Atomic scale redistribution of carbon during the transformation from austenite to martensite in steels

马氏体钢具有很高的应用价值，因为其强度、韧性和成形性可通过简单的工艺处理在很大范围内进行调整。马氏体钢的整体性能敏感地取决于马氏体相的体积分数、形态、成分和本征性能。这是由奥氏体向马氏体相变过程中的局部组织过程以及马氏体的后处理过程决定的。在此背景下，钢的马氏体相中碳原子的非平衡分布起着至关重要的作用。C原子在新形成的马氏体中是过饱和的，它们的浓度和分布对马氏体的强度和韧性起着决定性的作用。碳原子的分布(和势能有序化)发生在相变前沿，即奥氏体-马氏体界面处。这些过程，特别是碳扩散和界面流动性的基本动力学还没有被详细地理解到原子尺度，尽管这与具有定制机械性能的马氏体钢的设计高度相关。到目前为止，理论和实验研究都局限于理想化的材料体系或有限的联合结构和化学数据的分辨率。在拟议的项目中，我们的目标是通过对Fe-TM-C钢的补充方法，通过高分辨率的理论和实验研究相结合来解开这些悬而未决的问题。一个主要的焦点是马氏体-奥氏体界面对马氏体相变过程中碳的分布和再分布的影响。这不仅包括考虑和分析局域原子界面结构，而且还包括可能形成的界面态、中程结构调制和马氏体的四方形变(齐纳有序)。作为C在马氏体中以界面为主的结构和化学分布的竞争机制，C原子的有序-无序转变，在界面或块体上形成碳化物或其他高度有序的C排列，以及反转奥氏体的形成将被考虑在内。为此，我们将应用基于密度泛函理论(DFT)的从头算热力学和动力学，包括所有有限温度贡献、原子密度场(ADF)理论和由平均场方法完成的准粒子方法(QA)。在实验方面，将使用原子探针断层摄影术(APT)和(高分辨率)透射电子显微镜(TEM)来结合结构和化学数据。最后，将对选定的关键样品进行相关的TEM-APT分析，以获得关于局部结构和化学的完全相关数据。

Martensitic steels are of high applicational relevance due to their extraordinary strength and the adjustability of their strength, toughness and formability over a wide range by simple technological treatments. The bulk properties of martensitic steels are sensitively depending on the volume fraction, morphology, composition and intrinsic properties of the martensitic phase. These are determined by the local structural processes during the austenite-to-martensite transformation as well as the post-processing of the martensite. In this context, the non-equilibrium distribution of C atoms in the martensitic phase of steels plays a critical role. C atoms are super-saturated in freshly formed martensite and their concentration and distribution is decisive for the strength and toughness of the martensite. The distribution (and potential ordering) of carbon atoms happens at the transformation front, the austenite-martensite interface. These processes, and in particular the underlying kinetics of the C diffusion and the interface mobility are not yet understood in detail down to the atomic scale, although of high relevance for the design of martensitic steels with tailored mechanical properties. Both, theoretical and experimental investigations are so far restricted to either idealized material systems or limited resolution of joint structural and chemical data. Within the proposed project we aim to unravel these open questions by combining high resolution theoretical and experimental investigations in a complementary approach for Fe-TM-C steels. One major focus is on the role of the martensite-austenite interface on the C distribution and redistribution during the martensitic transformation. This includes the consideration and analysis of not only the local atomic interface structures, but also possibly formed interfacial states, medium range structural modulations and the tetragonal deformation of the martensite (Zener ordering). As competing mechanisms to the interface-dominated structural and chemical distribution of C in the martensite, the order-disorder transition of C atoms, the formation of carbides or other highly ordered C arrangements at the interface or in the bulk, and the formation of reverted austenite will be considered. To this end we will apply ab initio thermodynamics and kinetics based on density functional theory (DFT) including all finite temperature contributions, atomic density field (ADF) theory and the quasiparticle approach (QA) accomplished by mean-field approaches on the theoretical side. On the experimental side, atom probe tomography (APT) and (high resolution) transmission electron microscopy (TEM) will be used to combine structural and chemical data. Eventually, correlative TEM-APT analyses to achieve fully correlative data on the local structure and chemistry will be applied for selected key samples.