Distribution of Nb and Fe in a Zr-1%Nb-(Fe) alloy irradiated by protons and its relationship with irradiation defects

Fuel claddings and guide tubes, made of zirconium alloys, exhibit in-reactor deformation which is partly due to irradiation-induced growth. At high irradiation dose, growth breakaway occurs which is associated with the appearance of c-component loops. In the past the nucleation of c-loops has been correlated to the release of iron from Laves phases under irradiation. However, recent results show that increasing the iron content in Zr-1%Nb alloys tends to decrease the c-loop density and the irradiation induced growth rate. In order to understand these discrepancies, an extensive study was performed on the M5 alloy with Atom Probe Tomography (APT) and Analytical Transmission Electron Microscopy (ATEM). Proton irradiations were carried out at three fluences, with an energy of 2 MeV at 350°C. Native β-Nb particles showed a decrease in niobium content while Laves phases underwent a uniform amorphization with a loss of iron. Small and diffuse clusters of niobium and iron were formed everywhere in the matrix after irradiation. However, no niobium-rich nano-precipitates were seen using conventional TEM contrary to what is usually found in the literature. In addition, clusters forming intensive iron-rich bands were evidenced parallel to the basal planes around Laves phases for all the studied doses (3, 5 and 8 dpa). At 3 and 5 dpa, the iron rich band location is not correlated with c-loops but at 8 dpa iron segregation is found on c-loops. It is suggested that the iron lost by Laves phases first segregates on a-loop that are aligned along the basal planes, forming these iron-rich bands. At higher doses, iron also segregates on existing c-loops. This observation proves that c-loop nucleation is not directly enhanced by iron segregation, but there seems to be an indirect link between the loss of iron from precipitates and the c-loop formation.