Characterization of He Bubbles in Irradiated Aluminium with High Resolution Transmission Electron Microscopy and Electron Energy Loss Spectroscopy

DAVID RONDÓN; E. A. SÁNCHEZ; A. TOLLEY

Ciudad Autónoma de Buenos Aires

Congreso; CIASEM 2019; 2019

SAMIC

Irradiation effects in metallic alloys has been an active area of research since a few decades ago, and is actually stimulated by the demand of developing materials that should withstand mechanical integrity at higher temperatures and higher irradiation doses for structural components of Generation IV fissionreactors and for first wall components in fusion reactors [1].This work reports results of 20 keV He irradiations on hi gh purity aluminium at room temperature. The aim of the experiments was to study the formation of nanometer sized bubbles that cause swelling and embrittlement, and the effects of blistering that results from coalescence of bubbles and leads to surfaceero sion by exfoliation. The irradiation experiments were carried out in a 120 kV ion accelerator at Centro Atómico Bariloche. A Tecnai F20 transmission electron microscope equipped with a Quantum ER system was used for post irradiation characterization, combi ning High Resolution Imaging(HRTEM) and Electron Energy Loss Spectroscopy (Figure 1a shows faceted He bubbles in the aluminium matrix, imaged with HRTEM, produced by anirradiation fluence of 5.2 x 10 16 ions/cm 2 , equivalent to an irradiation dose of 3 displacements per atom (dpa). Figure 1b corresponds to a specimen irradiated to a fluence of 2.6 x 10 17 ions/cm 2 that corresponds to 15 dpa, where faceting is more marked. The mean equivalent diameter at 15 dpa (that of a spherical bubble with the sam e volume as the faceted bubble) was (2.4 ± 0.1) nm, and the width of the size distribution was 0.6 nm (figure 1c).Figure 1d shows three EELS spectra for comparison obtained in i) an unirradiated pure Al specimen; ii) an irradiated Al specimen to a dose o f 3 dpa and iii) an irradiated Al specimen to a dose of 15 dpa. In all spectra, typical Al volume plasmon peaks at energy losses of about 15 eV and 30 eV are clearlyobserved. In between these plasmon peaks, in the irradiated Al specimens a small peak can be observed. The inset in figure 1d shows a detail of the energy loss region between the plasmon peaks for the three specimens mentioned. The curves have been displaced for clarity. The small peak in the irradiatedspecimens corresponds to the excitation of the K shell of He. The energy of the small K shell He peak was found to vary between 21.5 eV and 24 eV. Such a variation in the energy of the K shell He peak in He bubbles formed by ion implantation has been previously reported by J ä ger and co workers [ 2], andwas attributed to differences in the gas pressure inside the bubbles.Summarizing the present results, He in irradiated Al has been detected by means of EELS. Further studies involving quantification of the He content as a function of the irradiat ion parameters are in progress.