The growth of helium bubbles in stainless steel at high temperatures

The influence of annealing time (1 to 1000 hours), applied tensile stress (0 to 130 MPa), temperature (973 to 1073 K) and helium content (10 to 300 a.p.p.m.) on the size distribution and density of helium bubbles within the grains of AISI type 316 stainless steel has been studied by TEM of α-implanted foil specimens. The results are compared with theoretical predictions assuming different growth mechanisms. The experimental data are best described by assuming that (1) the bubble coarsening is due to Ostwald ripening and (2) the helium in the bubbles obeys a highly non-ideal equation of state. From such a comparison an activation energy for helium permeation through the lattice of 3.5 eV can be deduced, which is comparable with theoretical estimates. It was further observed that tensile stresses can lead to bimodal size distributions, which are attributed to the presence of two mobile and supersaturated components (helium and vacancies) during the nucleation stage. The subsequent growth of matrix bubbles seems to be rather insensitive to stress, in contrast to the behaviour of grain boundary bubbles. Finally it could be shown that, within the covered parameter range, the implanted helium is entirely precipitated into bubbles with diameters > approx. 1.5 nm that are detectable by TEM. (author)