Transmission electron microscope studies of tantalum irradiated by MeV energy α particles

Abstract

Helium atoms introduced into metals by ion implantation can often cause microstructural changes, such as bubble formation and blistering on the surface and subsurface layers of the implanted materials. Also, accidental introduction of He atoms escaping from a plasma or by (n, AE) reaction occurring in a fusion reactor environment would make the problems of radiation damage of plasmafacing materials more serious. These problems induce void swelling, intergranular embrittlement and surface blistering or flaking, resulting from the development of defect structures such as nucleation and growth of cavities, dislocation loops and precipitates. High energy helium projectiles penetrating through metals can displace lattice particles from their sites, and when they are slowed down sufficiently they can be trapped in the lattice. In the process of interaction of helium ions with lattice particles, a displacement cascade generates over the entire helium projectile range and finally most of the helium ions are distributed near the end of their range. At this depth the helium has a high tendency to precipitate into bubbles due to its extremely small solubility in metals. The microstructures developed in various stages of helium interaction with metals are important, to allow fundamental understanding of the phenomenon of blister formation. In this letter we report the results of a transmission electron microscope (TEM) study into the subsurface structure of tantalum irradiated with 30 MeV helium ions to high dose. Earlier, we studied scanning electron microscope (SEM) observations of Ta foils bombarded by high MeV energy AE particles [1]. In the present case, a preliminary observation was carried out on the back of a Ta foil thickness 12.7 im, situated at a depth of about 25 im (two Ta foils of 12.7 im each) from the top surface. The projected range, (Rp), of the 30 MeV AE particles in Ta is large compared to the thickness of the specimens, and formation of bubbles in the foil is not feasible. However, the observed microstructures developed due to helium implantation in the present studies may expose a number of important stages leading to bubble, void or blister formation, and may lead to a better understanding of the mechanism involved in the processes. A stack of three Ta foils, each of thickness 12.7 im, was placed on an Nb foil of thickness 125 im. All of the foils were polished on both sides. The total thickness of the combination was 38 im Ta and 125 im Nb, which is approximately equal to the projected range of 30 MeV alpha in the combination as obtained by TRIM calculations [2] for the range. 30 MeV AE particles obtained from the Variable Energy Cyclotron of Calcutta were incident on these samples normally. The targets were mounted on a water-cooled copper flange to efficiently dissipate the heat developed due to beam hitting. The beam was collimated by a 3 mm diameter hole on an Mo disc and passed through a stainless steel cylindrical spacer of about 5 cm length. Thus, secondary electrons were properly suppressed and actual current and dose were obtained at the target. The maximum beam current at the target was 2 iA, which is equivalent to a current density of 30 iA cmy2. The total dose at the target was around 1 3 1018 AE cmy2, the vacuum in the target chamber was 6:66 3 10y7 KPa. After bombardment of the samples, sufficient time was allowed to cool down the activity of the samples. For preliminary investigation, an unimplanted and the third foil bombarded by AE particles were taken for the preparation of TEM samples in our new electrolytic jet thinning apparatus. The detailed description of the thinning process and the electrolytic jet thinning unit was presented by Chini et al. [3]. The electrolyte for jet thinning of Ta foils was a mixture of methanol (360 ml), butyl cellosolve (72 ml), H2SO4 (64.8 ml) and HF (21.6 ml). A nozzle diameter of 0.5 mm was chosen for thinning a 3 mm diameter disc punched out from the desired portion of the Ta samples. The polishing voltage, current range and time for the best results are listed in Table I. The thinning was performed at the front surface of the bombarded sample to ensure that the TEM observations were only conducted at the rear face of the third Ta foil. The samples thus prepared were examined in a Jeol JEM200CX TEM with a maximum resolution (point to point) of about 0.4 nm operating at 120–160 kV. Figs 1 and 2 show TEM micrographs of the Ta samples. Unbombarded Ta foils under TEM showed some black precipitates almost all over the sample.