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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

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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.