(332d) Molecular-Dynamics Analysis of the Mechanical Behavior of Plasma-Facing Tungsten

Investigating the impact of helium (He) ion implantation on the mechanical properties of tungsten is of utmost importance for evaluating tungsten as a plasma-facing component (PFC) in nuclear fusion devices. Toward this end, we carried out a systematic computational analysis of the mechanical behavior of plasma-facing component (PFC) tungsten (W) placing emphasis on how plasma exposure-related defects, namely, voids and helium bubbles, govern the mechanical and structural response to applied straining of the PFC material beyond the elastic deformation regime. Our analysis is based on molecular-dynamics (MD) simulations of uniaxial tensile straining tests (dynamic deformation tests at constant strain rate and temperature) using a properly parameterized machine learning Spectral Neighbor Analysis Potential (SNAP). Our computational models of PFC tungsten consist of regular arrays of He bubbles embedded in the tungsten lattice at different PFC matrix porosities.

We explore the effects on the mechanical behavior and structural response of our model PFC tungsten of a few key parameters such as porosity, bubble size, temperature, and He content. We find that there is a substantial softening of the elastic moduli, following an exponential scaling relation as a function of the porosity and the He atomic content. Beyond the elastic deformation regime, the presence of spherical defects (empty voids and He bubbles) of nanometer-scale size reduces the yield strength of tungsten according to an exponential scaling relation as a function of tungsten matrix porosity and He concentration. Detailed analysis of PFC tungsten structural response near the yield point showed that yielding is initiated by nucleation of twin regions as well as emission of dislocations from bubble surfaces, typically ½⟨111⟩ shear loops, which subsequently glide, grow, and may react to form ⟨100⟩ dislocations. Such twinning and dislocation dynamics facilitate substantial stress relief in the PFC crystal. After this stress relief stage, dislocation annihilation and twin boundary depletion reactions mediate a recovery stage in the PFC matrix, during which the stress increases upon continued applied straining. We also find that, at higher porosities, the higher He bubble density in the PFC crystal has a strong effect on twin boundary depletion during the recovery stage. In general, increase in the dislocation density and decrease in the areal defect density after the initial stress drop upon yielding make dislocation-driven deformation mechanisms dominate the mechanical response of the PFC crystal. Our analysis advances further our fundamental understanding of the mechanical behavior of PFC tungsten and elucidates the role that the most important plasma-related defects play in the mechanical and structural response of the PFC material to applied straining.