Solubility:

Hydrogen gas (involving tritium gas) dissolves to a few extents in most materials.  Tritium and deuterium behave such as protium, except for small isotopic effects.  Diatomic hydrogen molecules or Hydrogen atoms and a few larger hydrogen-bearing molecules dissolve interstitially; that is, that diffuses within a structure and locates among atoms or molecular frameworks. Since it dissolves then the hydrogen slightly disrupts the structural networks of the material and causes expansion.  The extent of the disruption, with the extent of chemical attractive/repulsive forces among the hydrogen and the material, determines a ultimate equilibrium state.