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Using in-situ transmission electron microscopy, we have directly observed nano-scale defects formed in ultra-high purity tungsten by low-dose high energy self-ion irradiation at 30K. At cryogenic temperature lattice defects have reduced mobility, so these microscope observations offer a window on the initial, primary damage caused by individual collision cascade events. Electron microscope images provide direct evidence for a power-law size distribution of nano-scale defects formed in high-energy cascades, with an upper size limit independent of the incident ion energy, as predicted by Sand et al. [Eur. Phys. Lett., 103:46003, (2013)]. Furthermore, the analysis of pair distribution functions of defects observed in the micrographs shows significant intra-cascade spatial correlations consistent with strong elastic interaction between the defects. PAC 61.72.J-,61.80.Az,68.37.Lp

a b s t r a c tIn-situ irradiations with 150 keV W þ ions have been performed on W and W-5wt.% (Re; Ta; V) alloys in a comprehensive study of the influences of irradiation temperature T irr , dose, alloying elements and grain orientations on radiation damage production and microstructural evolution. For T irr between 30 K and 1073 K, the first observable defects in pure W appeared at doses 0.01 dpa, and were most likely vacancy loops, with Burgers vectors predominantly of type b ¼ ½ <111>. With increasing T irr , the retained defect concentration decreased strongly and the maximum cluster size increased from~1300 point defects at 30 K to~2300 point defects at 1073 K. At all irradiation temperatures, the evolution of damage microstructures with dose from 0.1 to 1.0 dpa involved defect cluster migration, with mutual elastic interactions often leading to spatial inhomogeneities and loop reactions. In pure W, spatial ordering of loops was observed at doses >0.4 dpa and T irr ! 773 K in grains close to z ¼ <001>. No such ordering was found in similar grain orientations for the W-(Re; Ta) alloys, but it was found in the non-z ¼ <001> grains. Post-irradiation analysis on W and W-5 wt% (Re; Ta) at 1.0 dpa showed that ½ <111> and <100> loops of both vacancy and interstitial type were present, at number densities~10 15 loops m À2 . In all cases ½ <111> loops were dominant, the fraction of these with interstitial nature increased with T irr , and the proportion of <100> loops decreased with increasing T irr . Compared with pure W, microstructures in the W-(Re; Ta) alloys exhibited higher loop number densities and evolved more quickly with increasing dose towards damage saturation.

Using in situ transmission electron microscopy (TEM), we have observed nanometre scale dislocation loops formed when an ultra-high-purity tungsten foil is irradiated with a very low fluence of self-ions. Analysis of the TEM images has revealed the largest loops to be predominantly of prismatic 1/2〈111〉 type and of vacancy character. The formation of such dislocation loops is surprising since isolated loops are expected to be highly mobile, and should escape from the foil. In this work we show that the observed size and number density of loops can be explained by the fact that the loops are not isolated-the loops formed in close proximity in the cascades interact with each other and with vacancy clusters, also formed in cascades, through long-range elastic fields, which prevent the escape of loops from the foil. We find that experimental observations are well reproduced by object Kinetic Monte Carlo simulations of evolution of cascades only if elastic interaction between the loops is taken into account. Our analysis highlights the profound effect of elastic interaction between defects on the microstructural evolution of irradiated materials.

Transmission electron microscopy of high temperature annealing of pure tungsten irradiated by self-ions was conducted to elucidate microstructural and defect evolution in temperature ranges relevant to fusion reactor applications (500-1200°C). Bulk isochronal and isothermal annealing of ion irradiated pure tungsten (2 MeV W + ions, 500°C, 10 14 W + /cm 2 ) with temperatures of 800, 950, 1100 and 1400°C, from 0.5 to 8 h, was followed by ex situ characterisation of defect size, number density, Burgers vector and nature. Loops with diameters larger than 2-3 nm were considered for detailed analysis, among which all loops had b ¼ 1 2 h1 1 1i and were predominantly of interstitial nature. In situ annealing experiments from 300 up to 1200°C were also carried out, including dynamic temperature ramp-ups. These confirmed an acceleration of loop loss above 900°C. At different temperatures within this range, dislocations exhibited behaviour such as initial isolated loop hopping followed by large-scale rearrangements into loop chains, coalescence and finally line-loop interactions and widespread absorption by free-surfaces at increasing temperatures. An activation energy for the annealing of dislocation length was derived, finding E a ¼ 1:34 AE 0:2 eV for the 700-1100°C range.

The displacement damage induced in W and W-5 wt.% Re and W-5 wt.% Ta alloys by 2 MeV W + irradiation to doses 3.3×10 17-2.5×10 19 W + /m 2 at temperatures ranging from 300 to 750°C has been characterized by transmission electron microscopy. An automated sizing and counting approach based on Image J (a Java-based image processing program developed at the National Institutes of Health) [1] has been performed for all near-bulk irradiation data. In all cases the damage comprised dislocation loops, mostly of interstitial type, with Burgers vectors b = ½<111> (> 60%) and b = <100>. The diameters of loops did not exceed 20 nm with most being ≤ 6 nm diameter. The loop number density varied between 10 22 and 10 23 loops/m 3. With increasing irradiation temperature, the loop size distributions shifted towards larger sizes, and there was a substantial decrease in loop number densities. The damage microstructure was less sensitive to dose than to temperature. Under the same irradiation conditions, loop number densities in the W-Re and W-Ta alloys were higher than in pure W but loops were smaller. In grains with normals close to z = <001>, loop strings developed in pure W at temperatures ≥ 500°C and doses ≥ 1.2 dpa, but such strings were not observed in the W-Re or W-Ta alloys. However, in other grain orientations complex structures appeared in all materials and dense dislocation networks formed at higher doses.

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