Formation of metastable iron carbide phases after high-fluence carbon ion implantation into iron at low temperatures

Carbon ions were implanted with an energy of 100 keV into thin iron layers at temperatures of -70 ^∘{}C. Detailed analysis of structure was done by x-ray-diffraction synchrotron-radiation experiments. Phase formation was studied as a function of fluence from 1.2×{}10¹⁷ up to 1.2×{}10¹⁸ C⁺/cm². The existence of cementite ({θ}-Fe₃C) and other metastable phases {χ}-Fe₅{C}₂and η-{Fe}₂$C is demonstrated and explained by lattice-invariant deformation. The sequence of phase transformation during subsequent annealing to temperatures of up to 450 ^∘{}C was also investigated. The transformation sequence was found to end up in {θ}-Fe₃C, which is explained by a diffusion-determined process.