【作者】 李慧；

【导师】 周邦新；

【作者基本信息】 上海大学 ， 材料学， 2011， 博士

【摘要】 核电具有低污染、效益高的特点,在缓解能源危机的同时可以降低碳的排放,是当今世界上大规模可持续供应的主要能源之一。蒸汽发生器为压水堆核电站的主要构件。为满足苛刻条件,压水堆蒸汽发生器传热管材料早期曾使用奥氏体不锈钢。但奥氏体不锈钢发生过严重的应力腐蚀开裂（SCC）问题,后被耐热、耐腐蚀合金Inconel-600合金替代,后来600合金也出现过SCC问题。690合金被认为是目前最好的第三代压水堆蒸汽发生器传热管材料。如果要延长反应堆运行的服役寿命,以及继续提高反应堆运行的参数（如温度及压力）,690合金做成的传热管在寿命后期也可能出现失效问题。曾经做为蒸汽发生器传热管材料的合金都是Ni-Cr-Fe体系,其存在的问题也是相近的。沿晶界的腐蚀问题一直都是这些材料的主要失效原因,因此,有必要对Ni-Cr-Fe合金（690合金与304奥氏体不锈钢）晶界相关的问题进行研究,以进一步提高其服役寿命。本工作应用原子探针层析（APT）技术、高分辨透射电子显微镜（HRTEM）、扫描电子显微镜（SEM）与电子背散射衍射（EBSD）技术研究了690合金与304不锈钢中晶界处的偏聚情况、碳化物析出行为、及其对晶间腐蚀的影响。得出以下主要结论:（1）杂质原子或溶质原子向690合金与304不锈钢中同一条晶界处不同区域的偏聚倾向是不同的。杂质原子或溶质原子在晶界面上发生偏聚时并非是均匀分布的,存在一定的浓度起伏周期,周期变化的尺度约为7.2 nm。这种周期性的变化也反映了晶界结构的特征。（2）在晶界处Cr23C6形核之前,C原子与Cr原子会向690合金与304不锈钢的晶界处偏聚,形成共偏聚区。但是它们不是正好偏聚在晶界的核心区域,而是共偏聚在晶界的一侧,晶界处于此侧晶粒的高指数面。在C-Cr共偏聚区内,因偏聚而多余的Cr原子数可以多于形成碳化物所需要的Cr原子数,但是如果C的浓度过低会使碳化物无法形核。当晶界处C含量大于0.3 at.%时才会形成C-Cr共偏聚区,而在晶界处C含量小于0.2 at.%时观察不到C-Cr共偏聚区。（3）碳化物在晶界上析出时,在晶界面处于高指数晶面的一侧晶粒中形核,且与其具有共格的取向关系。碳化物会向与其无共格取向关系一侧生长得更快一些。碳化物生长过程中,碳化物与基体之间的界面可分为平直与弯曲。在基体与碳化物无共格取向关系一侧,碳化物与基体之间的界面主要为弯曲片段,界面无特定取向。在基体与碳化物存在共格取向关系一侧,随着时效时间的延长,碳化物与基体之间平直的界面片段越来越多,这些平直的界面处在{111},{002},{011}等低指数面。（4）在碳化物与基体平直的界面附近,观察到Cr元素在低指数晶面上周期性偏聚。在弯曲界面附近,基体与碳化物之间存在过渡相,过渡相尺寸最宽仅约30nm,这种过渡相为密排六方结构,其晶格常数为:与基体具有（111）γ//（0001）_hcp,（1-1-1）γ//（11-2-1）_hcp,（200）γ//（11-22）_hcp的取向关系。（5）晶界附近贫Cr区的Cr浓度梯度也是影响其耐晶间腐蚀性能的重要原因,而不仅是贫Cr区的深度。经过晶界工程（GBE）处理后,样品的耐晶间腐蚀能力明显提高,GBE样品的腐蚀失重要比非GBE样品的明显低。GBE样品晶间腐蚀主要沿晶粒团簇外围的随机晶界扩展。晶粒团簇脱落困难,能够阻止晶间腐蚀向样品内部扩展,有效的保护下层显微组织。更多还原

【Abstract】 Nuclear power has the features of low pollution and high efficiency. It eases theenergy crisis, and reduces the carbon emissions at the same time. Steam generatorsare the main components of pressurized water reactor （PWR） nuclear power plants.To meet the harsh environmental conditions, austenitic stainless steels have beenused as steam generator tube materials in PWRs earlier. However, the austeniticstainless steels generally damaged by severe stress corrosion cracking （SCC）.Thenthe corrosion resistant Inconel-600 alloy was used as an alternative material. But,SCC also appeared in Alloy 600. Alloy 690 is considered as the best steam generatortube materials for the third generation PWRs.But, prolonging service lifetime and improving performance of steam generatortubeshave been demanded by the nuclear energy industry. The steam generatortubesmade of Alloy 690 may also failure in the late service life. The alloys whichonce were used as steam generator tube materials are all Ni-Cr-Fe alloys. Thesealloys generally meet similar problem, e. g. corrosion along the grain boundary is amain failure reason of these alloys. Therefore, it is necessary to study the grainboundary related properties of Ni-Cr-Fe alloys, e.g. Alloy 690 and 304 austeniticstainless steel. This study is helpful to further improve the service life of Ni-Cr-Fealloys.The atom probe tomography, high resolution transmission electron microscopy,scanning electron microscopy and electron backscatter diffraction techniques wereused to study the grain boundary segregation, carbide precipitation, and their effectson intergranular corrosion resistance in Alloy 690 and 304 austenitic stainless steel.The following conclusions can be drawn:（1） Solute or impurity atoms have different tendency to segregate at differentregions in the same grain boundary both in Alloy 690 and 304 austenitic stainlesssteel. The segregated atoms do not randomly distribute in the grain boundary region.There is a ⁷.2 nm concentration fluctuation periodicity. The periodic distribution ofsegregated atoms is also the reflection of the structure of grain boundaries. （2） Before the Cr23C6 nucleated at grain boundary, C atoms and Cr atomsco-segregated to the grain boundaries in Alloy 690 and 304 autenitic stainless steel.The C and Cr atoms co-segregated at one side of the grain boundaries while not thegrain boundary core regions, the grain boundaries lying on the high indexed crystalplane of that side of grain. The number of excess Cr atoms in the C-Crco-segregation zone is more than that demanding for the nucleation of carbide, butthe number of excess C atoms is too low to nucleate carbide. If the concentration ofC is more than 0.3 at. %, the C-Cr co-segregation will form at the grain boundary.When the concentration of C is less than 0.2 at. %, the C-Cr co-segregation zone cannot be detected at grain boundaries.（3） Carbides nucleate on the grain which grain boundary plane lying on the highindex crystal plane, and have coherent orientation relationship with it. Carbides growfaster into the grain which does not have coherent orientation relationship with it. Thephase interface between carbides and matrix can be divided into flat and curvatureparts.Most of incoherent phase interfaces are curvature and not lying on typicalcrystal planes.More and more flat interfaces form at the coherent interface duringprolonging the aging time. These flat phase interfaces generally lye on {111}, {002},{011} crystal planes and other low index crystal planes.（4） The Cr atoms are observed to be periodically segregate on low index crystalplanes near the coherent phase interfaces. The periodic segregation leads to atransition phase forming near the curvature coherent phase interfaces. The transitionphase is less than 30 nm in width. This transition phase has hexagonal close packstructure with the lattice constants:And the transition phase has （111）γ// （0001）_hcp, （1-1-1）γ// （11-2-1）_hcp, （200）γ//（11-22）_hcp orientation relationship with the matrix.（5） Not only the Cr concentration level, but also the Cr concentration gradientsin the Cr depletion zone neat the grain boundary are the key factors that influencethe resistance to intergranular corrosion of the grain boundary. The grain boundaryengineering （GBE） treatments can improve the resistance to intergranular corrosion of the materials significantly. The weight losses of GBE specimens weresignificantly reduced than that of Non-GBE specimens. The intergranular corrosiongenerally penetrates along the outer random grain boundaries of the grain clusters inGBE specimens. The grain clusters are hard to drop, and the lower layer of thematrix can be protected.更多还原