Ultrafine-grained Tungsten by High-Pressure Torsion - Bulk precursor versus powder processing route

The continuous enhancements and developments in the field of power engineering, as well as the uprising of nuclear fusion technology, demand novel high performance materials featuring exceptional strength and damage tolerance as well as durability in harsh environments. Ultra-fine grained bulk materials fabricated by high-pressure torsion, exhibiting a grain size less than 500 nm are promising candidates for these applications. Tungsten, the material of choice for plasma-facing materials in fusion reactors, is expected to exhibit even more enhanced properties by precise doping with impurity atoms, strengthening grain boundary cohesion. In order to allow this meticulous control of chemical composition, in-house mixing of the raw material powders is preferable to use of commercially available alloys. Several challenges arise in powder processing of tungsten via high-pressure torsion, originating in the intrinsic strength and high melting point of the material, and in the affinity of the powder to oxygen. Strategies to overcome these problems will be addressed in this work. Furthermore, we compare ultra-fine grained tungsten produced from a bulk precursor to that from the developed powder approach regarding microstructural features, hardness and rate-sensitive properties. The powder route showed promising and widely comparable results to the material processed from bulk tungsten, rendering it an effective way for fabricating ultra-strong tungsten, while keeping the additional ability to accurately control chemical composition and tailor grain boundary segregation states.