Fracture toughness evaluation of UFG tungsten foil

The fracture behaviour of pure, ultrafine grained 100 μm tungsten foil was investigated as a function of testing direction and temperature in the range from −196 °C to 800 °C. To study the influence of the anisotropic microstructure on the fracture process, the single-edge-notched specimens were extracted in three different crack orientations relative to the rolling direction. This thorough investigation shows the positive impact of deformation induced grain refinement through extraordinary values of fracture toughness and a reduction of the ductile-to-brittle transition temperature to about room temperature. Furthermore, it was demonstrated that the grain shape anisotropy and a strong rotated cubic texture are decisive factors for anisotropic fracture properties. Fracture surface investigations reveal distinctive behaviour with an increase in temperature. The pronounced transition in failure mode was observed going from brittle, transcrystalline fracture at −196 °C towards pronounced delamination at intermediate temperatures and to ductile failure at highest temperatures. The measurement of the crack tip opening displacement for specimen fracturing in a completely ductile manner enables the determination of more accurate fracture toughness in the elastic plastic regime.