Small-scale fracture mechanical investigations on grain boundary doped ultrafine-grained tungsten

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 root m. The samples doped with Hf even improved the fracture toughness to values of up to 27 MPa root 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 devel-opment 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.

Automated summary

The brittleness of tungsten is a challenge for its application as a divertor material in nuclear fusion reactors. Grain refinement is a promising strategy to increase its fracture toughness, but it also promotes intercrystalline crack growth. This study explores the use of grain boundary doping with boron and hafnium to enhance the fracture toughness of ultrafine-grained tungsten. The results show that doping with boron and hafnium can improve the fracture toughness of ultrafine-grained tungsten, with values up to 27 MPa root m.