The inherent brittleness of the refractory metal tungsten represents a major challenge for its application as divertor material in future nuclear fusion reactors. Grain refinement to the ultrafine-grained regime is a promising strategy to increase the fracture toughness of W, but it also promotes intercrystalline crack growth. Therefore, the strengthening of grain boundary cohesion in W is of great importance. In this work, grain boundary doping with B and Hf, two elements that were identified in previous work to increase bending strength and ductility, is applied to ultrafine-grained W. The fracture toughness is measured utilizing small-scale testing techniques. Fracture mechanical experiments on the microscale provide a plethora of challenges to correctly assess size-independent toughness values, which are presented and discussed within this work. It was found that the toughness of W can be under- and overestimated, depending on the sample dimensions and plastic zone size. When assessing the valid and size-independent fracture toughness measured for the differently doped W specimen, doping with the strengthening element B maintained the already remarkably high toughness of the undoped ultrafine-grained W of around 20 MPa√m. The samples doped with Hf even improved the fracture toughness to values of up to 27 MPa√m. Hence, the effects of GB doping on the fracture toughness of ultrafine-grained W are explored, while simultaneously the influence of sample dimensions on measured fracture toughness is discussed. These insights are expected to have a great impact on the development of superior materials for use in harsh environments, as well as the application of small-scale fracture mechanical experiments, as used, for example, in the assessment of control samples in nuclear technology.

难熔金属钨固有的脆性对其在未来核聚变反应堆中作为偏滤器材料的应用构成了重大挑战。晶粒细化至超细晶粒状态是提高 W 断裂韧性的一种有前途的策略，但它也会促进晶间裂纹的生长。因此，增强 W 中的晶界凝聚力非常重要。在这项工作中，将 B 和 Hf 的晶界掺杂应用于超细晶粒 W，B 和 Hf 这两种元素在之前的工作中被确定可以提高弯曲强度和延展性。断裂韧性是利用小规模测试技术测量的。微型断裂力学实验为正确评估与尺寸无关的韧性值提供了大量挑战，在这项工作中提出和讨论。结果发现，W 的韧性可能被低估和高估，具体取决于样品尺寸和塑性区尺寸。在评估为不同掺杂的 W 样品测量的有效且与尺寸无关的断裂韧性时，掺杂强化元素 B 保持了 20 MPa√m 左右的未掺杂超细晶粒 W 已经非常高的韧性。掺杂 Hf 的样品甚至将断裂韧性提高到高达 27 MPa√m 的值。因此，探索了 GB 掺杂对超细晶 W 断裂韧性的影响，同时讨论了样品尺寸对测量的断裂韧性的影响。预计这些见解将对开发用于恶劣环境的优质材料产生重大影响，