The character of dislocation structure evolution in nanocrystalline FCC Ni-Co alloys prepared by high-energy mechanical milling

The process of dislocation structure evolution and nanocrystalline structure formation during high-energy mechanical milling (MM) was studied in Ni-Co alloys with different stacking fault energy (SFE). X-ray patterns were analysed to determine the lattice microstrain, average crystallite size, probability of the stacking faults and the lattice parameter. Comparison with some other methods of inducing plastic deformation (open-die forging and filing), as well as the analysis of temporal changes in the structural parameters during MM, allowed us to elucidate the sequence of events leading to the formation of a nanocrystalline structure. It was found that SFE is the key factor controlling the process of dynamic recovery. It significantly influences the kinetics of the nanocrystalline structure formation, as well as the steady state structural parameters observed at very long milling times. An anomalous dependence of the lattice microstrain on SFE was also observed. This effect is discussed using a model of the nanocrystalline state composed of dislocation pile-ups separated by high-angle boundaries.