Radiation anneal hardening of neutron irradiated niobium

The radiation anneal hardening of niobium neutron-irradiated to a dose of 3.7 x 1024 n/m2 (E > 1.60 x 10-13 J) was studied by the internal friction measurement and the tensile test. The niobium wires (dia.: 1.25 mm) used in this study were Marz Grade Niobium (Nb > 99.96 wt%, Ta: 0.03 wt%) obtained from Material Research Corporation. The specimens were annealed at 2330 K for 5.4 x 103 s in vacuum of 1.3 x 10-5 Pa. Oxygen, nitrogen and carbon contents analyzed chemically after the annealing were 70 wt ppm, 30 wt ppm and trace, respectively. Two peaks of radiation anneal hardening were observed at temperatures of about 470 and 670 K. The internal friction measurement revealed that the 470 K peak of the hardening was mainly attributed to the formation of complex defects, which were radiation induced defects combined with oxygen atoms, and that the 670 K hardening peak was mainly caused by the complex defects formed by the combining of nitrogen atoms and irradiation induced defects. In the longer time annealing, the increase of yield strength by post irradiation annealing increased with increasing concentration of complex defects. (author)