稀土钇对UNS N10003合金在高温熔盐环境下蠕变行为的影响以及相关机理研究

The molten salt nuclear power systems are operated in the harsh environment of high temperature, strong fluoride salt corrosion, low stress and so on. Among them, the structural material of loop and reaction vessel parts of long service in the environment, the mechanical property in high temperature molten salt environment of which will determine the service life of structure material, which affects the service life of the reactor. UNS N10003 alloy (Ni-Mo-Cr-Fe-C) is one of the candidate materials that are most close to the service requirements of molten salt nuclear energy system, however, its existing creep performance is difficult to meet the working temperature requirement of the new generation of nuclear power system with a higher temperature (> 750 degrees). This problem is expected to be solved by the modification of rare earth yttrium microalloying. The creep experiments are carried out on the high temperature molten salt environment creep testing machine. Combined the synchrotron radiation and transmission electron microscopy methods, fine local structure of fracture crack, grain-boundary, phase-boundary and corrosion film of yttrium doped UNS N10003 alloys are observed before and after the high temperature creep experiment, to in-depth study the effects and mechanisms of rare earth elements Y on creep behavior of UNS N10003 alloy in high temperature fluoride salt, and reveal the microstructure evolution and damage mechanisms of yttrium modified UNS N10003 alloys in the service environment of molten salt nuclear energy systems. It is desirable to synthesize a new UNS N10003 alloy containing rare earths yttrium with independent intellectual property rights in the laboratory scale, and lay a foundation for the preparation of nickel-based alloys which are suitable for applying to a new generation of molten salt reactors.

熔盐核能系统工作在高温、强氟盐腐蚀、低应力等恶劣环境下，反应堆熔盐回路和反应容器部位结构材料长期服役于该环境下，那么结构材料在高温熔盐环境下的力学性将能决定其服役寿命，进而影响反应堆使用寿命。UNS N10003合金是最接近熔盐核能系统服役要求的候选材料之一，但其现有高温蠕变性能难以满足新一代核能系统更高（＞750℃）的工作温度要求。通过稀土钇微合金化改性有望解决这一难题。本项目利用高温熔盐环境蠕变试验机进行蠕变实验，将同步辐射与透射电镜等表征手段相结合，观察掺钇UNS N10003合金在高温熔盐中蠕变断裂前后的断口、裂纹、晶界、相界以及腐蚀膜的精细局部. 深入研究钇对UNS N10003合金在高温熔盐中蠕变行为的影响及作用机制，揭示钇改性UNS N10003合金在熔盐核能系统服役环境中的微观组织演化规律及损伤机理，在实验室规模合成有自主知识产权的新型含钇的UNS N10003合金。

熔盐核能系统用结构材料工作在高温、强氟盐腐蚀、低应力等恶劣环境下，复杂恶劣的服役环境势必会降低结构材料的使用寿命，而结构材料的使用寿命将直接决定核能系的统服役寿命，所以结构材料的选择对于熔盐堆至关重要。目前，熔盐堆用结构材料是UNS N10003合金，但是该合金的在高温熔盐中的抗高温蠕变性能并不能完全满足新一代熔盐堆的需求，需要研发比其更耐高温熔盐腐蚀、更抗高温蠕变的合金结构材料。本课题提出一种痕量稀土钇改性UNS N10003合金，该合金具有非常好的抗高温氧化、抗高温熔盐腐蚀性能，同时还具有比原UNS N10003合金更优异的力学性能。通过本课题的研究，发现在700-750℃高温熔盐环境中，改性合金的抗高温蠕变性能得到显著的提高，在应力为190MPa时，蠕变断裂寿命提高30%左右，断裂延伸率提高45%左右。并且将同步辐射与透射电镜等常规表征手段相结合，对合金断裂后的微观组织进行观察，发现改性合金中的钇能够与熔盐中的氧化性杂质结合，形成一层有效降低铬元素流失的膜，从而减弱材料表面在加载有应力的情况下由于腐蚀造成的裂纹损伤，降低主裂纹的扩展速率，从而增强合金在高温熔盐中的抗蠕变性能。通过该课题的研究，明晰了合金结构材料在高温熔盐中的蠕变机制以及钇改性UNS N10003合金在高温熔盐中的抗高温蠕变的作用机制。目前该合金已经做到了20公斤级，热处理及加工工艺也得到定型，在实验室规模基本完成了改新型含钇合金的制备工作，在工艺放大过程中还有很长很曲折的路要走，但是目前取得的进步足以支撑后续工作的顺利开展，使其能够在新一代熔盐堆构件中使用的步伐又向前迈进一步。