Thermal stability of nanocrystalline structure in niobium processed by high pressure torsion at cryogenic temperatures

Evolution of structure of Nb subjected to high-pressure torsion (HPT) in liquid nitrogen and further annealing in the temperature range of 100–600 °C has been studied by transmission electron microscopy (TEM). HPT at the cryogenic temperature of 80 K results in the formation of nanocrystalline structure in Nb, with crystallite sizes of about 75 nm and the record-breaking microhardness of 4800 MPa. The structure obtained is stable at room temperature but possesses relatively low thermal stability, namely, it undergoes recrystallization at lower temperatures than the structure after the conventional deformation or room-temperature HPT.