Magnetism in iron alloys: thermodynamics, kinetics and defects

该项目的重点是用于高温、高强度和强磁应用的三种铁基合金：Fe-Cr、Fe-Mn和Fe-Co。由于磁性的作用，需要一种基于先进建模方法的创新材料设计来控制这些材料的关键性能。设计战略需要结合(I)确定原子特征的准确方法和(Ii)有效的粗粒化来获取目标物理性质并进行材料成分的筛选。对于前者，密度泛函理论(DFT)已被证明是非常成功的工具。然而，对于铁基合金，一个关键的瓶颈是磁有序、激发和相变对热力学、缺陷和动力学性质的影响。因此，需要一个完整而准确的磁性模型来解决材料设计方面的挑战：与Fe-Cr的化学分解有关的抗辐射损伤，铁素体Fe-Mn的晶界脆化和奥氏体Fe-Mn的高强度，以及如果不适当地考虑磁效应，就不能完全理解Fe-Co中的α和伽马相的相序和相对稳定性。首先，我们用密度泛函理论解决这一挑战，利用铁中磁性处理的最新进展，对具有点/扩展缺陷的磁性多组分系统进行精确建模，并超越标准的共线近似。其次，我们将开发新的方法，通过充分考虑磁性对缺陷性质、扩散和微观结构演变的影响，在(I)高精度电子计算和(Ii)大规模原子热力学和动力学模拟之间架起桥梁-这是决定性的。对于后者，将在密度泛函的基础上发展基于晶格的有效相互作用模型(EIMS)和紧束缚模型(TB)，包括磁性组态、激发和相变。这将使我们能够对有限温度下磁性对铁基合金各种性质的作用提供一致的描述。这将使我们能够进一步优化控制相关性质的关键参数，如Fe-Cr的相分解、Fe-Co的相有序化或Fe-Mn的晶界解聚。该项目将对块状合金和按需生长的晶间/相间边界进行专门的实验，这些实验对于验证理论预测的稳健性至关重要。所选的三种合金表现出不同的磁性行为。本文提出的方法可推广到其他磁性材料的建模。我们的模拟结果将导致热力学和扩散数据库和工具(如DICTRA)的改进，这些数据库和工具目前在工业研发中经常使用，但目前在解释磁性方面存在困难。

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.