Non-Equilibrium Microstructures in Iron-Carbon Thin Films: Co-Evolution of Carbon Clusters and Ferrite Nanograins

This study investigates how carbon influences the microstructure formation in Fe-C thin films produced by room-temperature sputter deposition. Using 4D-scanning transmission electron microscopy and atom probe tomography, we examined films with varying carbon concentrations (1.3 to 17.1 at.% C). The results show that low-carbon films form crystalline ferrite microstructures, while high-carbon films remain amorphous. At intermediate carbon concentrations, a unique two-step process occurs: small ferrite seeds form in the amorphous film, grow to a critical size, and then undergo a decomposition process that creates nanometer-scale carbon-rich clusters within a carbon-depleted matrix. These clusters contain sub-nm thick nanofaceted regions of higher enrichment, with concentrations typical of transitional carbides, supporting the emerging view that binary iron carbides evolve through a series of transient states. The clusters co-evolve with ferrite nanograins, likely mediated by the Gibbs adsorption effect, linking nanoscale carbon heterogeneities to structural development. The resulting nanostructured films demonstrate remarkable hardness, comparable to advanced nanostructured steels. This study sheds light on how microstructures form in carbon-supersaturated Fe-C systems and may inspire new methods for creating efficient nanomaterials.