Microstructural evolution in beryllium by fusion-relevant low energy helium ion irradiation

Abstract

An in situ transmission electron microscopy (TEM) study was performed to investigate the microstructural changes in beryllium due to helium-ion implantation. Beryllium samples were irradiated by 8 keV helium ions up to 10 22 He +/m 2 at various temperatures between room temperature and 873 K. At all the temperatures examined here, dense tiny bubbles were primarily formed in beryllium above a threshold fluence. The size of the tiny bubbles was almost independent on temperature and was at most 2 nm in diameter. The threshold fluence was slightly dependent on irradiation temperature. More significant effect of irradiation temperature was the growth of the bubbles. Above 673 K, the bubbles were observed to grow up to the size of several tens to hundreds of nanometers in diameter. Cavities with hexagonal shape were also observed above 773 K. After the irradiation and simultaneous TEM observation, surfaces of the samples were also investigated by scanning electron microscopy (SEM). Blisters, exfoliation, flake and pinholes were observed to be formed on the surfaces, and they strongly depended on temperature. Thermal desorption spectroscopy (TDS) was also measured using a quadrupole mass analyzer (QMA) to investigate the amount of helium gas released from the irradiated samples as a function of annealing temperature. The TDS spectra showed that most of the helium gas was released at lower annealing temperature for the samples irradiated at higher fluence. Interrelation among the microstructural evolution in beryllium, the changes in surface shape and the behavior of helium gas implanted in the sample was discussed.