Journal of Microscopy, Vol. 00, Issue 0 2017, pp. 1–8 doi: 10.1111/jmi.12662 Received 24 July 2017; accepted 8 October 2017 Characterisation of lattice damage formation in tantalum irradiated at variable temperatures I. IPATOVA ∗ , † , P.T. WADY †, S.M. SHUBEITA †, C. BARCELLINI ∗ , A. IMPAGNATIELLO ∗ & E. JIMENEZ-MELERO ∗ ∗ School of Materials, The University of Manchester, Manchester, U.K. †Dalton Cumbrian Facility, The University of Manchester, Moor Row, U.K. Key words. Dislocation analysis, electron microscopy, nuclear materials, radiation damage, tantalum, void formation. Summary The formation of radiation-induced dislocation loops and voids in tantalum at 180(2), 345(3) and 590(5)°C was assessed by 3MeV proton irradiation experiments and subsequent dam- age characterisation using transmission electron microscopy. Voids formed at 345(3)°C and were arranged into a body centred cubic lattice at a damage level of 0.55 dpa. The low vacancy mobility at 180(2)°C impedes enough vacancy clus- tering and therefore the formation of voids visible by TEM. At 590(5)°C the Burgers vector of the interstitial-type dislocation loops is a<100>, instead of the a/2 <111> Burgers vector characteristic of the loops at 180(2) and 345(3)°C. The lower mobility of a<100> loops hinders the formation of voids at 590(5)°C up to a damage level of 0.55 dpa. Introduction The continuous exposure of structural materials to intense ra- diation fields causes atomic displacement cascades that gener- ate a high density of vacancies and self-interstitial atoms (SIAs) in equal proportions. Those lattice point defects can evolve into a range of defect structures such as dislocation loops, nanoclusters, stacking fault tetrahedra or voids (Was, 2007; Odette et al., 2008). The long-term lattice damage brings along phenomena such as radiation-induced hardening or void swelling that compromises the integrity of key structural components in applications such as nuclear reactor cores or targets in neutron spallation sources (Yvon & Carr´ e, 2009; Azevedo, 2011; Zinkle & Was, 2013). Radiation-induced void swelling was initially reported in face-centred cubic metals and alloys, such as austenitic stainless steel or nickel (Cawthrone & Fulton, 1967; Norris, 1971). Material candidates for future nuclear reactor systems and higher-output targets for spal- lation sources are mainly based on body-centred cubic (bcc) Correspondence to: I. Ipatova. School of Materials, The University of Manchester, Manchester M13 9PL, U.K. Tel: +44 78 49290480; fax: +44 161 275 4865; e-mail: iuliia.ipatova@postgrad.manchester.ac.uk metals due to their enhanced radiation tolerance and, in par- ticular, to their higher void swelling resistance (Singh & Evans, 1995; Stork et al., 2014). Tantalum currently stands out as an advanced high-temperature material candidate due to its additional benefits of a high fluence threshold for He+ ion- induced surface nanostructuring, high water corrosion resis- tance, good workability, and ductility retention at relatively high radiation damage levels (Chen et al., 2003; Nelson et al., 2012; Novakowski et al., 2016). Void formation and swelling in bcc metals is interpreted in terms of the biased absorption of SIAs at defect sinks such as dislocation loops, and the evolution of the resultant vacancy excess in the matrix into vacancy clusters and eventually voids (Evan & Foreman, 1985; Trinkaus et al., 1993). This overall process depends on the mobility of SIAs, vacancies and defect sinks, and therefore on temperature. However, the existing experimental evidence of the formation and evolution of SIA and vacancy arrangements at variable temperatures in tan- talum is still fragmented. Earlier radiation damage studies in tantalum bombarded with neutrons up to a fluence of 2.5 × 10 22 neutron cm –2 at a temperature of 585°C reported the appearance of voids ordered on a bcc superlattice. Those voids randomise at higher irradiation temperatures up to 1050°C and are absent at 425°C (Wiffen, 1977). Heavy ion bombard- ment of tantalum induces the formation of vacancy clusters that evolve into randomly distributed voids at temperatures higher than 400°C (Yasunaga et al., 2000). In this work we aimed to assess the formation of radiation-induced dislocation structures in tantalum at variable temperatures and their im- pact on the occurrence, or otherwise, of void formation. We used a 3 MeV proton beam produced by an electrostatic par- ticle accelerator as a surrogate of neutron bombardment in void formation studies, because it generates a uniform radia- tion damage profile through a sample thickness of 30 μm. Ion irradiation experiments have been performed successfully since 1970s on metallic materials to mimic void swelling induced by neutron damage (Nelson et al., 1970; Mazey, 1990). C  2017 The Authors Journal of Microscopy C  2017 Royal Microscopical Society