[en] Elastic after-effects in W single crystals with various orientations were measured below 50 K after fast neutron irradiation at about 5 K. Two prominent relaxation peaks were observed at 5.5 K and 17.6 K. These two peaks correspond, respectively, with the 8 K and 27 K internal friction peaks in previous high frequency measurements. The 5.5 K peak recovered monotonicly in recovery stage I (approximately 18 K). On the other hand, the 17.6 K peak showed a transient growth in stage I and then steadily recovered in stage IIsub(a) (approximately 28 K). From these results, and their dependences on irradiation fluence and orientation, it was concluded that the 5.5 K and 17.6 K peaks are, respectively, associated with a stress-induced rotation of free <110> split type interstitials and of di-interstitials with <110>-symmetry. Anisotropy ratios and activation parameters of these defects were determined more accurately than in the previous work. Both activation energies for rotation and for migration showed a somewhat broad distribution for the 5.5 K and 17.6 K peaks. (author)