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 but most probably are split interstitials. Defect asymmetry and activation energies for reorientation ($E_r=0.066\mathrm{}\pm 0.01$ eV) and annealing ($E_a=0.11\mathrm{}\pm 0.01$ eV) are determined for the 30°K process. A discussion of internal-friction background increase with heavy irradiation is also presented.