Thermal stability of nanocrystalline Nb produced by severe plastic deformation

Nanocrystalline niobium with a grain size of the order of magnitude of 100 nm has been produced by the method of severe plastic deformation (high-pressure torsion). The evolution of its structure at various annealing temperatures in the range of 400-800 degrees C has been studied using transmission electron microscopy. It has been found that at 400-600 degrees C, recovery processes take place. Grain boundaries become thinner and more uniform, the dislocation density in the bulk of grains decreases, while the grain size does not increase, which means that the nanocrystalline structure is stable in this temperature range. An increase in the annealing temperature to 700 degrees C leads to an intense recrystallization. Grain size increases by an order of magnitude, and the size-distribution width grows noticeably. The microhardness changes in accordance with the structure evolution.