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

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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.