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. © 1969 The American Physical Society.