Investigation of the Effects of Hydrogen Atoms Concentration on the Tungsten Sigma 5 (310) Symmetric Tilt Grain Boundary

Abstract

The objective of this work is to investigate the effects of the concentration of hydrogen atoms at the tungsten Σ5 (310) grain boundary (GB) on the GB energy and the GB pulling force. Tungsten (W) is the most preferred plasma facing material (PFM) for the future nuclear fusion reactors such as in the proposed DEMO (demonstration power plant) and ITER (international thermonuclear experimental reactor — which is at the moment the largest Tokamak nuclear fusion reactor under construction in the world. Tungsten is considered as a PFM because of its excellent thermal properties, low-sputtering yield and high melting point. However, hydrogen (H) atoms have an affinity for tungsten grain boundary, and are trapped there permanently. In addition, it makes it prone to failure. W will be exposed to extremely high fluences of H isotopes. The low-energy H isotopes will be retained in the tungsten material leading to the formation of blisters in W and causes degradation of the mechanical and thermal properties of W. Therefore for safety reason, the effect of hydrogen atoms at the tungsten Σ5 (310) symmetric tilt-GB needs to be investigated. This will greatly aid in better designing of fusion wall materials. In addition, the full understanding of the tungsten grain boundary energy and the force required to pull the grain boundary apart will also help in better material selection. Classic molecular dynamics method was used for the investigation. LAMMPS-a sophisticated, classical atomic and molecular dynamics modelling and simulation software, is a vital tool adopted for the investigation of the effects of hydrogen atoms in tungsten Σ5 (310) symmetric tilt-GB. Tersoff potential for W-H interactions was used for the modelling. The size of the simulation box is 10 × 100 × 10 lattices and it consists of 21262 W atoms. Periodic boundaries were used for all sides of the system. Conjugate gradient method was used for the minimization. The trajectories of the atoms were visualized using visual molecular dynamics (VMD) software. The GB energies are calculated to be −924.060 J/m2, −923.898 J/m2 and −743.414 J/m2 for pure W, W with one H atom and W with 30 H atoms are at the GB respectively. In addition, the forces required to separate the GB apart are 0.0279551eV/Ang for pure W, 0.024789eV/Ang and 0.0185eV/Ang for W with one H and 30 H atoms at the GB respectively. The result shows that hydrogen acts as a grain boundary embrittler and weakens the GB strength. The GB energy reduces as the concentration of hydrogen at the tungsten GB increases. In addition, the more the hydrogen atoms at the tungsten GB the lesser the value of the force required to pull the GB apart. However, the GB energies and pulling forces starts increasing slowly when H atoms exceed a certain number depending on the H atoms distribution around the W GB.