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

Subject Area Thermodynamics and Kinetics as well as Properties of Phases and Microstructure of Materials
Mechanical Properties of Metallic Materials and their Microstructural Origins
Term from 2018 to 2023
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 406912286
 
Final Report Year 2024

Final Report Abstract

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 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 carbon (C) atoms in the martensitic phase of steels plays a critical role. It is also determined by the processes happening at the transformation front, the austenite-martensite interface. These ordering processes were 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 this ANR-DFG project we have addressed these open questions by combining high resolution theoretical and experimental investigations in a complementary approach for Fe-TM-C steels. This included the consideration and analysis of not only the local atomic interface structures, but also possibly formed interfacial states. As competing mechanisms to the interface-dominated structural and chemical distribution of carbon in the martensite, the order-disorder transition of carbon atoms, the formation of carbides or other highly ordered carbon arrangements at the interface or in the bulk, and the formation of reverted austenite were considered. One major focus has been on the role of the martensite-austenite interface on the C distribution and redistribution during the martensitic transformation. More generally, incorporating C atoms in a quasi-particle approach, the simulation of C segregation to structural defects became possible. In addition, we have studied the long-range (Zener) ordering of randomly distributed C atoms in martensite, showing that it is energetically favorable, although short-range C-C interaction hinders it. Furthermore, the impact of alloying elements on C clustering in the martensite has been investigated. A major achievement of the project was the development of a variety of methods to address to complex energetics and kinetics in these microstructures. This includes on the theoretical side the development of the metadynamics method to consider C ordering at ambient temperatures and on the experimental side the development of an analysis tool for the identification of the orientation relationship at the austenite/martensite interface from electron back-scatter diffraction (EBSD) measurements. The results of this project form a fundamental contribution to the development of new martensitic steel products for the automotive industry (more resistant, therefore lighter components), contributing to the reduction in carbon footprint.

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