The diffusion of point defects to the foil surface during irradiation damage experiments in the high voltage electron microscope

Abstract

Abstract The high voltage electron microscope can be used to simultaneously produce and observe irradiation damage structures in thin foils of many materials, including in particular the formation of voidage at elevated temperatures. In such experiments some of the vacancies and interstitial atoms produced in the damage process will be lost by diffusion to the foil surface, so that the results may not be characteristic of the bulk material if the foil is too thin. The steady state point defect concentrates built up in a thin foil have been computed as a function of the foil thickness, defect production rate and vacancy jump frequency, taking into account the effect of the mutual recombination of vacancies and interstitial atoms. The coupled diffusion equations for the point defect concentrations have been solved numerically for a thin foil, assuming the foil surfaces to be perfect sinks for the point defects, and the results for any material are expressed in terms of a single dimensionless parameter. The computions show that for a foil containing few internal sinks, which is the most demanding case, defect concentrations typical of the bulk material can be achieved within an appreciable fraction of a 0.5μm thick foil, for temperatures up to about one-half the absolute melting temperature. Foils of this thickness can readily be penetrated by high voltage electrons for most materials. The loss of point defects to the foil surface is shown to be further reduced when the foil contains a high density of internal sinks (e.g. dislocations, voids, precipitate particles) and there is an additional improvement at very high temperatures due to the effect of thermal vacancies.