超细晶铁素体/马氏体钢中Mn的异常快速扩散及其对LBE氧化机制的影响

Reduced activation ferritic/martensitic steel would suffer from liquid metal corrosion and liquid metal embrittlement in contact with liquid lead-bismuth eutectic (LBE). An effective method to improve the compatibility of liquid LBE with steel is controlling the oxygen concentration dissolved in LBE, so that a protective oxide layer is formed on the surface of steel to prevent the dissolution of steel into liquid LBE. Most importantly, formation of a stable and dense oxide scale on surface of steel could provide a better protection against LBE. The presence of grain boundaries facilitates the formation of a protective oxide scale, which may provide a new method to improve the compatibility of liquid LBE with steel. During our previous oxidation test in liquid LBE, the ultrafine-grained 9Cr2WVTa steel presents a better oxidation resistance in liquid LBE, which is related to the enrichment of Mn in the oxygen diffusion zone and the inner oxide scale/outer oxide scale interface. Although the Mn content is low in the ultrafine-grained 9Cr2WVTa steel, an anomalous enrichment of Mn is observed in the oxide scale during oxidation test in liquid LBE and the mechanism is still unclear. In the present study, effect of defect types including vacancies, dislocations, substructure and grain boundaries, on the diffusion behaviors of Mn was investigated. On this basis, effect of Mn content on the formation and growth of oxide scale of ultrafine-grained 9Cr2WVTa steel during oxidation test in liquid LBE was studied. Thus, the anomalous diffusion behaviors of Mn and its effect on the oxidation mechanism in liquid LBE were clarified, which may be beneficial to the improvement in the compatibility of liquid LBE with reduced activation ferritic/martensitic steel.

低活化铁素体/马氏体钢在与液态LBE（铅铋共晶合金）接触时会发生腐蚀和脆化等现象。控制液态LBE中的氧分压，使材料表面生成保护性氧化膜可将材料与LBE隔离开来，以改善材料与LBE相容性，其中，氧化膜的致密性是决定性因素。合理利用晶界等可提高氧化膜的致密性，有望为提高抗LBE氧化性能提供新方法。前期发现超细晶9Cr2WVTa钢展现出优异的抗LBE氧化性能，这与氧扩散区、以及内层氧化膜/外层氧化膜界面处Mn的明显富集有关。超细晶样品中较低的Mn含量却引起氧化膜中Mn的异常富集，其机制尚不清楚。本项目通过控制钢中空位、位错、亚结构、晶界等缺陷数量，研究缺陷类型对Mn扩散行为的影响，阐明Mn的异常快速扩散机制；在此基础上，研究Mn对超细晶材料LBE氧化过程中氧化膜形成及生长行为的影响，揭示Mn的异常快速扩散行为对LBE氧化机制的影响，为改善低活化铁素体/马氏体钢与LBE相容性提供理论基础。

利用晶界等可提高材料表面氧化膜的致密性，有望为提高抗液态铅铋腐蚀性能提供新方法。本项目围绕超细晶铁素体/马氏体钢氧化膜中Mn的异常富集现象。利用冷旋锻变形处理技术，澄清了严重塑性变形和后续退火过程中位错密度、晶界/亚晶数量、碳化物分布的演变规律，实现了不同缺陷类型和数量的控制，并制备出具有超细晶结构的样品。设计了不同Mn含量的钢，采用高温空气氧化试验，澄清了Mn含量对元素扩散行为的影响。通过对比分析回火态样品、变形态样品和超细晶样品的高温空气氧化行为，澄清了位错密度、晶界数量等缺陷对元素扩散行为的影响。基于元素扩散行为研究，提出了Mn的异常快速扩散机制。在此基础上，澄清了Mn含量和晶粒尺寸对铅铋腐蚀行为的影响，揭示了Mn异常快速扩散行为对耐铅铋腐蚀性能的影响机制。

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