Department of Materials, University of Oxford, Oxford OX1 3PH, UK.
Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA.
Nat Commun. 2017 Jun 30;8:15942. doi: 10.1038/ncomms15942.
Nuclear-grade graphite is a critically important high-temperature structural material for current and potentially next generation of fission reactors worldwide. It is imperative to understand its damage-tolerant behaviour and to discern the mechanisms of damage evolution under in-service conditions. Here we perform in situ mechanical testing with synchrotron X-ray computed micro-tomography at temperatures between ambient and 1,000 °C on a nuclear-grade Gilsocarbon graphite. We find that both the strength and fracture toughness of this graphite are improved at elevated temperature. Whereas this behaviour is consistent with observations of the closure of microcracks formed parallel to the covalent-sp-bonded graphene layers at higher temperatures, which accommodate the more than tenfold larger thermal expansion perpendicular to these layers, we attribute the elevation in strength and toughness primarily to changes in the residual stress state at 800-1,000 °C, specifically to the reduction in significant levels of residual tensile stresses in the graphite that are 'frozen-in' following processing.
核级石墨是当前和潜在下一代全球裂变反应堆的一种至关重要的高温结构材料。了解其耐受损伤的特性，并辨别在服役条件下损伤演变的机制是至关重要的。在这里，我们在环境温度到 1000°C 的温度范围内，使用同步加速器 X 射线计算微断层扫描对核级 Gilsocarbon 石墨进行了原位力学测试。我们发现，这种石墨的强度和断裂韧性在高温下都得到了提高。虽然这种行为与在更高温度下平行于共价-sp 键合的石墨烯层形成的微裂纹闭合的观察结果一致，这些微裂纹可以容纳垂直于这些层的十倍以上的热膨胀，但我们将强度和韧性的提高主要归因于 800-1000°C 时残余应力状态的变化，特别是在加工后“冻结”的石墨中显著的残余拉伸应力水平的降低。
