(388b) Elastic Properties of Plasma-Exposed Tungsten Predicted By Molecular-Dynamics Simulations

Studying 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 goal, we have performed systematic molecular-dynamics computations of the elastic properties of single-crystalline tungsten containing regular periodic arrangements of structural defects, voids and over-pressurized He nanobubbles, related to plasma exposure of PFC tungsten in nuclear fusion devices. Our computations reveal that the empty voids are centers of dilatation resulting in development of tensile stress in the tungsten matrix, whereas He-filled voids (nanobubbles) introduce compressive stress in the plasma-exposed tungsten. We find that the dependence of the elastic moduli of plasma-exposed tungsten, namely, the bulk, Young, and shear modulus, on its void fraction follows a universal exponential scaling relation. We also find that the elastic moduli of plasma-exposed tungsten soften substantially as a function of He content in the tungsten matrix, following an exponential scaling relation; this He-induced exponential softening is in addition to the softening caused in the matrix with increasing temperature. A systematic characterization of the dependence of the elastic moduli on the He bubble size reveals that He bubble growth affects significantly both the bulk modulus and the Poisson ratio of plasma-exposed tungsten, while its effect on the Young and shear moduli of the plasma-exposed material is weak.

Having established that the elastic properties of PFC tungsten depend on porosity, corresponding to the volume fraction of He-filled bubbles in the tungsten matrix, and the He bubble size, we expect that the scaling relations derived for the elastic moduli as a function of porosity and bubble size will be valid for the actual near-surface region of plasma-exposed tungsten, which contains spatially random distributions of irregularly shaped bubbles with a narrow size distribution. Specifically, we expect our findings based on a regular bubble array in tungsten to be in semi-quantitative agreement with the elastic properties of actual PFC tungsten to within the statistical errors related to the actual bubble size and spatial distribution. Our findings contribute significantly to our fundamental understanding of the mechanical response of PFC materials upon plasma exposure as well as to the development of a structure-properties database in support of predictive modeling of the dynamical response of PFCs in nuclear fusion devices.