Reduction in Ductility of Austenitic Stainless Steel after Irradiation

Abstract

RECENTLY it has been established that the reduction in ductility of austenitic stainless steel in the temperature range 650°–850° C after neutron irradiation is caused primarily by the thermal neutrons^1,2. The decrease in ductility occurs in the non-uniform portion of the stress–strain curves and the fracture is intergranular^2–5. The elevated temperature ductility cannot be recovered even by annealing for 0.5 h at temperatures above 1,000° C (refs. 4 and 6). Furthermore, the magnitude of the effect is associated with the boron-10 content of the steel⁷. It appears most probable, therefore, that one or both of the products of boron-10 (nα) lithium-7 transmutation are responsible for this effect. A mechanism, based on the presence of helium at the grain boundaries, has been suggested⁸ recently to explain the decrease in ductility and the tendency to intergranular fracture, and after some conditions of irradiation helium bubbles have been observed at the grain boundaries⁹, using transmission electron microscope techniques. However, neither the relative nor additive effects on the elevated temperature ductility is known for such small amounts of helium and lithium as those formed by the thermal neutron transformation of the boron-10 isotope present in the steel.