FORMATION OF METASTABLE IRON CARBIDE PHASES AFTER HIGH-FLUENCE CARBON-IONS IMPLANTATION INTO IRON AT LOW-TEMPERATURES

Carbon ions were implanted with an energy of 100 keV into thin iron la yers at temperatures of -70 degrees C. Detailed analysis of structure was done by x-ray-diffraction synchrotron-radiation experiments. Phase formation was studied as a function of fluence from 1.2X10(17) up to 1.2X10(18) C+/cm(2). The existence of cementite (theta-Fe3C) and other metastable phases chi-Fe5C2 and eta-Fe2C is demonstrated and explaine d by lattice-invariant deformation. The sequence of phase transformati on during subsequent annealing to temperatures of up to 450 degrees C was also investigated. The transformation sequence was found to end up in sigma-Fe3C, which is explained by a diffusion-determined process.