Development, validation and application of a magnetic bond order potential for the Fe-C system

铁碳体系属于技术上最重要的二元体系，因为它是钢的基础，是当今社会的主要材料之一。在对该体系的物理、热力学和力学性质的理论认识方面取得了很大进展。然而，在具有复杂化学和微观结构的现代钢中，明确的原子尺度过程建模仍然是一个重大的挑战。原因是大多数这些过程是由化学和磁相互作用之间的微妙相互作用所控制的。本研究项目的主要目标是开发一种最先进的磁键序势（BOP），它将能够定量描述Fe-C系统的原子尺度行为。开发的防喷器模型应该能够覆盖整个组成范围，从铁基体中稀C浓度的单个元素到形成不同碳化物相的大C浓度。由于bop是由量子力学的严格粗粒化过程导出的，因此它们隐含地包含起源于电子结构水平的现象，如磁性。同时，它们以多体原子间势形式的实空间公式使它们的计算效率很高。这种物理精度和计算效率的结合使它们能够用于复杂构型和条件的原子研究，例如，扩展晶体缺陷的性质或相界面的运动，这不仅控制了Fe-C系统中结构和磁性相变的机制，而且还控制了其宏观力学行为。

The iron-carbon system belongs to the technologically most important binary systems since it is the basis of steel, one of the main materials in today’s society. A great deal of progress has been made in theoretical understanding of the physical, thermodynamic and mechanical properties of this system. However, explicit modelling of atomic-scale processes in modern steels with complex chemistries and microstructures still presents a significant challenge. The reason is that most of these processes are governed by a subtle interplay between chemical and magnetic interactions.The primary objective of this research project is to develop a state-of-the-art magnetic bond-order potential (BOP) that will be capable of describing quantitatively the atomic-scale behavior of the Fe-C system. The developed BOP model shall be able to cover the whole composition range from individual elements over dilute C concentrations in the Fe matrix up to large C concentrations where diverse carbide phases are formed. Since BOPs are derived by a rigorous coarse-graining procedure from quantum mechanics they implicitly contain phenomena originating at the electronic structure level such as magnetism. At the same time, their real-space formulation in the form of many-body interatomic potentials makes them computationally efficient. This combination of physical accuracy and computational efficiency enables them to be employed in atomistic studies of complex configurations and conditions, for instance, properties of extended crystal defects or motion of phase interfaces, that govern not only the mechanisms of structural and magnetic phase transitions in the Fe-C system but also its macroscopic mechanical behavior.