(323b) Thermomechanical Properties of Plasma-Facing Tungsten

Investigating the impact of helium (He) ion implantation on the thermomechanical properties of tungsten is of utmost importance for evaluating tungsten as a plasma-facing component (PFC) in nuclear fusion devices. To this end, here, we report results on the thermomechanical behavior of single-crystalline tungsten containing structural defects, voids and over-pressurized He nanobubbles, related to the conditions of plasma exposure in nuclear fusion devices. We develop PFC models as regular distributions of spherical nanoscale-size He bubbles in the tungsten matrix to investigate and predict the thermomechanical response under realistic plasma exposure conditions. Based on molecular-dynamics (MD) simulations using a properly parameterized machine learning Spectral Neighbor Analysis Potential (SNAP), we characterize the response of tungsten to plasma exposure and determine the elastic properties, tensile yield strength, and thermal expansion coefficient of PFC tungsten as a function of its porosity and He content.

We find that the Young modulus of PFC tungsten softens substantially as a function of porosity, following an exponential scaling relation, in addition to the softening caused in the tungsten matrix with increasing temperature [1]. The presence of these nanoscale spherical defects reduces the yield strength of tungsten in a monotonically decreasing fashion, obeying an exponential scaling relation as a function of tungsten matrix porosity and He concentration [1]. Detailed analysis of the structural response of PFC tungsten near the yield point reveals that yielding is initiated by emission of dislocation loops from bubble/matrix interfaces followed by gliding and growth of these loops and dislocation reactions. Moreover, dislocation gliding on twin systems nucleates twin regions in the tungsten matrix. These dynamical processes reduce the stress in the matrix substantially [1].

We also distinguish between two approaches of filling the bubbles with He, where the amount of He in the bubble can or cannot vary with temperature. In the former case, the thermal expansion coefficient decreases monotonically with increasing the porosity and He content of the tungsten matrix, while in the latter case, the thermal expansivity increases monotonically with increasing porosity and He content [2]. The latter condition, where the He content in the bubble is determined at the implantation temperature and remains constant with varying temperature in the tungsten matrix, is consistent with He species transport in PFC tungsten and implies the development of biaxial compressive thermal strains in the PFC material that contribute to accelerating the growth of a nanostructure on PFC tungsten surfaces.

Our analyses advance the fundamental understanding of thermomechanical properties in PFC tungsten and contributes to the development of a thermophysical property database for properly incorporating effects of realistic particle and heat loads into modeling the dynamical response of PFC tungsten under fusion reactor operating conditions.

[1] A. Weerasinghe, E. Martinez, B. D. Wirth, and D. Maroudas, ACS Appl. Mater. Interfaces 15, 8709–8722 (2023).

[2] A. Weerasinghe, B. D. Wirth, and D. Maroudas, J. Appl. Phys. 132, 185102 (2022).