| Molten-salt reactor (MSR), one of the most promising next generation reactors,has incomparable advantages: inherent safety, fission fuel sustainable utilization,producing less long-lived wastes, excellent heat transfer characteristics and so on.Thorium-based Molten Salt Reactor technology is an important way to supplylong-term nuclear energy for the future. Hastelloy N alloy used as the structuralmaterial in MSR contacts directly with fluoride salts. Fission product Te leads tointergranular embrittlement of Hastelloy N alloy, which seriously affects the servicelife of Hastelloy N. In this study, the electroplating method was chose for preparingsamples, and the research is mainly concerned on microstructure evolution, varietiesand stability of nickel telluride, transformation of the tensile formability, andintergranular diffusion depth. A detailed analysis was taken to understand thediffusion behavior of Te in nickel and nickel chrome alloys and embrittlementmechanism. This study is helpful to improve Hastelloy N to resist intergranular brittle.Diffusion conditions (temperature, time, concentration of Te) will influence thetypes of reaction products and stability between tellurium and nickel. When theconcentration of Te is0.5mg/cm2, The main product is NiTe0.67or NiTe0.7below900C, which has good thermal stability. While no reaction layer is detected at1000C. When the concentration of Te is higher than10mg/cm2, the reaction layercomes off easily, and many types of nickel telluride are discovered, such as NiTe2,NiTe, NiTe0.77, Ni3Te2and so on. With the increase of temperature, nickel telluride ofthe hexagonal structure (NiTe2, NiTe) and orthogonal structure (NiTe0.77) are notstable and slowly change into monoclinic structure (NiTe0.67).Diffusion conditions will affect the microstructure of nickel diffused of Te. Thereaction product is lamellar structure. Grain bundaries in edge are etched moreseriously than that in center in nickel when temperature below900C, and which isincreasingly severe with temperature and time increasing. While the corrosionresistance of nickel is not change obvious when increased concentrations of Te.However, the whole grains and the grain bundary near the surface are corroded at1000C.Compared with the nickel without Te, the ultimate tensile strength(UTS) andbreaking elongation of samples with Te decrease dramatically. UTS is from380MPa down to200MPa and breaking elongation is from0.52to0.27between500C and1000C. UTS decline from340MPa to285MPa, whereas the elongation is0.45andnot change obvious in the diffusion time range of24h-1000h. Fracture of the alloywith Te exhibited brittle fracture with intergranular section near the surface andtransgranular deformation in the center. The depth of intergranular fracture graduallyincreases with temperature and time rising.The diffusion of Te in nickel base alloy are mainly vacancy diffusion. Diffusiondepth increases with the temperature and the time increasing. Te diffuses easily intonickel along the grain boundary below900C. While at1000C, the thermal vibrationof the matrix atoms is much more severe because of higher temperature. As a result,lattice diffusion becomes obvious and Te distributes equally into matrix. Theintergranular fracture depth (x) is measured by observing the fracture surface oftensile test specimens. Through fitting the linear relationship between (/) and1/, it is calculated that the diffusion activation energy of Te in Ni is152kJ/mol inthe temperature range from500C to1000C, and the formula of diffusion coefficientis=0.×10xp (10/) cm2/s.For the nickel chrome alloy，the interface products is NiTe0.67, while nochromium tellurium was found. The element of Cr is deficient in the edge of the alloy.The average size of the grain and the diffusion depth of Te in nickel chrome alloy aregradually decrease with the increasing of Cr content. When the content of Cr is higherthan15%, the alloy shows obvious effect to resist the diffusion of Te along the grainboundary. |
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