Irradiation Damage in Tungsten alloys

适用于核聚变反应堆偏滤器的材料需要能够承受高热负荷(高达20 mW/m2)、高瞬时应力水平和来自中子的高水平辐射损伤。这使它成为任何材料都必须生存的最极端的环境之一。领先的候选材料是钨，因为它的熔点高，并且具有良好的抗等离子侵蚀性能。然而，它的机械性能很差，而且缺乏关于它们如何被中子辐射降解的知识。众所周知，中子辐照对钨有两个主要影响。1)破坏晶格，导致缺陷，如空位、位错环和空洞；2)形变到不同的元素，导致高达5%的钨变成Re、Om、Ta和其他次要元素。原始元素和变形元素与晶体缺陷相互作用导致的微观组织的聚集和演化决定了重要的性能，如延性和热导率。到目前为止，大多数研究都集中在纯钨或二元合金上，因此多元素之间的相互作用相对未知。特别是，不同的转化产物如何相互作用尚不清楚。有些是集群所知的，但另一些可能是反隔离的。如果要开发能够包括真实的变形微结构的辐射损伤过程的模型，就需要了解这一点。将对两类辐照样品进行研究：1)离子辐照样品，它们被预合金化，以预测在未来的聚变反应堆中将产生WReTaos水平。这些样品已经在美国的LANL和英国的萨里接受了辐射，以及2)来自裂变反应堆(HFR荷兰、SCK-CEN比利时和波兰玛丽亚)的中子辐照样品。离子辐射如何模拟中子辐射是一个悬而未决的问题，我们将通过这个项目来回答。这个项目将使用原子探针断层扫描(APT)在单原子长度尺度上进行化学分析，并将观察到的化学结构与透射电子显微镜中成像的晶体缺陷联系起来。重点将是开发相关技术，以直接成像瞬变显微镜和APT中相同的缺陷。中子辐照样品将使用CCFE和牛津大学新安装的用于活性材料研究的设备进行研究。这包括第一个活跃的原子探测器的国家。微结构损伤对机械性能的影响将使用新开发的纳米压痕方法来绘制大范围(几平方毫米)的地图。这将允许理解钨合金中离子和中子损伤之间的差异，并开发使用替代辐照来预测中子辐照微结构的方法。学生将接触到广泛的实验技术和数据分析方法，并学习活性材料的工作。该项目与能源和核聚变EPSRC领域保持一致。

Materials suitable for use in the divertor of a nuclear fusion reactor need to be able to withstand, high heat loads (upto 20MW/m2), high transient stress levels, and high levels of irradiation damage from neutrons. This makes it one of the most extreme environments for any material to have to survive in. The leading candidate material is tungsten due to its high melting point, and good resistance to plasma erosion. However its mechanical properties are poor and there is a lack of knowledge on how they are degraded by neutron irradiation. It is well know that neutron irradiation will have two major effects on the tungsten. 1) damage to the crystal lattice leading to defects such as vacancies, dislocation loops and voids and 2)transmutation in to different elements which leads to upto 5% of the tungsten becoming rhenium, osmium, tantalum and other minor elements. The clustering and evolution of the microstructure due to the original and transmuted elements interacting with the crystallographic defects determines important properties like ductility and thermal conductivity. The interaction of multiple elements is relatively unknown as most research to now has focused on pure tungsten or binary alloys. In particular how the different transmutation products interact with each other is unclear. Some are known to clusters but others may antisegregate. A knowledge of this is need if models of the irradiation damage process are to be developed that can include realistic transmutation microstructures. Two sorts of irradiated samples will be studied 1) ion irradiated samples which are pre-alloyed to have WReTaOs levels predicted to be produced in future fusion reactors. These have been irradiated at LANL in the USA and Surrey in the UK and 2) neutron irradiated samples from fission reactors (HFR Holland, SCK-CEN Belgium and Maria, Poland). How well the ion irradiations mimic the neutron irradiation is an open question and one we will answer with this project.This project will use atom probe tomography (APT) to perform chemical analysis at the single atom length scale and relate the chemical structures observed to crystallographic defects imaged in the transmission electron microscope. There will be a focus on developing correlative techniques to directly image the same defects in TEM and APT. Neutron irradiated samples will be studied using newly installed equipment at CCFE and Oxford for the study of active materials. This includes the countries first active atom probe. The effect the microstructural damage has on mechanical properties will be studied using newly developed nanoindentation methods to map large areas (several mm2). This will then allow an understanding of the differences between ion and neutron damage in tungsten alloys and to develop methods to predict neutron irradiated microstructures using surrogate irradiations. The student will be exposed to a wide range of experimental techniques and data analysis methods and learn to work on active materials. This project is aligned with the energy and nuclear fusion EPSRC areas.