Effect of the degree of deformation on the structure and thermal stability of nanocrystalline niobium produced by high-pressure torsion

The effect of the degree of deformation by high-pressure torsion (using five and ten revolutions of anvils) on the structure of niobium and its thermal stability under annealing has been studied using transmission electron microscopy. By deformation with five revolutions, nanocrystalline niobium with a grain size of about 100 nm has been produced, but increasing degree of deformation (using ten revolutions of anvils) has not resulted in a further structure refinement. Besides, the structure obtained at the higher degree of deformation is more imperfect and exhibits lower thermal stability. In the first case (deformation with 5 revolutions of anvils), structure recovery processes have been observed in niobium at 400–600°C, and only at 700°C intense recrystallization accompanied by an increase in the grain size by an order of magnitude occurred; after deformation with ten revolutions of anvils, the structure is thermally unstable already at 500°C, and after annealing at 600–700°C it becomes completely recrystallized. Along with studying structure evolution, changes in the micro-hardness have been investigated and a correlation between this parameter and specific features of the structure has been established.