Helium in metals: effect on mechanical properties

Helium is produced in solids under irradiation by reactions of the type (n,α), (p,α), etc. or by α-implantation. Because of its vanishingly small solubility, helium precipitates into bubbles which in turn can cause drastic changes in the mechanical properties of solids, particularly at high temperatures. In this review, typical examples of high temperature embrittlement by helium under static (creep) and cyclic (fatigue) loads are presented with emphasis on austenitic and martensitic steels for fusion application. The experimental results (lifetime, ductility and helium bubble microstructure as functions of stress, temperature, helium content, etc.) are discussed in the light of theoretical models treating the sequence of processes which lead from the production of helium via the formation of bubbles and their stable and unstable growth to the final failure of the materials