key features of Fe-Mn alloys are identified, and the model is validated at low temperature by reproducing ab-initio predictions. Using this model coupled to Monte Carlo simulations, we simulate the chemical evolution of Fe-Mn properties depending on temperature and Mn concentration while relaxing the magnetic structure on-the-fly. In order to illustrate the validity and the applicability of the model, we examine certain finite temperature properties of bcc Fe-Mn alloys such as the concentration dependence of the Curie temperature or the temperature evolution of the mixing energy and the atomic short-range order. Since diffusion in Fe and Fe-Mn alloys is frequently ruled by vacancy mechanism, we also propose a formalism to take explicitly into account the properties of vacancies in the interaction model and the effect of local magnetic state on these properties. By performing tracer diffusion Monte Carlo simulations, this approach predicts the temperature dependence of self-diffusion in bcc Fe in excellent agreement with experimental results. This prediction directly includes the deviation from Arrhenius law around the Curie temperature and the change of slope between the ferromagnetic and paramagnetic regimes. The accuracy of the widely used Ruch model is discussed in the light of the present results. Finally, we apply this approach to the diffusion of a Mn solute in bcc Fe at the infinite dilution limit. Using both tracer diffusion simulations and the LeClaire model, we show that the acceleration of diffusion near TC is highly reduced in the case of Mn compared to the self-diffusion of Fe, in good agreement with experimental observations, which is a step forward compared to the available theoretical literature. This work provides some new insights on the effects of magnetism in bcc Fe and Fe-Mn alloys, both at ground-state via DFT calculations and at finite temperatures using Monte Carlo simulations with a newly developed effective interaction model that takes explicitly into account the magnetic and chemical variables of the system. (author)