Loading rate dependence of the fracture toughness of polycrystalline tungsten

Crucial issues for the application of tungsten as a structural material for future fusion reactors are its inherent brittleness at low temperatures and its relatively high brittle to ductile transition temperature. The transition temperature is not an invariant material property but depends strongly on the loading rate and the fabrication route. To gain insight into the controlling factors of the brittle to ductile transition in polycrystalline tungsten, a comprehensive fracture mechanical study was setup for a common commercial material. To account for the anisotropic microstructure, three different crack orientations with respect to the rolling were tested from -150 deg. C to 950 deg. C. Additionally, the loading rate was varied over two orders of magnitude. The results confirm that the transition temperature follows an Arrhenius relationship, giving an activation energy for the transition process. This activation energy can be employed to predict fracture toughnesses in a wide range of temperatures and loading rates.