Fe-C nanograined alloys obtained by high-pressure torsion: Structure and magnetic properties

The microstructure, phase composition, Mössbauer spectra and magnetic properties of nine binary Fe-C alloys with carbon concentrations between 0.05 and 1.7 wt.% were studied in the as-cast state, after a long annealing at 725 °C and after high-pressure torsion (HPT) at ambient temperature and 5 GPa with five anvil rotations (shear strain about 6). The grain size after HPT was in the nanometer range. Only Fe₃C (cementite) and α-Fe remain in the alloys after HPT. It was also shown that the less stable Hägg carbide (Fe₅C₂) and retained austenite disappear, and phase composition closely approaches the equilibrium corresponding to the HPT temperature and pressure. Measurements of saturation magnetization and Mössbauer effect reveal that the amount of cementite decreases after HPT. The reason for partial cementite dissolution is the formation of the carbon-rich segregation layers in the ferrite grain boundaries.