Dynamic HVEM Observations of Metals and Alloys during in-Situ Helium Ion Irradiations
A 200 kV ion accelerator, with a PIG source for accelerating ionized gases, was coupled to a 500 kV transmission electron microscope in order to investigate the effects of helium irradiation on the development of microstructure in candidate fusion reactor first wall material. The microstructural evolution in nickel and in type 316 stainless steel bombarded by 80 keV helium ions was monitored via video recordings. The effect of specimen thickness and temperature and the effect of plastic deformation during irradiation was considered. In order to conduct these experiments a quantitative load-elongation hot tensile stage was constructed with a capability for monitoring specimen current continuously during the experiment. It was found that surface exfoliation and blistering may occur in specimens thinner than the mean projected range of helium ions. However, in the thinnest specimen regions, surface layer removal is replaced by the bending of individual grains. Blistering apparently can occur when underpressurization of bubbles exists. Plastic deformation during helium ion irradiation can lead to denuded layers adjacent to some grain boundaries and can produce removed surface layers whose thickness is several times greater than the project mean range of the incident helium ions.
The role of implanted gas and lateral stress in blister formation mechanisms
Abstract In this paper the parts played by gas and lateral stress in blister formation on metal surfaces after helium implantation are critically reviewed. Although measurements show the existence of lateral stresses in implanted surfaces, an analysis indicates that these stresses can play little part in blister formation. Conversely, there is a strong case for a gas driven model. One possible mechanism, that involving the interbubble fracture of overpressurised helium bubbles, is outlined and revised to take recent measurements into account.
HELIUM EMBRITTLEMENT IN TYPE 304 STAINLESS STEEL.
Abstract Helium was uniformly injected into small tensile samples of type 304 (austenitic) stainless steel to concentrations of 1 × 10−7 and 3 × 10−5 atom fraction helium. Subsequent tensile testing above 540 °C revealed progressive ductility loss—as measured by total elongation at rupture—with increasing temperature. This effect was more severe in samples with the high helium content where elongations at 760 °C were a third or less of those of control samples. Yield and tensile strengths remained unaltered by the presence of helium. Above 650 °C, grain boundary sliding, which results in intergranular cracking, becomes important. The cracks begin as voids on carbide particles which act as obstacles to grain boundary sliding. In the presence of helium, bubbles attach themselves to these carbide particles and serve as void nuclei, thereby accelerating the process. Bubbles were also more prevalent on grain boundaries, dislocations and inclusion particles than isolated in the matrix.