Tungsten-based materials are candidates for various high temperature applications such as future fusion reactors due to their excellent high temperature properties, however, the fracture behavior of polycrystalline tungsten and tungsten alloys is not well understood. These materials show the typical change in fracture behavior of body-centered cubic (bcc) metals - from ductile at elevated temperatures to brittle at low temperatures. This ductile-to-brittle transition (DBT) is well above room temperature and much higher compared to many other bcc metals like for example α-Fe. Grain size, grain shape, dislocation density, texture, chemical composition and grain boundary impurities are thought to have a large effect on the fracture toughness especially at temperatures below DBT. The limited ductility in the low temperature regime is one of the confining factors in the use of tungsten for structural applications.
In this thesis the fracture behavior of tungsten and tungsten alloys was investigated in a temperature range from room temperature up to 800°C. Main work was done on technically pure tungsten with a special focus on the fracture resistance of recrystallized and various deformed microstructures. Samples with different crack plane orientations were used to analyze the influence of grain shape. Furthermore, the effect of grain boundary impurities on the fracture behavior was investigated by Auger electron spectroscopy.
It will be shown that polycrystalline tungsten exhibits two coexisting types of fracture for most investigated samples - intergranular as well as transgranular. The varying amount of each type is mainly dependent on the grain shape and the crack propagation direction, therefore, the fracture toughness varies significantly with the testing direction. Impurities do not show a pronounced influence on the type of fracture while an increasing dislocation density due to deformation improves the fracture resistance. Furthermore, it will be shown that the fracture resistance increases with crack extension, known as R-curve behavior of a material. As a consequence the fracture toughness cannot be characterized solely by a single value, a resistance curve - R-curve - is necessary.