Neutron irradiation has an adverse effect on the tensile ductility of austenitic steels and a nickel-base alloy at test temperatures above about 600 C. n contrast to the fast-neutron-induced, low-temperature irradiation hardening and embrittlement of materials, the loss in ductility at elevated temperatures is a function of the thermal neutron dose, and the effects cannot be recovered by post-irradiation annealing at temperatures up to about 1200 C.
Similar detrimental effects on the elevated-temperature tensile ductility are produced by irradiation at ambient temperatures and 650 C, respectively. The irradiation-induced loss in the elevated-temperature tensile ductility of a 20Cr-25Ni-Cb steel is preceded by the formation of fine carbide (and nitride) precipitates on dislocations and in grain boundary regions.
The deleterious effects of irradiation on the high-temperature properties are also manifested in post-irradiation stress-rupture and tube-burst tests. For example, the elongations to fracture and the rupture lives may be reduced in stress-rupture tests at 650 C.
The thermal neutrons are effective in producing the elevated-temperature loss in ductility by forming new atoms in the lattice by transmutation processes. The results of tension tests at 750 C on samples of 20Cr-20Ni, titaniumstabilized steels impregnated with B10 or B11 show that the loss in ductility can be correlated with the number of helium or lithium atoms, or both, produced by the B10(n,α)Li7 reaction.
The magnitude of the irradiation-induced changes in the elevated-temperature properties can be controlled to some extent by an initial cold-working and annealing treatment.