【作者】 贾彦彦；

【导师】 徐洪杰；

【作者基本信息】 中国科学院研究生院（上海应用物理研究所） ， 核技术及应用， 2013， 博士

【摘要】 熔盐堆是六种第四代核反应堆之一，具有固有安全性、核燃料可持续利用、核废料少、热转化效率高等众多优点，钍增值燃料的熔盐堆技术是解决未来核能燃料短缺的重要途径之一。Hastelloy N合金作为熔盐堆的结构材料，与液态氟盐燃料直接接触。燃料盐中的裂变产物碲（Te）导致Hastelloy N合金晶间脆化，严重影响合金的使用寿命。本论文利用在纯镍及镍铬合金表面电镀Te的方法，结合金相显微镜（OM）、扫描电镜（SEM），电子探针（EPMA）同步辐射X射线微区元素荧光分析（μ-XRF）、X射线衍射（XRD）等分析方法，系统研究了Te在镍及镍铬合金体系中的微观组织演化规律、镍碲反应产物种类及稳定性、拉伸性能的变化及Te的扩散行为和沿晶扩散深度，并对Te在镍及镍铬合金中的扩散和脆化机制进行了较深入的讨论。这一研究对于改善和提高Hastelloy N的抗晶间脆化性能具有重要的理论和工程意义。不同的扩散条件（温度、时间、Te浓度）会影响镍碲界面产物的类型和稳定性。Te浓度为0.5mg/cm2时，随扩散温度的升高（<900℃），镍碲扩散系统的界面反应产物未发生明显变化，为NiTe0.67或NiTe0.7，而且该产物有良好的热稳定性；当温度升到1000℃时，没有界面反应产物出现。Te浓度为10mg/cm2时，样品表面出现脱层现象，有多种镍碲化合物出现，如NiTe2、NiTe、NiTe0.77、NiTe0.67等。随着温度的升高，六方结构（NiTe2、NiTe）和正交结构（NiTe0.77）的镍碲化合物不稳定，慢慢转变为稳定的单斜结构的镍碲化合物（NiTe0.67）。扩散条件不同时Te对纯镍微观组织影响不同。界面反应层为片层状碲化镍。当温度低于900℃时，Te主要对纯镍晶界产生影响，近表层的晶界比中心的晶界耐蚀性差，并且随扩散温度升高，纯镍近表层晶界受侵蚀现象越来越严重。当温度升到1000℃及以上时，Te的扩散致使纯镍近表面的整体区域（包括晶界和晶内）耐蚀性变差。随着扩散时间的延长，纯镍近表层晶界受侵蚀现象也越来越严重。而随表面Te浓度增加，纯镍近表层晶界耐蚀性变化不明显。Te在纯镍中的扩散会影响纯镍的拉伸性能。与无Te样品相比，镀Te纯镍样品室温拉伸性能随着扩散温度升高下降更明显。扩散温度在500℃-1000℃之间，镀Te的纯镍样品极限抗拉强度由380MPa急剧下降到200MPa，延伸率由0.52下降到0.27。扩散时间在24h-1000h之间，镀Te的纯镍样品极限抗拉强度由340MPa下降到285MPa，延伸率在0.45左右基本不变。镀Te样品室温下的拉伸断口为混合型断口，靠近表层附近为沿晶脆性断裂，靠近中心区域为穿晶断裂；并且随扩散温度升高或扩散时间的延长，沿晶脆性断裂倾向加剧。Te在纯镍中主要以空位扩散机制为主。Te在纯镍中的扩散深度随扩散温度的升高而增加，随扩散时间的延长而增加。扩散温度小于900℃时，Te主要沿晶界快速通道扩散，沿体内晶格扩散较弱；当温度升高到1000℃及以上时，原子振动加剧，晶格扩散占主导地位，晶界扩散强度相对变弱，Te在纯镍中发生均匀扩散。通过测量Te扩散后纯镍的沿晶脆断深度，再根据扩散公式拟合(/)与1/的线性关系式得出Te在纯镍中沿晶界的扩散激活能Q=153.5kJ/mol，在中低温时Te在纯镍中沿晶界扩散系数随温度变化的表达式为=0.×10xp (10/) cm2/s。对于镍铬二元合金，Cr元素的引入使合金发生晶格畸变。Te与镍铬合金的界面反应产物主要为NiTe0.67，没有发现铬碲化合物出现。合金近表层有贫Cr现象出现。随Cr含量的升高，合金晶粒逐渐减小，晶界贫Cr现象越来越明显，Te在镍铬合金中的扩散深度也逐渐降低。当合金中Cr含量高于15%时，合金中基本无Te扩散现象出现。更多还原

【Abstract】 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.更多还原