SMALL-ANGLE NEUTRON-SCATTERING FROM HELIUM BUBBLES IN METALS

Small angle neutron scattering (SANS) has been widely used in investig ating 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 p owerful technique, very sensitive to microstructural changes and its u se was of fundamental importance to show the bimodal distribution of t he bubble population: in the vicinity of grain boundaries and free sur face and inside grains, respectively. Here the most important applicat ions of the SANS technique to the study of the helium bubbles in impla nted 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 (MA NET) with a more complicated structure have been successfully investig ated. 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 bubb le size distributions have been determined, from which parameters such as mean radius and density of the bubble population have been calcula ted. The gas pressure inside the bubbles was also determined by the co ntrast variation technique in SANS and by a computational procedure, a nd an excellent agreement was found between the results. These results show a marked overpressure inside the bubbles as compared to thermody namical equilibrium values of about 3 GPa. A comparison with results o btained by other techniques confirms the validity of SANS, which has t o be considered as a complementary technique for its indirect image of the sample.