Refractory Metals: On-Demand Oral Presentations
Sponsored by: TMS Structural Materials Division, TMS: Refractory Metals Committee
Program Organizers: Eric Taleff, University of Texas at Austin; Lauren Garrison, Commonwealth Fusion Systems; Alexander Knowles, University of Birmingham

Monday 8:00 AM
March 14, 2022
Room: Advanced Materials
Location: On-Demand Room

Thermal Stability of Thin Rolled Potassium-doped Tungsten Sheets at Temperatures between 1100 °C and 1400 °C: Ditlev Tarras Madsen1; Umberto Ciucani1; Andreas Hoffmann2; Wolfgang Pantleon1; 1Technical University of Denmark; 2Plansee SE
    Microstructural changes which occur in plasma-facing tungsten at the high operation temperature in fusion reactors might be prevented by potassium doping. The thermal stability of thin cold-rolled tungsten sheets containing 80 ppm potassium is characterized. Mechanical degradation is quantified by micro hardness testing; microstructure and texture evolution are investigated using electron backscatter diffraction. During annealing, the potassium-doped tungsten sheets undergo extended recovery. No evidence of primary recrystallization is found in the investigated temperature range, but occasionally secondary recrystallization close to the surface. Annealing at the highest temperatures causes a significant loss in hardness during the first two hours, but only smaller changes further on, indicating that recrystallization is impeded through immobilizing high angle boundaries by potassium bubbles. The recovery kinetics for potassium-doped tungsten fits nicely the master curve of recovery obtained on pure tungsten sheets confirming recovery as dominating mechanism occurring unaffected by potassium doping.

Grain Boundary Segregation Engineering in Technically Pure Molybdenum Examined via Three-point Bending Tests: Severin Jakob1; Thomas Weissenboeck1; Anton Hohenwarter1; Alexander Lorich2; Wolfram Knabl2; Reinhard Pippan3; Helmut Clemens1; Verena Maier-Kiener1; 1Montanuniversität Leoben; 2Plansee SE; 3Erich-Schmid-Institute of Materials Science, Austrian Academy of Sciences
    Molybdenum has highly advantageous functional and high-temperature properties. However, plastic deformation is limited due to its tendency for brittle, intercrystalline failure, especially at low temperatures. It is well known that segregations have a decisive effect on grain boundary cohesion. An advanced approach for the improvement of the boundaries is segregation engineering, e.g. the deliberate addition of cohesion enhancing elements. To investigate crack formation, three-point-bending tests on recrystallized commercially pure and boron micro-doped molybdenum were conducted between -28°C and RT. The tensile-loaded top surface of the specimens are examined post-mortem close to the final fracture plane via scanning electron microscopy. The occurring, mainly intergranular, separations of grains are investigated for distinct features such as crystallography and length of open boundaries. The chemical composition of the interface is complementary measured by atom probe tomography. Necessary requirements for a direct comparison between the material variants and the effect of boron doping are discussed.

Tungsten Grain Optimization and Composite Fabrication for Use in Fusion Reactors: Lauren Garrison1; John Echols1; Nathan Reid1; 1Oak Ridge National Laboratory
    Tungsten will be a plasma-facing material in future fusion reactors. Its properties, mechanical and gas retention especially, depend heavily on its processing history including grain size and dislocation density. To form a full plasma-facing component, tungsten must be joined with other materials, which can further alter its microstructure. This work’s purpose is twofold: 1) examine tungsten with varying grain sizes and textures (powder sintered and rolled; forged; and e-beam additive manufactured), and 2) develop tungsten joints and composites relevant for fusion (W-SiC, W-Cu, and W-steel). The fabrication of the joints and composites will be described. Unirradiated materials were examined with scanning electron microscopy, electron backscatter diffraction, and miniature three-point bend tests. The mechanical properties are linked to the grain characteristics and/or interface properties. This development and characterization set the stage for the future neutron irradiation of these materials in the High Flux Isotope Reactor.

Studies on the Phase Formation of Cobalt Contacted with Zinc Vapour: Melanie Leitner1; Eva Gerold1; Stefan Luidold1; Christoph Czettl2; Christian Storf2; 1Montanuniversitaet Leoben; 2CERATIZIT Austria GmbH
    The main components of cemented carbides are cobalt and tungsten carbide, with cobalt being the binder metal and tungsten carbide the hard phase. To process these materials and recover their recyclable components the zinc process is used. In the first step, hard metals react with zinc at 900–1000 °C and form intermetallic Co-Zn phases. This leads to a volume expansion, resulting in a destruction of the bond between cobalt and tungsten carbide. Investigations with liquid zinc have been carried out, but due to the better kinetics and, according to the literature, higher decomposition rate, this research focuses on gaseous zinc. To gain profound knowledge these investigations work with a simplified system of the main components in the recycling process. Pure cobalt reacts with vaporous zinc under variation of different process parameters. This study focuses on the formation of intermetallic phases of Co-Zn diffusion couples.

Development of Novel Chromium-superalloys Strengthened by B2 Intermetallic Precipitates: Kan Ma1; Jóhan Magnussen1; Anke Ulrich2; Till König2; Mathias Galetz2; Alexander Knowles1; 1University of Birmingham; 2DECHEMA-Forschungsinstitut
    Common to many high temperature applications, increases in operating temperature are eagerly sought to improve the thermal efficiency of concentrated solar plants (CSPs). However, increasing operating temperature creates new challenges for the materials employed. Chromium (Cr) offers significant advantages for advanced CSPs with its high melting point, low price and good resistance in corrosive environments. However, the mechanical properties of Cr-based materials still need improvement to match current nickel superalloys, both for creep resistance and low temperature toughness. Here we explore the ‘bcc-superalloy’ design concept by introducing ordered-bcc (B2) NiAl precipitates into disordered-bcc (A2) Cr matrix to give Cr-superalloys that are hoped to confer attractive properties. This work addresses the demonstration and optimization of B2-A2 microstructures. The influence of alloying content, alloying element and thermal treatment on the morphology, distribution and volume fraction of precipitates is investigated alongside their influence on mechanical and environmental properties.