Anelasticity study of self-interstitials in tungsten

The internal friction and dynamic modulus of W single crystals and polycrystal were measured after fastneutron irradiation near liquid-helium temperature. Two prominent relaxation peaks were observed at 8 and 27 K (at vibrational frequencies ≅500 Hz). The 8- and 27-K peaks annealed out, respectively, in the recovery stages I (∼ 18 K) and $I{I}_a\left(\sim 30 \mathrm{K}\right)$ increased in height almost linearly with irradiation dose, and were not affected by preirradiation doping. Their orientation dependence suggests that the defects of these peaks have a maximum strain field along the crystallographic direction. From these results it is proposed that the origin of these two peaks is a stress-induced rotation of free split-type self-interstitials (8-K peak) and of and of di-interstitials (27-K peak). Anisotropy ratios and activation energies for rotation of these defects are determined. Many minor peaks were also observed in the temperature range 7 to 310 K. Properties of these peaks suggest that some of them are attributable to interstitials trapped by impurity atoms and the others to interstitial clusters. These results are discussed in comparison with the resistivity recovery stages.