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 .