Small angle neutron scattering from helium bubbles in metals

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Small angle neutron scattering (SANS) has been widely used in investigating defects in metals, and in particular, to characterize the helium bubble population in implanted materials. The main advantage in using SANS is the non-destructive feature of the tests and the quantitative results obtained by averaging over a large sample volume. SANS is a powerful technique, very sensitive to microstructural changes and its use was of fundamental importance to show the bimodal distribution of the bubble population: in the vicinity of grain boundaries and free surface and inside grains, respectively. Here the most important applications of the SANS technique to the study of the helium bubbles in implanted materials are reviewed. Most of the work has been done on nickel samples, but also a ternary alloy Fe-Ni 15%wt-Cr 15%wt and a steel (MANET) with a more complicated structure have been successfully investigated. Different annealing treatments, isothermal and isochronal, were investigated in order to determine the active mechanisms of the bubble coarsening and their activation energies. From the SANS data the bubble size distributions have been determined, from which parameters such as mean radius and density of the bubble population have been calculated. The gas pressure inside the bubbles was also determined by the contrast variation technique in SANS and by a computational procedure, and an excellent agreement was found between the results. These results show a marked overpressure inside the bubbles as compared to thermodynamical equilibrium values of about 3 GPa. A comparison with results obtained by other techniques confirms the validity of SANS, which has to be considered as a complementary technique for its indirect image of the sample.