利用同步辐射散射和衍射研究金属中的氦脆问题

As the energy demand rises worldwide, electricity coming from nuclear power plants has become a more environmentally friendly and economically viable substitute for fossil. China is developing Thorium fueled molten salt reactor. In molten salt reactor environment, nickel based alloy is the primary choice of structural material due to its corrosion resistance to the salt. However, because of alpha decay induced by neutron irradiation, helium will accumulate in the metal during its service in the reactor. These helium atoms can aggregate and form nanosized bubbles. Eventually the metal will become brittle as large amount of helium bubbles exist in it. This phenomenon is called helium embrittlement. So it is important to understand the existent form of helium atoms in nickel based alloy and its correlation with the mechanical properties. This research will induce helium into nickel via ion accelerator. In this way the energy and number of helium atoms are controllable. The size and concentration of helium bubbles can be obtained by synchrotron X-ray scattering. The deformation of the crystalline structure can be probed by micro-focus diffraction. The information about the change of the micro-structure of the metal, combined with the information about the change of the mechanical properties such as hardness and elastic module, can be used to study the mechanism of the formation of helium bubbles and how it affects the mechanical properties, and hence propose a possible direction to improve the nickel based alloy for its application in reactors.

出于能源发展的战略需求，我国正在研发钍基熔盐堆，以促进核电的应用。在熔盐堆中，镍基合金因为比较耐熔盐腐蚀，成为首选的结构材料。但是在反应堆的服役环境里，镍基合金因为受到中子辐照和裂变产物的影响，会有大量的氦进入金属内部，这些氦聚集形成氦泡，会造成材料的脆化，又称氦脆。所以研究不同的辐照条件下氦在金属里面的存在方式及其和宏观力学的联系是有很大的应用价值的。本项目的研究拟采用粒子加速器往金属镍里面注氦，然后通过同步辐射散射来获得氦泡的浓度、大小，微聚焦衍射来获得镍晶格的应力变化。通过分析这些微观结构变化的信息，再结合测量注氦后的镍的硬度、弹性模量等宏观力学性质的变化，来研究在辐照环境中，镍基金属的氦脆的形成条件和机制，从而进一步提出镍基合金改性的方向。

在熔盐堆中，镍基合金因为比较耐熔盐腐蚀。成为首选的结构材料。但是在反应堆的服役环境里，镍基合金因为受到中子辐照和裂变产物的影响，会有大量的氦进入金属内部，这些氦聚集形成氦泡，会造成材料的氦脆。因此本项目采用小角度X射线散射（SAXS）测量和透射电子显微镜（TEM）表征在氦离子辐照下在镍箔中产生的氦泡的大小分布， SAXS获得的气泡尺寸的对数正态分布与TEM结果非常吻合。研究了纯Ni和Ni-1wt%SiCNP在600℃下He辐照后结构的演变，结果发现同种材料中氦泡尺寸随辐照剂量增大，且Ni-1wt%SiCNP材料中氦泡的尺寸小于纯Ni中氦泡的尺寸，数量密度大于纯Ni中氦泡的数量密度，说明纳米弥散相的加入能有效抑制氦泡的生长。利用同步辐射掠入射XRD的方法研究镍基合金在He,Ni等离子辐照后晶格常数的变化，发现晶格畸变是由辐照产生的空位和间隙子的竞争导致的, 同时发现不同能量、不同离子辐照对镍基合金的损伤程度及位点不一样。