Free Migration of Interstitials in Tungsten

The internal friction and dynamic modulus of W single crystals were measured after neutron irradiation near liquid-helium temperature. The results on dislocation pinning and the associated relaxation peaks suggest that in W the $〈110〉$ split interstitials perform a free migration at ∼ 15 K (substage ${\mathrm{I}}_1$) and the ∼ 30-K stage (substage ${\mathrm{I}}_2$) is associated with detrapping of the $〈110〉$ split interstitials from impurity atoms.