Stability and evolution of non-equilibrium states in the iron carbon system
by Jonas Arlt
Date of Examination:2024-12-16
Date of issue:2025-03-20
Advisor:Prof. Dr. Cynthia A. Volkert
Referee:Prof. Dr. Cynthia A. Volkert
Referee:Prof. Dr. Astrid Pundt
Referee:Prof. Dr. Baptiste Gault
Persistent Address:
http://dx.doi.org/10.53846/goediss-11155
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Abstract
English
The engineering of metastable microstructures has opened new avenues in advanced materials development. In Fe-C-based alloys, the drastic reduction in carbon solubility during the fcc-to-bcc transformation, in combination with controlled thermal annealing, has been leveraged to design novel steel classes such as nanostructured dual-phase steels and nanoprecipitate-rich maraging steels. However, the fundamental mechanisms governing the underlying structural and phase transformations remain only partially understood. Carbon plays a pivotal role, influencing phase stability, defect structures, and decomposition pathways. In particular, its tendency to order within the bcc iron lattice and segregate to defects defines the microstructural evolution of Fe-C alloys. Recent theoretical studies suggest that spinodal decomposition in carbon-supersaturated ferrite can drive α''-Fe16C2 cluster formation, offering a self-assembly route to nanoscale microstructures. However, experimental studies on highly supersaturated Fe-C systems remain scarce. This work systematically investigates the formation and evolution of non-equilibrium microstructures in carbon-supersaturated Fe-C using atom probe tomography (APT), scanning transmission electron microscopy (STEM), and in-situ transmission electron microscopy (TEM) annealing. Combining these techniques enables nanoscale resolution of both chemical composition and microstructure. Two distinct routes were employed to induce extreme supersaturation: 1. Explosive welding of carbon steel plates, where rapid solidification following a high-impact collision results in melt pockets with carbon concentrations far exceeding equilibrium solubility. 2. Sputter deposition of high-carbon Fe-C thin films, where kinetic constraints during film growth trap carbon in the bcc ferrite matrix, reaching ~10 at.% C – nearly 1000 times the room-temperature solubility limit. In high-carbon Fe-C films, a two-stage growth process leads to a metastable microstructure where plate-shaped α''-Fe16C2 clusters, spaced 5–10 nm apart, evolve in direct correlation with columnar ferrite nanograins. The clustering process follows a spinodal decomposition mechanism, with in-plane ring-like arrangements. These clusters contain nanosized layers, where carbon concentrations approach those of transitional η- and ε-carbides, supporting the view that carbon clusters act as intermediate states in the carbide precipitation pathway in supersaturated Fe-C. The strong correlation between carbon clustering and nanograin formation is most plausibly explained by the Gibbs adsorption effect, which reduces defect formation enthalpies, promoting the formation of extended defects. Nanoindentation measurements reveal that these films possess exceptional mechanical properties, with a hardness of 21(2) GPa, exceeding martensitic Fe-C and closely approaching the strength of nanostructured pearlite wires – one of the strongest ductile bulk materials known. In-situ TEM annealing of Fe-C films with ~11 at.% C reveals that the nanocolumnar ferrite microstructure is stable up to 270°C, beyond which carbide precipitation initiates. By 300°C, cementite grains emerge along nanograin boundaries, indicating a transformation from α'' clusters to transitional carbides and eventually to cementite. Above 375°C, the nanostructure rapidly breaks down, mirroring the tempering behavior of Fe-C martensite. These findings establish a direct link between decomposition in sputtered Fe-C films and thermally treated Fe-C alloys, reinforcing the universality of carbon clustering in Fe-C systems. Explosive welding creates melt pockets with a nanoscale microstructure rich in extended defects and plate-like carbon clusters, resembling those found in low-temperature tempered martensites. The clusters, spaced ~10 nm apart and reaching peak carbon concentrations of 7–9 at.% C, align parallel or perpendicular to one another, consistent with a spinodal formation mechanism. The presence of clusters along dislocation lines suggests that defects actively contribute to clustering. Notably, this metastable microstructure has remained stable for over 35 years at room temperature, underscoring its remarkable stability. This work reveals that carbon clustering in supersaturated Fe-C is not merely a byproduct of defect segregation but may actively drive defect formation. In particular, the findings suggest that in high-carbon Fe-C thin films, clustering itself triggers extended defect formation – providing direct evidence for chemically driven defect formation, a concept that has recently gained interest in materials science. Furthermore, the striking similarities between clustering in sputtered films, explosively welded steels, and tempered martensites suggest a shared local minimum in Gibbs free energy, independent of the initial processing route. Beyond fundamental insights, these findings have practical significance for energy-efficient nanomaterials synthesis. Unlike severe plastic deformation (SPD) techniques, which require significant mechanical energy input, nanostructure formation in high-carbon Fe-C thin films occurs spontaneously during deposition, providing a low-energy route to nanoscale material design. Moreover, controlled annealing could enable tailored nanostructured Fe-C composites, opening pathways for next-generation wear-resistant coatings. In summary, this work advances our understanding of non-equilibrium microstructure formation in supersaturated Fe-C solid solutions, offering insights into the fundamental mechanisms governing carbon clustering and its coupling to defect evolution. These findings inspire new approaches for the energy-efficient production of nanostructured Fe-C materials, with applications ranging from wear-resistant coatings to ultra-high-strength steels.
Keywords: atom probe tomography (APT); scanning transmission electron microscopy (STEM); Fe-C thin films; spinodal decomposition; metastable phases; non-equilibrium microstructures; nanostructured metals; carbon-supersaturated ferrite; carbon clustering; iron carbide precipitation sequence; Gibbs adsorption effect; chemically driven defect formation; in-situ transmission electron microscopy
