Effects of High Energy He+ Ions Irradiation on the Micro-structures of Pure Metals (Zr, Fe and Al)

Abstract

The study of radiation damage in nuclear materials has been an active area of research during last few decades. These studies have facilitated in providing useful information about different materials that can be used in irradiation environment. Therefore, a considerable interest has been raised in this area to investigate the radiation induced damage in structural materials that are used in the advanced nuclear reactor systems (Gen IV fission and fusion). Helium and protons are produced by (n, p), (n, α) reactions when the core structural components are irradiated with neutrons. These helium ions can significantly influence the integrity of structural components through the physical sputtering, formation of voids, surface swelling, ridges and bubbles in the structural materials. To the best of our knowledge, the surface and structural features of nuclear structural materials such as Zirconium (Zr), Iron (Fe) and Aluminum (Al), irradiated with 18 MeV helium have not been reported in the past. In this work, we reported the effects of 18 MeV helium ions irradiation on the surface and structural properties of Zr (HCP), Fe (BCC) and Al (FCC) at different fluences. The aim of this work is to explore the helium ions irradiation induced defects generation at 18 MeV and their impact on the structural stability of the nuclear metals. Besides, the comparison of HCP, BCC and FCC metals after helium ions irradiation has been particularly highlighted in this dissertation. Pure Al samples were irradiated by 18 MeV helium using MC-50 cyclotron accelerator at different fluences ranging from 1 × 1010 to 1 × 1016 ions/cm2. The range of the helium ions penetration was measured to be 157-160 µm, depending on the surface, which is in good agreement with the SRIM simulation results. The FESEM results revealed the Morphological features of ion irradiated Al contain incoherent structure, ridges, cavities and exfoliational sputtering due to melting, re-deposition and re-solidification process of target material whereas the XRD results indicated the significant change in the deflected intensities of crystallographic planes, especially in the peak intensity of (311). Full width half maximum (FWHM) of the irradiated specimens are also changed. However, these changes are due to the competition between defect generation and defects annealing process. In addition, the study of 99.99 % pure Fe was conducted with 18 MeV helium ions with fluences in a range from 1 × 1010 ions/cm2 to 1 × 1016 ions/cm2. SRIM estimated penetration depth of 18 MeV He+ in iron was found to be 68.2 μm that quite accurately matched with experimentally calculated projectile range. FESEM results revealed that sputtered and recoiled Fe atoms from the surface were deposited on the surface in the form of irregular shape incoherent structures which instigates spherical/circular craters, nano and micro size small pits all over the surface due to melting, re-deposition and re-solidification process. XRD results depicted a variation in the diffraction peaks intensities, peak shifting and broadening of the diffraction peak on the (110) plane, which is the most compact plane of the BCC structure, probably due to the residual stresses made by implanted helium atoms are responsible this shift. Surface and structural features of Fe were correlated with each other to elucidate the changes induced by helium ion beam irradiation. Samples of pure Zr were irradiated by 18 MeV helium (He+) ions in the fluence range from 1 × 1010 ions/cm2 to 1 × 1016 ions/cm2 at 373 K by using Cyclotron Accelerator. The experimentally calculated penetration depth of the helium ions in Zr (100 ~102 μm) was found to be in good agreement with the SRIM Monte Carlo simulations. FESEM results revealed that the helium ion irradiation significantly affected the surface of the Zr specimens by producing defects on the surface in the form of black spots, bubbles, bubbles assist blistering and cracks. At the lowest fluence of 1 × 1010 ions/cm2, the nano-size bubbles were observed inside these black spots. Later on, by increasing irradiation fluence density and size of these bubbles increased. However, at a highest fluence of 1 × 1016 ions/cm2 the excessive pressure inside the bubbles ultimately disrupted the bubbles, resulting in the form of cracks on the surface. The AFM results indicate the surface roughness of the Zr is slightly increased at lower fluences (1 × 1010 ions/cm2 to 5 × 1013 ions/cm2), whereas exponential increase is observed by increasing the irradiation fluence from 1 × 1014 ions/cm2 to 1 × 1016 ions/cm2. The high-resolution X-ray diffraction (XRD) results revealed that no additional peaks were found but the peak intensity variations were clearly observed mostly in the basal plane (0002) due to the irradiation. The transmission electron microscopy (TEM) results reported a significant decrease in the grain size after the He+ irradiation. The values of grain size, calculated using the TEM, were found to be in good agreement with the crystallite size calculated using the XRD analysis.