Development of microstructure and irradiation hardening of Zircaloy during low dose neutron irradiation at nominally 358 °c

Wrought Zircaloy-2 and Zircaloy-4 were neutron irradiated at nominally 358 °C in the high flux isotope reactor (HFIR) at relatively low neutron fluences between 5.8 × 10²² and 2.9 × 10²⁵ n/m² (E > 1 MeV). The irradiation hardening and change in microstructure were characterized following irradiation using tensile testing and examinations of microstructure using Analytical Electron Microscopy (AEM). Small increments of dose (0.0058, 0.11, 0.55, 1.08, and 2.93 × 10 ²⁵ n/m²) were used in the range where the saturation of irradiation hardening is typically observed so that the role of microstructure evolution and 〈a〉 loop formation on irradiation hardening could be correlated. An incubation dose between 5.8 × 10²³ and 1.1 × 10²⁴ n/m² was needed for loop nucleation to occur that resulted in irradiation hardening. Increases in yield strength were consistent with previous results in this temperature regime, and as expected less irradiation hardening and lower 〈a〉 loop number density values than those generally reported in literature for irradiations at 260-326 °C were observed. Unlike previous lower temperature data, there is evidence in this study that the irradiation hardening can decrease with dose over certain ranges of fluence. Irradiation induced voids were observed in very low numbers in the Zircaloy-2 materials at the highest fluence.