Neutron-induced void formation in 300 series stainless steels was simulated by 1000-keV electron bombardment of thin foils in a high-voltage microscope. Stereomicroscopy was used to determine the effect of surface proximity and to establish minimum foil thickness criteria for irradiations that would yield results characteristic of bulk material. Irradiation of specimens which are too thin yields misleading results, including an apparent saturation of swelling which actually is an artifact of the surface influence. Bulk-characteristic irradiations lead to volume increases as large as 100 percent without saturation.

Void nucleation in these irradiations was very sensitive to the initial microstructure. The thickness of zones denuded of voids at specimen surfaces was found to be very sensitive to temperature and surface conditions, particularly oxidation. The denuded zone width appears to be controlled by the effective vacancy migration behavior.