Micromechanical Creep - Improving Experimental and modelling capabilities

本项目旨在开发一种在辐照损伤下测量堆芯内核构件蠕变变形的方法。蠕变变形是指材料在高于材料熔点一半的温度下，在载荷作用下发生的随时间变化的永久变形。蠕变变形对航天推进和能源发电等高温工程部件的结构完整性起着至关重要的作用。虽然已知高温工作是核裂变电厂主要的寿命限制因素之一，但对核聚变相关材料的研究却很少。尽管事实是，任何未来的核聚变动力系统都依赖于能够承受一些最极端工程环境的材料的发展。这些包括高达1500摄氏度的温度，高能中子的高通量以及等离子体嬗变和注入产生的气体元素的影响。由于努力尽量减少这种反应堆产生的核废料，可用于结构部件的元素是有限的，在许多情况下，缺乏对以太纯材料或合金中发生的基本变形过程的了解，重要的是这些过程如何受到温度，辐射损伤和气体含量的影响。由于中子辐照活动需要很长的时间，再加上与活性材料相关的成本和工作的困难，需要开发在微观尺度上表征辐照材料的可靠方法。这样做有两个好处。首先，它允许从小体积中子辐照材料中返回最大的数据。其次，它允许使用重离子辐照来模拟中子损伤。在这种情况下，虽然损伤与中子相似，并且可以在更短的时间内形成，但损伤仅超过10微米。这就排除了传统机械测试方法的使用。虽然了解微尺度塑性变形所需的方法已经发展得很好，但微断裂测试却落后了。虽然人们已经在开发微机械方法来测量所有主要的机械性能，包括断裂应力和韧性、屈服行为和弹性性能。关注蠕变的努力要少得多。这是因为传统的微机械方法只能在室温下工作。本项目旨在建立一个实验验证的微尺度蠕变模型。这将被纳入一个更大的项目，旨在为整个核工业开发多尺度模型。我们现在有能力在牛津的三个不同的系统上进行常规测试，最高可达1273K。其中一个系统允许使用二次电子显微镜同时成像，用于基于图像的分析。该项目将使用这些纳米压头与新型和新设计的微尺度测试样品，使用聚焦离子束加工切割，以提供简单应力状态下的良好描述几何形状。将对熔融材料（最初是低活化钢和304不锈钢，这两种材料在核领域也有更广泛的兴趣）进行原位和非原位蠕变试验，然后通过发展离散位错动力学模型和有限元分析来验证结果。一旦在未辐照材料上校准，该方法将应用于辐照钢。这将是第一次对辐照材料进行微尺度蠕变试验。这些数据将用于为正在开发的高比例模型提供信息，作为更广泛的EPSC资助的一部分。该项目由EPSRC聚变能科学与技术CDT资助。该项目属于EPSRC能源研究领域。

This project aims to develop a method for measuring creep deformation of in-core nuclear components in the presence of irradiation damage. Creep deformation is time-dependent permanent deformation of materials under load nominally at temperatures higher than half the material melting point. Creep deformation plays a crucial role in the structural integrity of engineering components that work at high temperature such as those in aerospace propulsion and energy generation. While it is known to be one of the main life limiting factors of nuclear fission power plants that work at high temperature little work has been applied to fusion relevant materials. This is despite the fact that any future nuclear fusion power systems rely on the development of materials which can withstand some of the most extreme engineering environments. These include temperatures up to 1500oC, high fluxes of high energy neutrons and effects of gaseous elements produced by transmutation and implantation from the plasmas. Due to efforts to minimise the production of nuclear waste by such reactors the elements which may be used in structural components is limited and in many cases there is a lack of understanding of the basic deformation processes occur in ether pure materials or alloys and importantly how these are effected by temperature, radiation damage and gas content. Due to the long time periods required for neutron irradiation campaigns, plus the associated cost and difficulty of working with active materials there is a need to develop robust methods for the characterisation of irradiated materials on the microscale. There are two advantages to this. Firstly it allows the maximum data return from small volumes of neutron irradiated materials. Secondly it allows the use of heavy ion irradiation to mimic neutron damage. In this case while the damage is similar to that of neutrons and can be built up in much shorter time frames the damage is only over a few 10's of microns. This precludes the use of traditional mechanical testing methods. While the methods required for understanding micro-scale plastic deformation are well developed micro-fracture testing has lagged behind. While there have been extensive efforts to develop micromechanical methods for measuring all major mechanical properties including, fracture stress and toughness, yield behaviour and elastic properties. Much less effort has been focused on creep. This is due to the fact that traditional micromechanical methods have only been able to operate at room temperature. This project will aim to develop an experimentally validated model of creep deformation on the microscale. This will be fed into a bigger project aiming to develop multiscale model for the complete nuclear industry.We now have the ability to carry out these tests routinely up to 1273K in Oxford on three different systems. One of these systems allows simultaneous imaging using the secondary electron microscope, for imaged based analysis. This project will use these nanoindenters in conjunction with novel and newly designed micro-scale test specimens cut using focused ion beam machining to provide well describe geometries with simple stress states. Creep tests will be performed, both in and ex-situ, on fusion materials (initially reduced activation steels and 304 stainless steel, which are both also of wider interest in the nuclear field) and then the results validated by development of discrete dislocation dynamics modelling and finite element analysis. Once calibrated on unirradiated materials the method will be applied to irradiated steels. This will be the first time microscale creep tests on irradiated materials has been undertaken. This data will be used to inform high scale models being developed as part of a wider EPSC grant. This project is funded by the EPSRC CDT in Science and Technology of Fusion Energy. This project falls within the EPSRC Energy research area.