We have investigated microstructure and microchemistry of precipitates and <c> dislocation loops in high-burnup M5® using (scanning) transmission electron microscopy ((S)TEM) equipped with energy dispersive X-ray spectroscopy (EDS). Two (S)TEM lamellae were made by cryo-FIB from the same cladding sample. The Nb-rich native precipitates were found in the metal, in the suboxide and in the oxide. Upon diffraction analysis, most of the Nb-rich native precipitates in the metal matrix remain as β-Nb phase, while no β-Nb native precipitates were found in the oxide. Nearby the oxide and metal (O/M) interface, the native precipitates in the oxide were already oxidized into t-NbO₂ phase. At further distance away from the O/M interface, the Nb-rich native precipitates were gradually oxidized and became amorphous. Besides the native precipitates, Nb-rich irradiation induced precipitates (IIPs) were found in the metal matrix. Using g = <0002> vector for imaging, the length of the IIPs was aligned with <c> dislocation loops. However, no Nb segregation to the <c> dislocation loops themselves was observed. For the first time, we report that IIPs indeed exist in the oxide but only within about 1.5 µm away from O/M interface. However, the oxidation state of the IIPs in the oxide is still unclear. The presence of both native precipitates and IIPs in the oxide may indicate the Nb concentration in the oxide solid solution remain low nearby the O/M interface, which may explain the reduced corrosion kinetics of in-pile M5®. On the other hand, no IIPs were observed in the oxide at further distance and this may indicate that they have eventually dissolved back into the oxide. A few mechanisms related to IIPs stability in the oxide are presented.

我们使用配备了能量色散X射线光谱（EDS）的（扫描）透射电子显微镜（（S）TEM）研究了高燃耗M5®中沉淀物和<c>位错环的微观结构和微观化学。通过cryo-FIB从同一覆层样品中制备出两个（S）TEM薄片。在金属，次氧化物和氧化物中发现了富含Nb的天然沉淀物。通过衍射分析，金属基质中的大多数富Nb天然沉淀物保持为β-Nb相，而在氧化物中未发现β-Nb天然沉淀物。在氧化物和金属（O / M）界面附近，氧化物中的天然沉淀物已被氧化成t-NbO ₂相。在距O / M界面更远的地方，富Nb的天然沉淀逐渐被氧化并变成非晶态。除了天然沉淀物外，在金属基质中还发现了富含Nb的辐照诱导沉淀物（IIP）。使用g = <0002>矢量进行成像，将IIP的长度与<c>位错环对齐。但是，没有观察到Nb偏析到位错环本身。我们首次报告，氧化物中确实存在IIP，但仅在距O / M界面约1.5 µm的范围内。但是，氧化物中IIP的氧化态仍不清楚。氧化物中天然沉淀物和IIP的存在可能表明氧化物固溶体中的Nb浓度在O / M界面附近仍然很低，这可能可以解释桩内M5®降低的腐蚀动力学。另一方面，在更远距离的氧化物中未观察到IIP，这可能表明它们最终已溶解回到氧化物中。提出了一些与IIP在氧化物中的稳定性有关的机理。