(335a) Molecular Dynamics Simulations of Hydrogen Retention in Single-Crystal and Polycrystalline Tungsten

When tungsten is exposed to hydrogen plasma, it is known to form blisters. The observation of blisters is difficult to explain in light of the fact that electronic structure calculations and molecular dynamics simulations show that hydrogen atoms embedded in tungsten repel each other. We study hydrogen blistering through molecular dynamics, and have verified the absence of hydrogen self-clustering, even at times on the order of hundreds of nanoseconds. Instead, hydrogen atoms are observed to push nearby hydrogen atoms out of the way, resulting in each hydrogen atom being effectively isolated from other hydrogen atoms. Nudged elastic band calculations of hydrogen entering a void show that as the number of hydrogens in the vacancy increases, the barrier becomes larger for any new hydrogen that could enter. Instead, we expect that grain boundaries may be needed for the formation of blisters. Our simulations of hydrogen near grain boundaries show that there is indeed a tendency for hydrogen to collect along grain boundaries, and that hydrogen does not leave the grain boundary plane or diffuse at significant rates once situated there. There is also a significant drift force that causes hydrogen to be attracted to grain boundaries, which we quantify. Finally, we investigate the mechanical properties of grain boundaries with hydrogen adsorbed on them to determine whether blisters might form by way of delamination of the grain boundaries under stress.