Refractory Metals: Tungsten, Molybdenum, and Niobium
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 8:30 AM
February 28, 2022
Room: 252B
Location: Anaheim Convention Center

Session Chair: Lauren Garrison, Oak Ridge National Laboratory; Gary Rozak, H.C. Starck Solutions

8:30 AM  
Grain Boundary Doping as Effective Method to Improve Mechanical Properties in Ultra-fine Grained Tungsten: Daniel Kiener1; Michael Wurmshuber1; Simon Doppermann1; Stefan Wurster2; Severin Jakob1; Markus Alfreider1; Klemens Schmuck1; Rishi Bodlos3; Lorenz Romaner1; Verena Maier-Kiener1; Helmut Clemens1; 1University of Leoben; 2Erich Schmid Institute of Materials Science; 3Materials Center Leoben
    Despite its many favorable properties for high-performance applications, tungsten oftentimes cannot tap its full potential due to its inherent brittleness and low formability. Especially ultra-fine grained tungsten, a promising candidate for nuclear fusion applications due to its excellent performance in irradiative environments, suffers from a low-energy intercrystalline fracture. A key factor to reinforce these weak grain boundaries is to increase the grain boundary cohesion, e.g. using doping elements. In this work, several ultra-fine grained tungsten samples, doped with elements that were identified to strengthen grain boundaries via density functional theory, were produced from powders using severe plastic deformation. Extensive microstructural and micro-mechanical characterization confirmed that hafnium and boron can simultaneously increase strength, ductility and toughness of tungsten. Furthermore, an additional heat treatment of the boron-doped sample allows for more grain boundary segregation, which improves the mechanical properties and overall damage tolerance even further, suggesting a pathway to adaptive tungsten alloys.

8:50 AM  
Realizing Sub-micron Tungsten and Tungsten Composite Microstructures via Deformable Punch Spark Plasma Sintering: Nachiket Shah1; Nathan Madden1; Sergei Zvenigorodsky2; Zachariah Koyn2; Jessica Krogstad1; 1University of Illinois at Urbana-Champaign; 2Energy Driven Technologies LLC
    Tungsten is an attractive refractory material for future fusion reactor applications because of its high melting point, good thermal conductivity, and high sputter threshold. While dense polycrystalline tungsten has been fabricated before using powder processing methods, the potential for finer grain structures possible via a lower temperature processing regime has not been fully explored. In this work, a modified high pressure die stack setup was used in a spark plasma sintering (SPS) machine to manufacture high density, sub-micron grained pure tungsten samples. Processing parameters such as pressure and hold temperature were systematically varied to observe changes in microstructure and mechanical properties. Additional compositional regimes were explored by incorporating composite-forming carbide additives. Our results show that sintered tungsten samples with reasonably high densities (>80%) and high microhardness values (up to 387 kgf/mm2) can be fabricated at pressures between 500-750MPa and at temperatures under a third of its melting point (<0.3Tm).

9:10 AM  
NanoPhase Separation Sintering in Mo and Mo-W Based Systems: Christian Oliver1; Christopher Schuh1; 1Massachusetts Institute of Technology
    Molybdenum and Molybdenum-Tungsten alloys have great potential for future nuclear power and thermal propulsion applications due to their high thermal stability, high thermal conductivity and corrosion resistance. However, Mo and Mo-W based alloys, often produced through a powder metallurgy route, can be difficult to produce, requiring high temperatures or pressure-assisted techniques such as hot isostatic pressing (HIP) to obtain dense parts. Nanophase Separation Sintering (NPSS) offers a way to design Mo-based alloys to exhibit low temperature pressureless sintering, involving the transient formation and resolutionization of second phase necks in the solid state. Experiments demonstrate that the addition of Cr as a sintering aid facilitates NPSS, obtaining near full density at low temperatures without applied pressure. Thermodynamic considerations for composition design for NPSS are also presented and experimentally evaluated.

9:30 AM  
Nanostructured Tungsten Alloys for Nuclear Fusion: Neal Parkes1; Alexander Knowles1; Chris Hardie2; 1University of Birmingham; 2UK Atomic Energy Authority
     Nuclear fusion offers a utopian vision for large-scale low-carbon energy production. Although great progress has been achieved on the underpinning plasma physics, engineering materials are increasingly the limiting factor. Tungsten is the leading candidate for the first wall and divertor for its high melting point and sputtering resistance. However, tungsten’s low ductile to brittle transition temperature (DBTT) and poor irradiation tolerance makes its use a major issue. One approach to improve its mechanical properties is through nanostructuring and grain refinement. Here we explore novel tungsten alloys that exploit phase decomposition to create nano-sized precipitates and grains. Alloys produced through arc melting and powder metallurgy methods, were characterised by EBSD, EDS and x-ray diffraction. Thermal heat treatments were applied to confirm the stable phases, and to promote phase decomposition & nanostructuring. The novel microstructural characterised are sought to gain improved radiation resistance, and mechanical properties at room temperature.

9:50 AM Break

10:05 AM  
Tensile Properties of Molybdenum & ODS MoLa Sheet at Elevated Temperatures: Alex Xie1; Jacqueline Foradora1; Gary Rozak1; Mike Stawovy1; 1H.C. Starck Solutions
    Pure molybdenum and ODS Moly Lanthana are applied is specialized furnace applications. The tensile properties of 1.0 and 0.5 mm sheets were evaluated with a “hot-grip” high temperature test system. The test temperature ranged from room temperature to above the recrystallization transition to a maximum of 1550C . The strain rate was varied from the ASTM E21 standard. The strength is more is more strain rate sensitive when the temperature is lower. The ODS Molybdenum-Lanthana sheet is mores strain rate sensitive that the pure molybdenum. Test specimen fracture surface was evaluated for understanding of related mechanism.

10:25 AM  
Bending Creep Deformation of ODS MoLa with 0.6 vs 1.1 wt.%La: Brandon Kenny1; Gary Rozak2; Jacqueline Foradora2; Alex Xie3; 1Miami University; 2H.C. Starck Solutions Euclid; 3H.C. Starck Solutions Taicang
    Resistance to elevated temperature creep deformation is an important characteristic for high temperature furnace applications. This work evaluated creep deformation of ODS Molybdenum-Lanthna sheet at two Lanthanum concentrations 0.6 & 1.1%. Testing methods are applied for screening the effects of process variations on the deflection resistance capabilities in sheet bending of molybdenum and Mo alloys at temperatures up to 1550C. The first stage of the testing evaluated the room and elevated temperature tensile properties of commercially available Mo & Mo alloys that are applied in high temperature furnace. Second Stage of testing evaluated the bending deformation in a multi-cycle dead weight 3-point bending configuration of ODS Moly-Lanthana. Sheet material was produced via powder metallurgy technique with thermal mechanical processing. Results of this testing will provide insight on the temperature-stress level capability for designing applications of these materials in furnaces.

10:45 AM  
Flow Behavior and Associated Microstructures of Niobium at 1200 to 1500°C: Emily Brady1; Eric Taleff2; 1Exponent; 2University of Texas at Austin
    The tensile flow behavior of a Type 2 niobium sheet material, per ASTM B393-18, was evaluated at temperatures from 1200 to 1500°C and true-strain rates of 10-3 and 10-4 s-1 in a vacuum environment. Microstructures before and after tensile testing were characterized using scanning electron microscopy and electron backscatter diffraction. Microstructural features characterized include grain size, microtexture, and structures produced during deformation at elevated temperature. The flow behaviors and microstructures of this material are compared to those of a Type 1 niobium material; the Type 1 material has a lower impurity content than the Type 2 material. The Type 2 material is more resistant to grain growth and presents higher flow stresses than the Type 1 material. Differences in microstructure between the Type 1 and Type 2 materials are used to rationalize the differences in flow behaviors observed at elevated temperatures.