L12纳米析出相诱发面心立方高熵合金点蚀的机制

High-entropy alloys (HEAs) exhibit excellent comprehensive properties such as high strength, high ductility and high corrosion resistance, making them promising application prospects in nuclear safety, ship materials and aerospace areas. Pitting is the main corrosion failure form for HEAs, and the difference of compositions and microstructures between the precipitates and matrix phases can cause the preferential selective dissolution of the precipitates during corrosion process. However, the pitting mechanism of HEAs induced by the two-phase synergistic action is still unclear, needed to be studied in depth. In this project, L12-type nano-precipitates with controllable composition and size will be prepared in [(FeCoNi) or (FeCoCrNi)]AlxTiy face-centered cubic (FCC) HEAs. The electron work functions of L12 phase and FCC matrix phase will be determined by combining the first-principles calculation and atomic force microscope (AFM) measurement, and then the electron work function - composition (Al, Ti) three-dimensional diagram will be established to evaluate the galvanic corrosion tendency of the two phases. The effect of size and volume ratio of L12 phase on the current (or potential) variation rule at the L12/FCC phase interface areas during the pitting, induced by two-phase galvanic corrosion synergistic action, will be investigated by the microelectrochemical technique such as SVET. Combining with the characterization of evolution rule of passive film characters including compositions and microstructures at pitting area by in-situ TEM, 3DAP and XPS, the nucleation, growth and development mechanisms of HEAs pitting will be revealed. Based on the above research, the composition (Al and Ti) - L12 nano-precipitate features (size, volume ratio) - passive film character - pitting resistance relationship will be explored. This project will provide theoretical basis for the design and preparation of nano-precipitation strengthening HEAs with excellent pitting resistance.

高熵合金拥有高强度、高韧性、高耐蚀性等优异综合性能，在核安全、轮船材料及航空航天等领域应用前景广阔。点蚀是高熵合金的主要腐蚀失效形式，在腐蚀过程中析出相与基体相由于成分等差异会发生选择性溶解，但两相协同作用诱发点蚀的机制还有待深入研究。本项目拟在FeCoCrNiAlxTiy面心立方高熵合金中制备成分和尺寸可控的L12结构纳米析出相。通过第一性原理计算和AFM测量建立纳米相和基体相电子功函数-成分（Al和Ti）三维图，判断两相电偶腐蚀趋势；利用SVET等微区电化学技术研究纳米相尺寸等特征对两相电偶腐蚀协同作用诱发点蚀过程中两相界面处电流/电势变化规律的影响。结合原位TEM、3DAP等对点蚀区域钝化膜成分、结构等特性演变规律的精确表征，揭示两相电偶腐蚀诱发点蚀的形核、生长和发展机制，探索成分-纳米相特征-钝化膜特性-耐点蚀性能关联性，为设计制备耐点蚀性能优异的纳米析出强化高熵合金提供理论依据。

析出强化面心立方高熵合金因其优异的强韧性和良好的软磁等功能特性而有望成为新型的结构-功能一体化合金，但析出强化相导致的局部腐蚀问题限制了此类合金在苛刻服役环境中的应用。在本项目中，我们以(FeCoCrNi)100-x-yAlxTiy面心立方合金为研究对象，通过成分和组织调控，系统研究了析出相特征（种类、体积比、尺寸和形状等）对高熵合金耐点蚀行为的影响，探讨了析出相与基体相之间的微电偶腐蚀效应及其诱发合金点蚀形核的机制。研究结果表明，析出强化面心立方高熵合金的耐点蚀性能显著优于其它含Al、Ti的双相高熵合金以及316L奥氏体不锈钢，通过调控纳米相特性甚至可以使合金点蚀电位高于600 mV。然而，胞状纳米相在晶界附近的析出会降低其耐点蚀性能。同时，通过实验和第一性原理计算发现析出相与基体相之间的微电偶腐蚀作用是合金发生点蚀的主要原因。在微电偶腐蚀效应和Cl-的协同作用下，两相界面附近的析出相（阳极）表面钝化膜优先破裂成为点蚀形核点。通过本项目的研究，我们初步明确了合金成分-析出相特征-微电偶腐蚀效应/钝化膜特性-合金点蚀行为内在联系，为设计兼具高强韧和高耐点蚀性能的纳米强化面心立方高熵合金提供数据支撑和理论依据。