Redistribution of alloying elements in Zircaloy-2 after in-reactor exposure

Abstract

h i g h l i g h t sWe have conducted an atom probe tomography study on a Zircaloy-2 fuel cladding material that has been subjected to 9 annual cycles of in-reactor exposure.The results show that large numbers of nanosized Fe and Cr rich particles have precipitated along parallel planes in the matrix. Fe and Sn are seen to segregate to ring-shaped features that are interpreted to be c-loops. A sub-oxide of approximate composition ZrO is found at the metal-oxide interface. a b s t r a c tAn atom probe tomography study of the microstructure of a Zircaloy-2 material subjected to 9 annual cycles of BWR exposure has been conducted. Upon dissolution of secondary phase particles, Fe and Cr are seen to reprecipitate in large numbers of clusters and particles of 1-5 nm sizes throughout the Zr metal matrix. Fe and Sn were observed to segregate to ring-shaped features in the metal that are interpreted to be <c>-component vacancy loops. This implies that these two elements play a major role in the irradiation growth phenomenon in Zr alloys, which is believed to be caused by the formation of <c>-loops. Similarly to autoclave-corroded Zr alloys, the formation of a sub-oxide layer of approximate composition ZrO was observed. On the other hand, no oxygen saturated metal phase was detected underneath the oxide scale.

Effect of neutron and ion irradiation on the metal matrix and oxide corrosion layer on Zr-1.0Nb cladding alloys

“…Similar observations were reported on low tin ZIRLO ® after neutron and proton irradiation, where not only is Fe depleted from the Zr-Nb-Fe SPP but the Fe initially decorating β-Nb particle surfaces is also dissolved into the matrix [47]. After irradiation the displaced Fe can decorate nearby dislocation loops after both neutron [28] and proton irradiation [7].…”

“…This will create high concentrations of point defects that will enhance the rate of solute diffusion within the matrix so displaced Nb atoms can quickly return to their equilibrium state in the SPPs [83]. A key fact that is missing in developing an understanding of the RIP phenomenon in these alloys is accurate data on the concentration of Nb in metastable solid solution in the irradiated metal matrix that will require the use of techniques such as APT [28].…”

“…A similar analysis of the E635 (Zr-1%Nb-0.35%Fe-1.2%Sn) alloy containing predominantly Zr-Nb-Fe second phase particles (SPPs) showed that they transform to β-Nb after irradiation, with the Nb content increasing from 35 at% to 50 at% while Fe decreases from 20 at% to 5-10 at% [27]. These observations imply that after irradiation the Nb content in the matrix tends to decrease, but that Fe can be displaced into the metal matrix where it can decorate dislocation loops after both neutron [28] and proton irradiation [7].…”

Investigating the thermal stability of irradiation-induced damage in a zirconium alloy with novel in situ techniques

“…This irradiation-induced precipitation may provide barriers to dislocation loop glide and therefore increase the stability of the dislocation structure compared to alloys that show little irradiation-induced precipitation. This phenomenon may explain the lower temperature required for annealing in the Zr-0.1Fe sample studied here, where we do not have such precipitates away from the sparsely distributed SPPs, compared to neutron-irradiated Zircaloy-2 where irradiation-induced clustering has been observed throughout the matrix [20,21,49]. A particular benefit of this approach is that any sample-to-sample variation is eliminated during such in-situ analysis, compared to ex-situ studies.…”

Modelling and experimental analysis of the effect of solute iron in thermally grown Zircaloy-4 oxides

“…Even when the SPPs are fully oxidized, not all the iron will be incorporated into the ZrO 2 , as iron from oxidizing SPPs has been observed to diffuse towards free surfaces, forming an iron oxide phase on the outer surface of the oxide or on internal crack faces [21,22]. Finally, radiation has a significant impact on the iron-containing SPPs, which have been shown to undergo amorphous transformation and also segregation and dissolution of iron into the matrix [23][24][25][26][27][28]. This may cause a greater amount of iron to be found in solid solution as compared with that observed for unirradiated material.…”