Influence of microstructure on the fracture toughness of tungsten alloys

Tungsten and tungsten alloys show the typical change in fracture behaviour from brittle at low temperatures to ductile at high temperatures. In order to improve the understanding of the effect of microstructure the fracture toughness of pure tungsten, potassium doped tungsten and tungsten with 1wt% La2O3 has been investigated by means of 3-point bending -, double cantilever beam - and compact tension specimens. All these materials show the expected increase in fracture toughness with increasing temperature. The experiments show that the grain size, texture, chemical composition, grain boundary segregation and dislocation density seem to have a large effect on fracture toughness below the DBTT. These influences can be seen in the fracture behaviour and morphology, where two kinds of fracture occur: on one hand the transcrystalline and on the other hand the intercrystalline fracture. Therefore techniques like electron backscatter diffraction, auger electron spectroscopy and x-ray line profile analysis were used to improve the understanding of the parameter influencing fracture toughness.