MECHANICAL PROPERTIES OF ZIRCALOY-2 NEUTRON IRRADIATED TO HIGH FLUENCE

Zircaloy-2 specimens neutron-irradiated at 70 °C to a fluence of 41.0 × 1025 n m−2 (E > 1 MeV) are deformed in tension at 23 °C and at 170 °C in the longitudinal and transverse directions, respectively. Neutron irradiation significantly increases the strengths of the material, accompanied by a corresponding decrease in ductility. The effect is such that the 0.2% yield stress (YS) and ultimate tensile strength are always greater in the longitudinal than in the transverse direction at low fluence (<35 × 1025 n m−2). While the longitudinal strength increases more rapidly with fluence than the transverse strength, they begin to converge at a fluence ≥20 × 1025 n m−2, becoming almost identical at a fluence >35 × 1025 n m−2. This indicates a change in deformation behaviour, from anisotropic to isotropic with increasing amounts of neutron fluence. The microstructure of the irradiated Zircaloy-2 (X-18) consists of a high density of 5–10 nm diameter a-type dislocation loops, with no observed c-component loops. Deformed longitudinal and transverse specimens of the X-18 that have been irradiated to fluence ≈25.9, 35.4, and 41.3 × 1025 n m−2 at 23 °C contain approximately an even distribution of dislocation-free channels on prismatic and first-order pyramidal planes, with a small fraction of second-order pyramidal channels. This is in agreement with the 0.2% YS in the longitudinal and transverse directions converging to the same value at high fluences.