Helium cluster nucleation and growth in implanted B$_4$C boron carbide
Résumé
When B4C boron carbide is irradiated in neutron reactors, high quantities of helium are produced due to the 10B(n,α)7Li neutron absorption reactions. It is well known that most helium remains in the material, this inducing swelling and cracking. However very few studies deal with the nucleation and growth of the helium clusters. In this paper, B4C is implanted with helium and its behaviour is studied mainly thanks to TEM observations. Different parameters are considered: helium concentration, implantation temperature, post-implantation annealing temperature, width of the implanted zone and overlap with ballistic damage simulating the fast neutron scattering. We observe that helium clusters form either on structural defects (grain and twin boundaries, precipitates) or thanks to irradiation damage induced nucleation. When implanted at low temperature and then annealed, nanometric bubbles form that eventually agglomerate as flat discs associated with strong strain fields. In the temperature range encountered in fast neutron reactors, those discs evolve toward parallel 20–50 nm diameter helium platelets. Grain boundaries are very efficient trapping sites, no evolution of the intergranular helium bubbles is observed up to 1200 °C. In the implanted zone, the platelets change into polyhedron bubbles over 1100 °C with a relaxation of the strain fields together with a strong density decrease. The addition of a ballistic damage increases the density of the helium bubble germs.