A review of recent theoretical developments in the understanding of the migration of helium in metals and its interaction with lattice defects

Recent theoretical calculations of the properties of rare gases, and in particular Helium, in the common f.c.c. and b.c.c. metals, are reviewed from the viewpoint of the investigator concerned with the behaviour of rare gas in such radiation damage processes as surface blistering and void swelling. Particular attention is paid to mechanisms by which Helium may migrate in a damaged metal lattice during irradiation and to the properties of small gas and vacancy clusters which may represent bubble or void nuclei. Initially the proposed rapid migration of interstitial Helium is discussed together with the substitutional de-trapping mechanism, whereby thermally activated Helium jumps from a substitutional to an interstitial position. This enables a mechanism of substitutional Helium diffusion to be proposed which may proceed at temperatures below those of self-diffusion. The formation, binding, migration and dissociation energies of gas-vacancy clusters have been reviewed. The relevance of the predicted trend towards the optimum stability of clusters composed of equal numbers of gas atoms and vacancies is discussed. The limited data available concerned with the binding of a Helium atom to a pure dislocation line is presented together with comments on the possible nature of the interaction of Helium with the dislocation jog.