EFFECT OF IMPURITY DOPING ON MECHANICAL PERFORMANCE AND MICROSTRUCTURE IN ULTRA-FINE GRAINED TUNGSTEN PROCESSED BY HPT

摘要

Tungsten is frequently considered as candidate material in high-temperature and high-performance applications due to its favorable physical properties, such as a high melting point, excellent intrinsic strength and good thermal conductivity. A comparably low fracture toughness and a high ductile-brittle transition temperature often limits the applicability and full potential of tungsten-based materials. Grain refinement by severe plastic deformation to the ultra-fine grained regime (100-500 nm) is known to improve strength as well as ductility and fracture toughness, but also for promoting intercrystalline fracture along the increased amount of grain boundaries. Enhancing the grain boundary cohesion by doping using, for example, carbon or boron might therefore lead a pathway to an additional improvement in fracture properties. In order to realize precise control of impurity content, the first challenge was to develop a fabrication route for ultra-fine grained tungsten starting from a material powder. Several issues arise from processing tungsten powder via high-pressure torsion due to the intrinsic properties of the material as well as the affinity of the powders to oxidize. These problems and their solutions are addressed in the first part of this work. To confirm the developed powder route in its eligibility, ultra-fine grained tungsten produced from a bulk precursor is then compared to the samples fabricated from powders regarding microstructural features and mechanical properties. Finally, after proving that both fabrication methods lead to comparable material properties, tungsten samples doped with various amounts of additional carbon (1-10 at.%) are fabricated and characterized extensively using nanoindentation and in-situ micromechanical testing. The effect of the carbon content on microstructure, mechanical properties and deformation behavior is thoroughly discussed in this work.