Ab initio based calculation of the stability of selected TCP precipitates in steels: Temperature and interface effects

第二相的析出对高温能源和运输材料的力学性能和长期稳定性有很大的影响。特别是，在许多技术相关的合金中，如Fe-Nb系，拓扑密排(TCP)析出物起着决定性的作用。虽然这些相通常在实验中被清楚地识别，但它们的模拟要么限于经验模型，要么限于T=0K时不同相的稳定性。因此，可靠的预测往往是不可能的，在什么情况下会在材料中形成析出相。在本项目中，我们将应用基于从头计算的模拟技术，以了解、计算和优化形成复杂沉淀相的热力学和微观结构条件。一方面，将高精度地确定形成自由能到熔点的所有熵贡献。另一方面，还将考虑基体材料的界面以及析出物与显微组织的相关性。与以前的方法不同，这两个方面的相互作用将在沉淀形成的模拟中得到逼真的映射。该项目的概念需要使用一系列方法，这是MPIE杜塞尔多夫和ICAMS Bochum两个小组共同努力实现的。密度泛函理论需要精确地考虑有限温度下的激发过程，而微观结构的描述是通过将电子结构粗化而得到的简化描述。键序势能的应用使沉淀相与基体材料的复合组织模拟成为可能。本项目的研究将集中于Fe-Nb系中Laves相的形成。这种材料体系具有很高的应用潜力，在实验上得到了很好的表征，但对沉淀形成的理解有限。借助该项目，将阐明不同相之间的竞争、进一步合金化元素对其稳定性的影响、预期的析出相的形态以及晶界的作用。除了直接解释最近对Fe-Nb中Laves相的实验测量之外，这些研究还将为理解不同材料体系中的TcP沉淀奠定基础。

The mechanical properties and long-term stability of materials for high-temperature energy and transportation applications are strongly influenced by the precipitation of secondary phases. In particular, topologically closed-packed (TCP) precipitates play a decisive role in many technologically relevant alloys like the Fe-Nb system. While these phases are often clearly identified in experiments, their simulation is either limited to empirical models or to the stability of separate phases at T=0K. Therefore, a reliable prediction under which circumstances the precipitate phases form in the material is often not possible. Within the present project, we will apply ab initio based simulation techniques in order to understand, calculate and optimise the thermodynamic and microstructural conditions for the formation of complex precipitate phases. On the one hand, all entropy contributions for the free energy of formation up to the melting point will be determined with high accuracy. On the other hand the interfaces and the correlation of precipitates and microstructure of the matrix material will be considered. In contrast to previous approaches, the interplay of these two aspects will be mapped realistically in the simulation of precipitate formation.The concept of the project requires the use of a hierarchy of methods, which is achieved in a joint effort of two groups at the MPIE Düsseldorf and ICAMS Bochum. Density functional theory is needed for the accurate consideration of excitation processes at finite temperatures, whereas the microstructural description is achieved by a coarsening of the electronic structure towards a simplified description. The application of bond-order potentials makes the simulation of the combined microstructure of precipitate and matrix material accessible. The investigations of the present project will be focussed on the formation of Laves phases within the Fe-Nb system. This material system has a high potential for applications, is experimentally well characterized, but suffers from a limited understanding of precipitate formation. With the help of the project the competition between different TCP phases, the impact of further alloying elements on their stability, the expected morphology of the precipitates and the role of grain boundaries will be clarified. In addition to the direct interpretation of recent experimental measurements for Laves phases in Fe-Nb, the investigations will form the ground to an understanding of TCP precipitates in various material systems.