Positron beam studies of void swelling in ion irradiated Ti-modified stainless steel G. Amarendra *, B.K. Panigrahi, S. Abhaya, Christopher David, R. Rajaraman, K.G.M. Nair, C.S. Sundar, Baldev Raj Materials Science Division, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, Tamil Nadu, India 1. Introduction Titanium modified D9 steels (15 wt%Ni, 14 wt%Cr) have been adopted as candidate materials for the fuel clad and the hexagonal wrapper for fuel subassemblies in fast breeder reactor applications [1]. Among neutron irradiation induced degradations of these structural materials, void swelling is the most important because it produces dimensional changes and limits the lifetime of the structural components [2,3]. This will in turn limit the achievable fuel burn-up in fast reactors, which is an important criterion for cost-effectiveness. In particular, the fuel clad tubes would experience elevated temperatures in the range of 673–973 K as well as intense neutron flux under steady state conditions. Therefore, it is very important to test these structural materials in terms of their degradations due to void swelling. However, producing a high damage level of 100 dpa corresponds to a neutron fluence of 10 23 ncm 2 , which would require very long irradiation times up to a few years in a reactor. Therefore, the alternate way of studying void swelling is to simulate the neutron induced damage by irradiating the sample with heavy ions using an accelerator. Since heavy ions are capable of producing higher dpa/s, the peak damage produced can go up to 100 dpa within a few hours. Therefore, materials screening for void swelling performance can be accomplished using Ni-ions from an accelerator. In this paper, we report for the first time the applicability of depth-resolved positron annihilation spectroscopy in establishing the temperature dependence of void swelling and the determination of the peak swelling temperature in ion irradiated D9 alloys. 2. Experimental details 18% cold worked D9 samples of size 15 mm  15 mm  1.5 mm were preinjected with helium at two different energies, 170 keV and 275 keV in order to obtain a uniform concentration of 100 appm. These samples were then subsequently implanted with 2.5 MeV Ni 2+ ions at temperatures ranging from 720 to 970 K in steps of 50 K to a dose of about 6.96  10 16 ions/cm 2 in a vacuum of 4  10 7 mbar, using a 1.7 MV Tandetron accelerator. This corresponded to 84 dpa at the damage peak and the range of 2.5 MeV Ni 2+ in stainless steels was 668 nm as calculated by TRIM program [4]. The temperature stability during irradiation was 2 K. The sample after the high temperature irradiation was quenched to the ambient temperature using a jet of helium gas cooled by a liquid nitrogen trap. Similar irradiation was also carried out on SS316 sample. Depth-resolved positron annihilation measure- ments were carried out on reference and irradiated samples at room temperature using a variable positron beam, by implanting positrons of tunable energy (0–22 keV) into the sample [5]. The Doppler spectrum is measured for each energy using a 25% efficient intrinsic germanium detector having an energy resolution of 1.4 keV at 662 keV. From the Doppler spectrum, a defect-sensitive lineshape S- parameter, is deduced which is defined as the ratio of the counts in the central region (511  1 keV) to the total counts under the peak (511  10 keV). Since, the S-parameter is very sensitive to open- volume defects, the presence of the vacancy defects results in an increase in the S-parameter value [6,7]. 3. Results and discussions The implantation damage produced by Ni ions extends up to a depth of about 800 nm, exhibiting a maximum around 500 nm. Applied Surface Science 255 (2008) 139–141 ARTICLE INFO Article history: Available online 16 May 2008 Keywords: Void swelling Ti-modified stainless steel Ion beam damage Positron annihilation ABSTRACT The void swelling behavior of heavy ion irradiated D9 steel has been investigated using variable low energy positron beam. The normalized defect-sensitive S-parameter shows up a large increase in the depth region corresponding to the maximum radiation damage as a function of irradiation temperature. From the variation of S-parameter as a function of irradiation temperature, the peak swelling temperature has been deduced and the results are discussed. ß 2008 Elsevier B.V. All rights reserved. * Corresponding author. E-mail address: amar@igcar.gov.in (G. Amarendra). Contents lists available at ScienceDirect Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.05.214