(335b) Mechanical Properties of Plasma-Exposed 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, here, we report results on the mechanical properties of single-crystalline tungsten containing structural defects, voids and over-pressurized He nanobubbles, related to the conditions of plasma exposure in nuclear fusion devices. Based on systematic molecular-dynamics computations using properly parametrized interatomic potentials, including recently developed machine learning potentials, we determine the elastic moduli and investigate the mechanical response of PFC tungsten beyond the elastic regime, computing mechanical properties such as tensile strength and characterizing in detail failure mechanisms upon introducing plasma-exposure-related defects into the tungsten structure.

Our simulations reveal that empty voids are centers of dilatation resulting in the development of tensile stress in the tungsten matrix, whereas He-filled voids (nanobubbles) introduce compressive stress in the plasma-exposed tungsten. We establish universal exponential scaling relations for the bulk, Young, and shear moduli, as well as the Poisson ratio of plasma-exposed tungsten as functions of its porosity. Furthermore, we find that the elastic moduli of plasma-exposed tungsten soften substantially as a function of He content in the tungsten matrix, also following exponential scaling relations, in addition to the softening caused with increasing temperature. A systematic characterization of the dependence of the elastic moduli on the He bubble size reveals that He bubble growth significantly affects 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.

Furthermore, analysis of the mechanical behavior of PFC tungsten reveals that the presence of voids reduces the ultimate tensile strength (UTS) of tungsten, which is further affected by increasing its porosity. Introducing He into the voids leads to further reduction of the UTS with increasing He content. We find that the presence of He bubbles in the tungsten matrix causes embrittlement, mediated by crack initiation at the bubble/matrix interface; thus, He bubble formation and growth has a significant impact on the fracture mechanics of PFC tungsten. Our findings contribute directly to the further development of a structure−properties database that is required for the predictive modeling of the dynamical response of PFCs in nuclear fusion devices.