Investigations on helium bubble structure in metals by neutron scattering and electron microscopy

For a quantitative understanding of the high temperature embrittlement of metals by helium, the knowledge of the helium bubble structure is of basic importance. This work intends to show that a combination of Small Angle Neutron Scattering (SANS) and Transmission Electron Microscopy (TEM) can yield detailed and reliableinformation about the bubble parameters. This was demonstrated through two examples: (1) Measurements of the coarsening of bubbles in a Fe-Ni(15%wt)-Cr(15%wt) alloy (as a model material for austenitic stainless steel), implanted with 1200 appm He at room temperature and isothermally annealed at 973 K up to 2780 hours. (2) Measurements of the coarsening of bubbles in a martensitic stainless steel (DIN 1.4914 = reference material of the Europ. Fusion Materials Program),containing 1200 appm He and annealed at 798, 948, 1098 and 1248 K. In both cases good agreement of the size distributions determined by SANS and TEM, respectively, has been found. For the FeNiCr alloy it could be shown that the coarsening occurs by bubble migration and coalescence with surface diffusion being the rate determining process. It was further found that the gas pressure in the bubbles is considerably higher than the equilibrium pressure. These findings are discussed in the light of theoretical models for bubble coarsening