Molecular dynamics simulation of mechanical behaviors in nanocrystalline tungsten

Tungsten is a leading candidate for plasma-facing materials in fusion reactors owing to its exceptional properties such as high melting point, low thermal expansion coefficient and low hydrogen isotope retention. For such applications, nanocrystalline forms are promising due to their enhanced irradiation resistance. However, the mechanical strength of these nanostructures, which is critical for their stability, deviates from the classical Hall-Petch relationship. Moreover, the experimental difficulty in fabricating nanocrystalline tungsten poses a significant challenge to its practical investigation. Therefore, this work focuses on understanding how the grain size influences the behavior of uniaxial compressive deformation of nanocrystalline tungsten through molecular dynamics simulations by varying grain sizes (6.08-18.11 nm) and temperature (10-1200 K). Firstly, we find that there are different deformation mechanisms in larger and smaller grains, leading to a change from Hall-Petch relation to inverse Hall-Petch relation. Secondly, for samples with larger grain sizes, dislocations dominate deformation process while for the smaller ones, it is the grain boundary-relating mechanisms such as twins and grain boundary sliding determine the deformation. Finally, we find temperature can influence the strength of the nanocrystalline tungsten by reducing the flow stress in all grain sizes through promoting dislocation generation. In conclusion, our work reveals the unique mechanics in the deformation of nanocrystalline tungsten and find the optimal grain size with strongest strength which agree with experimental results. We expect these results can accelerate the design of new fusion reactors by facilitating the development of W-based nanostructured alloys.

In this study, the Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) was used for molecular dynamics simulation under an Embedded Atom Method (EAM) potential. Atomsk was employed to construct the nanocrystalline tungsten models with grain sizes of 6.08-18.11 nm. The temperatures were set at 10-1200 K, with a time step of 1 fs. Periodic boundary conditions (PBCs) were applied in all three dimensions, and uniaxial compression was imposed along the x-direction at a constant strain rate of 1 × 10⁹ s^-1, with isothermal-isobaric (NPT) ensemble. Visualization and structural analysis were conducted using the Open Visualization Tool (OVITO).