(551g) Thermomechanical Properties of Plasma-Exposed Tungsten

Studying 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. In a previous study, based on nonequilibrium molecular-dynamics (MD) simulations, we reported that the presence of plasma-related structural defects in tungsten single crystals, namely, nano-scale sized voids and He nanobubbles, causes a significant reduction in tungsten’s lattice thermal conductivity, k, down to ~20% of that of perfect crystalline tungsten, with kdecreasing with increasing void size and increasing He pressure in the nanobubbles.

Here, we report computational results for the thermal expansion coefficient, α(T,p), of defect-free crystalline tungsten over a broad range of temperature, T(0 ≤ T ≤ 2000 K), and pressure, p(0 ≤ p ≤ 10 GPa). Based on isothermal-isobaric (NpT) MD simulations according to a reliable interatomic potential for well-converged supercell sizes, we demonstrate a ~40% reduction in the value of α at 1000 K when the pressure increases from 0 to 10 GPa; hydrostatic stresses of several GPa are expected to develop in plasma-exposed tungsten as a result of He implantation and formation of over-pressurized He nanobubbles. To assess our classical MD predictions, we have carried out first-principles computations of the thermal expansion coefficient of tungsten within density functional theory (DFT). These computations were based on free energy calculations according to density functional perturbation theory (DFPT) for determining vibrational entropies, with the free energies calculated within the quasi-harmonic approximation (QHA). Such first-principles results are used to both validate our classical MD predictions and contribute to the construction of a property database toward classical potential development. These thermal expansion coefficient results also can be utilized as input to continuum-scale models for calculations of thermal stresses in plasma-exposed tungsten and their contribution to the total stress in the plasma-facing material, an important component in developing models of PFC surface morphological evolution.

Furthermore, we report results of systematic MD computations of the elastic modulus of single crystalline tungsten containing nanopores; filling the nanopores with the proper amount of He to form helium nanobubbles is used to generate models of plasma-exposed tungsten and to assess the effect of He-ion irradiation on tungsten elastic properties. We conduct a systematic exploration of parameter space: the parameters varied include porosity (tungsten matrix density), temperature, bubble radius, bubble arrangement in regular arrays, and helium content. Our computations reveal that the dependence of the elastic modulus of nanoporous tungsten on its density follows a power law, similar to that of the modulus-density relation of natural cellular materials. We find that arrangement of voids/bubbles in various lattice configurations in the tungsten matrix does not have an important effect on the elastic modulus. We also find that filling the nanopores with helium increases the stiffness of PFC tungsten and decreases the sensitivity of its stiffness to porosity. At given temperature, we find that decreasing He nanobubble size leads to stiffening of PFC tungsten and reduces the sensitivity of its stiffness to porosity. Finally, we find that at given nanobubble size, raising the temperature leads to relative stiffening of PFC tungsten, with respect to the stiffness of perfect tungsten at each temperature, and reduces the stiffness sensitivity to porosity. Our findings enable the development of a quantitative database for the elastic modulus dependence on He content in PFC tungsten at various plasma exposure conditions, which constitutes another important component toward modeling and simulation of PFC surface morphological evolution.