Effect of neutron fluence on microstructure and mechanical properties of Al 5052 irradiated in the National Research Universal (NRU) reactor

In support of the fitness service assessment on the National Research Universal (NRU) vessel and full evaluation of its properties, an Al 5052 rod made from the same material and irradiated in the NRU reactor, was retrieved and sectioned for microstructure characterization and mechanical testing. Along its axial position, the rod was exposed to various levels of neutron fluence ranging from 0 to 11.9x10²⁶ n/m² (E 25 n/m² (E > 0.1 MeV). The estimated peak fluence in the NRU vessel is within this range, i.e., 2.4x10²⁶ n/m² (E 22 n/m² (E > 0.1 MeV) after 44 years of service. The hardness, chemical compositions, microstructure, tensile and fracture toughness properties were examined from different sections of the Al 5052 I-rod. The study shows that the hardness increased with continuous production of transmuted Si until it reached a plateau, when all of the dissolved Mg (2.5 wt%) was consumed to form radiation-induced Mg–Si precipitates. Excessive Si produced by transmutation primarily precipitated on grain boundaries. Increase in thermal fluence caused the changes in sizes and chemical compositions of the precipitates in the grain interior and boundaries. The yield stress and ultimate tensile strength increased with increasing thermal fluence, but uniform elongation and fracture toughness decreased. The current results show that the NRU vessel wall is expected to have sufficient ductility, strength and material integrity until the planned shutdown in March 2018, and improve the understanding the effects of neutron irradiation on Al 5000 series alloys, which are widely used as vessel materials in experimental reactors.