Designing Nanocrystalline Tungsten Alloys for the Extreme Environments of Future Fusion Reactors

The unique thermodynamic state occupied by nanocrystalline alloys presents opportunities for designing materials against instabilities while simultaneously enhancing their radiation tolerance through the deliberate introduction of defect sinks. Such instabilities are a particular concern for tungsten as a plasma-facing material (PFM) due to the demanding operating conditions involving high heat loads coupled with aggressive particle and neutron fluxes. Exploiting nanoscale grain boundary engineering in the design of tungsten as a PFM provides a potential pathway for enhancing its stability, mechanical performance, and radiation tolerance. In this presentation, we explore nanoscale grain boundary engineering approaches for the design of tungsten as a plasma facing material for fusion reactors. Dopant species and concentrations are first identified through lattice Monte Carlo modeling and used to guide powder metallurgy synthesis of ternary nanocrystalline tungsten alloys. Through synchrotron x-ray diffraction and small angle x-ray scattering experiments, we select optimized alloy chemistries containing nanoscale compositional heterogeneities for enhanced stability and sinterability. The effect of grain boundary doping on the coupling between microstructural evolution and irradiation damage state is then explored using heavy ion irradiation experiments. Defect evolution is mapped up to 20 dpa using in situ measurements and bridged to high-dose stability using ex situ experiments up to 400 dpa. The addition of grain boundary dopants is shown to stabilize the nano-alloy against irradiation induced grain growth, which is correlated to the evolving defect microstructure. Mechanistic insights on the role of grain boundaries in damage accumulation are gained using molecular dynamics displacement cascade simulations, demonstrating that the grain boundary character and damage proximity govern the balance between defect formation, migration, and recombination.