The effect of He þ irradiation on hardness and elastic modulus of Fe-Cre40 wt.% TiB 2 composite rod designed for neutron absorbing Przemyslaw Litwa a, * , Lukasz Kurpaska b , Robert A. Varin c , Krzysztof Perkowski d , Jacek Jagielski b, e , Stanislaw J  o  zwiak a , Tomasz Czujko a a Military University of Technology, S. Kaliskiego 2, 00-908, Warsaw, Poland b National Centre for Nuclear Research, A. Soltana 7, 05-400, Otwock/  Swierk, Poland c University of Waterloo, 200 University Ave., Waterloo, ON N2L 3G1, Canada d Institute of Ceramics and Building Materials, Poste ˛ pu 9, 02-676, Warsaw, Poland e Institute of Electronic Materials Technology, W olczy nska 133, 01-919, Warsaw, Poland article info Article history: Received 7 January 2017 Received in revised form 27 March 2017 Accepted 30 March 2017 Available online 31 March 2017 Keywords: He ion irradiation damage Titanium diboride (TiB 2 ) Structural composites Mechanical alloying Hot isostatic pressing (HIP) Mechanical properties abstract A composite rod, designed for the neutron absorption in nuclear research reactors or Nuclear Power Plants (NPPs), was synthesized by ball milling and subsequent Hot Isostatic Pressing (HIP-ing), similarly to the one reported in Ref. [3]. Its core consists of the oxide dispersion strengthened ferritic Fe-Cr matrix and 40 wt.% of TiB 2 reinforcement, surrounded by a Ti tubing/cladding with a functionally graded interface layer. The Specific Surface Area (SSA) for the ball-milled pre-alloyed powders increases from 0.64 to 2.92 m 2 /g and XRD shows that the crystallite/grain size of TiB 2 is ~38 nm. Berkovich nano- indentation study after irradiation with 160 keV He þ ions at a fluence up to 1  10 17 He þ /cm 2 shows an initial hardening effect at fluences of 1  10 15 and 1  10 16 ions/cm 2 . Hardness and elastic modulus of the Fe-Cr -TiB 2 core rapidly drops when the fluence reaches 1  10 17 ions/cm 2 . The Ti cladding seems to be relatively impervious to increased radiation fluence since its hardness and elastic modulus change very slightly with increasing ion fluence. The observed changes in the mechanical properties are discussed in terms of vacancy/dislocation loops and He bubble formation in the irradiated microstructure. Although the He-vacancy complexes are widely regarded as being immobile, it is hypothesized here, based on the grazing incidence XRD (GI XRD), that interstitial helium diffuses outward through the boundaries of the (100) hcp-TiB 2 nanograins. As a result, the relaxation of compressive strains due to high concentration of vacancies in a nanograin crystalline lattice finally leads to the hcp-TiB 2 unit cell contraction. © 2017 Elsevier B.V. All rights reserved. 1. Introduction In general, the control rods for a nuclear reactor are composed of the B 4 C ceramic neutron absorber pellets, which are placed into a 304 grade stainless steel tubing. Optionally, they may be sur- rounded by a zirconium alloy guide tube. However, the limited service life-time of control assemblies of typical Boiling Water Reactors (BWRs), makes their application quite ineffective in the nuclear fission reactors. This is caused by the radiation-induced swelling of B 4 C which may initiate a chemical reaction between B 4 C and steel [1] leading to a subsequent embrittlement. Furthermore, B 4 C has an abnormally low thermal conductivity (12 W/m.K) at operating temperatures and in the case of hot- pressed B 4 C it maintains high porosity level which may reach 2% [2]. These are very important issues for such material because control rods are exposed to a high-energy neutron flux. It is known that extra heat is being generated from neutron-alpha (n, a) re- actions induced by neutrons, causing temperature increase within the control rods. Therefore, different material has been proposed as a replace- ment. It is a Fe-TiB 2 composite [3]. It has been showed that nearly fully dense (98%), polycrystalline TiB 2 , is characterized by high hardness and thermal conductivity of 81 W/m.K at ~500  C [4]. Therefore, TiB 2 which is also compatible with B 4 C could be a suit- able dispersoid for increasing a thermal conductivity of B 4 C [5]. Considering a suitable metal matrix composite (MMC) with the B 4 C absorber, the 6061Al alloy is commonly used, although highly * Corresponding author. Department of Advanced Materials and Technologies, Military University of Technology, S. Kaliskiego 2, 00-908, Warsaw, Poland. E-mail address: przemyslaw.litwa@wat.edu.pl (P. Litwa). Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom http://dx.doi.org/10.1016/j.jallcom.2017.03.350 0925-8388/© 2017 Elsevier B.V. All rights reserved. Journal of Alloys and Compounds 711 (2017) 111e120