He ion irradiation damage in Fe/W nanolayer films Nan Li a , E.G. Fu a , H. Wang b , J.J. Carter c , L. Shao c , S.A. Maloy d , A. Misra e , X. Zhang a, * a Department of Mechanical Engineering, Materials Science and Engineering Program, Texas A&M University, College Station, TX 77843-3123, USA b Department of Electrical and Computer Engineering, Texas A&M University, College Station, TX 77843-3128, USA c Department of Nuclear Engineering, Texas A&M University, College Station, TX 77843-3133, USA d Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, NM 87545, USA e Materials Physics and Application Division, Los Alamos National Laboratory, Los Alamos, NM 87545, USA abstract We report on the evolution of microstructure and mechanical properties of Fe/W multilayers subjected to helium ion irradiations. Sputtered Fe/W multilayers with individual layer thickness, varying from 1 to 200 nm, were subjected to He + ion irradiation with a peak displacement per atom value of 6 at ambient temperatures. Helium bubbles, 1–2 nm in diameter, were observed in Fe and W, and more so along layer interfaces. The magnitude of hardness variation after radiation depends on the individual layer thickness. Radiation hardening is observed in specimens with individual layer thickness of P5 nm. At smaller layer thickness, the hardness barely changes. Analysis indicates that radiation hardening may originate mainly from dislocation loops and partially from He bubbles. Ó 2009 Elsevier B.V. All rights reserved. 1. Introduction High energy helium (He) ion irradiation of metals generates a large number of defects, including vacancies and interstitials, He bubbles and dislocation loops [1–3]. Radiation typically degrades the mechanical properties of metals, most notably an increase in yield strength, and significant loss of ductility (embrittlement) [4]. Radiation induced defects in metals are of great interest, be- cause these defects determine the performance of irradiated mate- rials in nuclear reactor environment. He bubbles and dislocation loops are two major types of radiation induced defects. The solid solubility of He in metals is very low [5]. Thus, at relatively low concentrations of implanted He, it is easy to form He-vacancy clus- ters, which act as the nucleus for He bubble formation [6]. Once nucleated, in order to maintain a mechanical equilibrium between their internal pressure and the sintering stress, 2c/r, where c is the surface energy and r is the bubble radius, the bubbles grow by absorbing He atoms and radiation induced vacancies [7]. High en- ergy He ion bombardment of metals also produces recoil intersti- tial metal atoms that collapse into prismatic dislocation loops. Different types of defects have different obstacle strengths for glide dislocations. In general, voids and large precipitates act like Oro- wan barriers and have large barrier strengths; small bubbles, small clusters and network dislocations have relatively small barrier strengths. Lucas reviewed the mechanical properties of austenitic stainless steels [8], and found that at low temperature (373 K), radiation hardening was dominated by Frank loops at low dose, and by the network dislocations at higher dose. At higher temper- ature, 673 K, voids and bubbles begin to contribute to hardening, especially at high dose. Other studies on irradiated 316LN stainless steel show, at approximately 1 at.% He concentration, dislocations and loops can be pinned by He bubbles in the lattice [9]. Recent studies have shown that interfaces in composite materi- als can act as sinks for radiation induced defects, promote recom- bination of unlike point defects, and result in enhanced radiation tolerance as compared to conventional single-phase bulk metals [10–12]. For instance, He ion irradiated Cu/Nb multilayer films with a few nm layer thickness seem to suppress the burst of He bubbles after annealing [10]. In this study, we chose Fe/W multi- layers for radiation damage studies. Compared to Cu, Fe and W have relatively high melting points, and more open crystal struc- ture, bcc vs. fcc. Molecular dynamics simulations suggest that the characteristics of interface could be a major factor in determining the accumulation of radiation damage in composite materials [13]. The lattice parameter difference between Fe and W is rather large (10%), and so the Fe/W interface is incoherent [14]. Such incoherent interface could enhance the capability of defect storage. The study will also allow a comparison of incoherent bcc/bcc Fe/W interface with incoherent fcc/bcc Cu/Nb interface. 2. Experimental procedures Fe/W multilayers were deposited by magnetron sputtering at room temperature on SiO 2 substrates. The vacuum chamber was evacuated to a base pressure less than 5  10 8 torr prior to depo- sition. The constituents within the multilayers have equal layer thickness, varying from 1 to 200 nm. The total film thickness was 0022-3115/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jnucmat.2009.02.007 * Corresponding author. Tel.: +1 979 845 2143; fax: +1 979 845 3081. E-mail address: zhangx@tamu.edu (X. Zhang). Journal of Nuclear Materials 389 (2009) 233–238 Contents lists available at ScienceDirect Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat