Research On Microstructure And Performance Of Zirconium Alloys Under Ion Irradiation

Zirconium alloys are widely used as cladding materials in light water reactors due to their low thermal neutron cross section, good corrosion resistance and proper mechanical properties. For a long service period, the microstructure and properties of zirconium alloys change gradually in the harsh in-pile environment including water, stress, corrosion medium, and intense neutron irradiation, which will directly affect the service behavior of zirconium alloy fuel claddings and the safety of nuclear power plants. Studies of microstructure evolution and performance under irradiation help us understand the irradiation damage mechanisms, then provide the basis for the design and development of new zirconium alloys to meet the growing requirements of the nuclear industry.The entire testing process of the neutron irradiation of materials is complicated, long and expensive. Even after neutron irradiation, it is difficult to study samples with residual radioactivity unless with special facilities. With the comparable damage between ion irradiation and neutron irradiation, simulation of neutron irradiation damage with ion irradiation is necessary to give an insight into the irradiation effects of zirconium alloys. The ion irradiation test is shorter in time, higher in efficiency, lower in cost, and an easy-to-handle process. It can be observed that in the harsh service environment, the neutron irradiation plays an important role in microstructural evolution and mechanical properties changes of zirconium alloys.In this study, Zr-1Nb and Zr-1Nb-0.05Cu alloys with simple alloying composition and advanced corrosion resistant were the main research objects, as long as the specially designed Zr-1 Nb-0.5Cu alloys. The slow positron annihilation technique, coplanar extremely asymmetric X-ray diffraction technique, transmission electron microscopy, nanoindentation and thermogravimetry measurements were used to study the microstructure evolution and performance of these alloys under 6.37 MeV Xe26+ ion irradiation. The main findings are shown as follows.(1) The investigation the indentation size effects (ISEs) in indentation measurements of unirradiated zirconium alloys was a fundamental issue for further evaluation of ion irradiation effect on zirconium alloys and other metals. The ISEs were studied for the Zr-1Nb-0.05Cu alloy in hardness measurements and indentation creep tests. It is found that the indentation hardness, activation energy and activation volume were dependent on the indenter displacement. The hardness was proportional to the inverse indenter displacement, this behavior was related with the strain gradient theory, and the characteristic hardness could be determined by the Nix-Gao model. In the indentation creep tests, both the activation energy and activation volume exhibited ISEs. It is shown that the dominating mechanism changed from diffusion through dense dislocation for small indents to dislocation glide for large indents.(2) The Zr-1Nb alloys were irradiated up to peak doses of 0.15,0.5,1.5 and 2.5 dpa at room temperature. During ion irradiation, the microstructure evolution was shown as follows:the mono-, di- and trivacancies were formed, but their concentration remained constant; dislocation loops were also introduced, and their linear density increased with dose. The increase in hardness as a function of dose followed a power law expression with the exponent of 0.46. The activation energy at initial depth of 200 nm was higher than the unirradiated one. After high temperature steam corrosion tests, there were more and larger cavities or microcracks in oxide film of irradiated samples. Therefore, the dislocation loops induced by ion irradiation, on one hand, served as obstacles to dislocation glide and resulted in an increase in hardness and activation energy, on the other hand, which served as diffusion channel to make a major contribution to the enhanced oxidation.(3) Zr-1Nb samples were irradiated to 2.5 dpa at about 630 K to investigate the role of temperature played in the ion irradiation. After high temperature irradiation, the vacancy clusters and dislocation loops annihilated. Therefore, the hardness and activation energy of high temperature ion irradiation decreased compared with the same dose irradiated at room temperature. Besides, there were larger microcracks in the oxide film of samples irradiated at high temperature.(4) Alloys with different Cu concentrations were irradiated to 2.5 dpa, and the effect of Cu on the formation of<c>-type dislocation loops was investigated. The results showed that Cu assisted the formation of <c>-type dislocation loops. Moreover, the in-situ TEM observations of <c>-type dislocation loops annealing implied that the <c>-type loops annihilated at temperature higher than 870 K. Additionally, the addition of appropriate amount of Cu improved the corrosion resistance to high temperature steam.