Magnetism in iron alloys: thermodynamics, kinetics and defects

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The project focuses on three iron-base alloys for high-temperature, high-strength and strong-magnet applications: Fe-Cr, Fe-Mn and Fe-Co. Because of the role of magnetism an innovative materials design based on advanced modeling approaches is necessary to control key properties of these materials. A design strategy requires the combination of (i) accurate methods to determine atomic features with (ii) efficient coarse-graining to access target physical properties and to perform the screening of materials compositions. For the former, density functional theory (DFT) has proven to be a highly successful tool. For Fe-based alloys, however, a critical bottleneck is the role that magnetic ordering, excitations and transitions have on thermodynamic, defect and kinetic properties. Therefore, a complete and accurate modeling of magnetism is needed to address the materials-design challenges: resistance to radiation damage related to the chemical decomposition in Fe-Cr, grain-boundary embrittlement in ferritic Fe-Mn and high-strength of austenitic Fe-Mn, and the phase ordering and the relative stability of alpha and gamma phases in Fe-Co cannot be fully understood without properly accounting for the magnetic effects. First, we approach this challenge with DFT by making use of recent progress in treating magnetism in iron to go towards an accurate modeling of magnetic multi-component systems with point/extended defects, and beyond the standard collinear approximation. Second, we will develop new methods to bridge between (i) highly accurate electronic calculations and (ii) large-scale atomistic thermodynamic and kinetic simulations for iron based alloys by - and this is decisive - fully taking into account the impact of magnetism on defect properties, diffusion and microstructural evolution. For the latter, lattice-based effective interaction models (EIMs) and tight-binding (TB) models will be developed based on DFT, including magnetic configurations, excitations and transitions. This will allow us to provide a coherent description of the role of magnetism on various properties of Fe-based alloys at finite temperature. It will further give us the ability to perform the optimization of key parameters controlling the relevant properties like phase decomposition in Fe-Cr, phase ordering in Fe-Co or decohesion of grain boundaries in Fe-Mn. Dedicated experiments in bulk alloys and along intergranular / interphase boundaries grown on demand will be performed in the project, which are essential for verifying the robustness of the theoretical predictions. The three chosen alloys exhibit a large variety of magnetic behavior. The methods developed in this proposal are transferable to the modeling of other magnetic materials. The results of our simulations will lead to the improvement of thermodynamic and diffusion databases and tools (such as DICTRA) that are nowadays routinely used in industrial R&D but that at present have difficulties in accounting for magnetism.