Stage-I Interstitials in Electron-Irradiated Tungsten

Single crystals and polycrystalline 5-mil tungsten wires of high purity were irradiated at 20^∘{}K with 2.5-MeV electrons. Stage-I defect behavior was studied by internal friction, dynamic Young's-modulus, and resistivity measurements. The friction and modulus samples were electrostatically driven at their fundamental flexural mode ({~}600 Hz). After irradiation, a prominent but transient internal-friction peak was observed at 30^∘{}K. Simultaneous resistivity and mechanical-property measurements during isochronal annealing indicates that the friction peak recovers during a major close-pair recovery substage, and that long-range interstitial motion occurs between 45 and 100^∘{}K. Arguments are presented which identify the stress-induced ordering of the interstitial members of close Frenkel pairs as the source of the 30^∘{}K relaxation peak. Single-crystal results on relaxation strength versus stress direction suggest that the interstitials are not 111> but most probably are 110> split interstitials. Defect asymmetry and activation energies for reorientation (Eᵣ=0.066±0.01 eV) and annealing (Eₐ=0.11±0.01 eV) are determined for the 30^∘{}K process. A discussion of internal-friction background increase with heavy irradiation is also presented.