Cr-coated Zr-4 alloy prepared by electroplating and its in situ He+ irradiation behavior

Abstract With the aim of improving the oxidation resistance of zirconium alloy claddings in light water reactors and enhancing the deposition efficiency and economics of surface coating, electroplating was used to deposit a Cr coating with a rate exceeding 12 μm/h on the surface of thermally treated Zr-4 alloy predeposited with an intermediate transition nickel layer. The evolution of helium bubbles in the cross-section of the Cr-coated Zr-4 alloy during 30 keV He+ irradiation at 400 °C was in situ analyzed at a newly constructed facility named the Xiamen Multiple Ion Beam In situ Analysis TEM Facility, which is the only running in situ irradiation facility at China. The thermal treatment induced chemical reactions between nickel and zirconium to form a multilayer structure (Ni5Zr, Ni7Zr2, Ni3Zr, Ni10Zr7, NiZr and NiZr2 in sequence) with metallurgical bonding. Meanwhile, the Cr coating combined with the Ni layer in a lattice-matching mode. No defects could be found at the interface between each layer. The different phases and interfaces showed different resistance abilities towards He+ irradiation, where the size and distribution characteristics of bubbles were different. A bubble-free zone could be observed adjacent to the interface between Cr and Ni layers; however, bubbles mainly aggregated at the interface between Ni and Ni5Zr layers. With the increase in helium concentration, both the size and density of bubbles increased in the Cr coating. Helium atoms precipitated preferentially along Cr grain boundaries to form bubbles and then further gathered to form short bubble microcracks. The results were discussed in terms of the energetics of the material system and the surface diffusion mechanism.