Grain-subgrain structure and vacancy-type defects in submicrocrystalline nickel at low temperature annealing

Scanning tunneling microscopy, positron annihilation and X-ray diffraction were applied for the study of annealing of submicrocrystalline nickel prepared by equal channel angular pressing. Several processes were revealed in the structure of submicrocrystalline nickel on different scale levels during annealing in the range ΔT = (20 ÷ 360) °C. A decrease of grain nonequiaxiality and further structure refinement were observed with a temperature increase in the range ΔT = (20 ÷ 180) °C. Subgrain growth with maximum =60 nm at 120 °C occurred on the lower scale level within the same temperature range. Grain growth and microstress decrease in submicrocrystalline nickel observed at T > 180 °C indicate the beginning of recrystallization. The main positron trap centers were identified in submicrocrystalline nickel within different temperature ranges. In as-prepared samples positrons are trapped at dislocation-type defects and vacancy clusters that can include up to 5 vacancies. At the annealing temperature ΔT = (20 ÷ 180) °C positrons are trapped at low-angle boundaries enriched by impurities. Within the range ΔT = (180 ÷ 360) °C the dominant trap is dislocations.