Just another WordPress site - Ruhr-Universität Bochum

Abstract

Fracture toughness of commercially pure polycrystalline tungsten was investigated by means of mechanical testing and numerical simulations. The material was produced in the powder metallurgical route followed by hot rolling leading to an elongated grain structure. Notched samples prepared in three orthogonal directions with respect to the rolling direction were subjected to three-point-bending tests at different loading rates in a wide range of temperatures from 120 to 1200 K. The measured fracture toughness is compared with the results obtained for single crystals. We found that the fracture toughness of the polycrystal exhibits a much smaller temperature dependence such that the polycrystal is significantly tougher at low temperatures, but more brittle than the single crystal above 300 K. We consider the opposite effects of toughening and embrittlement due to grain boundaries to originate from temperature-dependent interactions between grain boundaries and dislocations. At low temperature grain boundaries introduce additional dislocation sources along the crack front so that more shielding dislocations can be produced than in a single crystal. At elevated temperature, however, grain boundaries limit the dislocation mobility and confine the plastic zone by hindering dislocation motion. This idea is verified by performing dislocation dynamics simulations for polycrystals with different dislocation generation models and different grain sizes. The numerical analysis qualitatively agrees with the experimental findings.