Stage-I Interstitials in Electron-Irradiated Tungsten

Abstract

Single crystals and polycrystalline 5-mil tungsten wires of high purity were irradiated at 20\ifmmode^\circ\else\textdegree\fi{}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 (\ensuremath{\sim}600 Hz). After irradiation, a prominent but transient internal-friction peak was observed at 30\ifmmode^\circ\else\textdegree\fi{}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\ifmmode^\circ\else\textdegree\fi{}K. Arguments are presented which identify the stress-induced ordering of the interstitial members of close Frenkel pairs as the source of the 30\ifmmode^\circ\else\textdegree\fi{}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 (${E}_{r}=0.066\ifmmode\pm\else\textpm\fi{}0.01$ eV) and annealing (${E}_{a}=0.11\ifmmode\pm\else\textpm\fi{}0.01$ eV) are determined for the 30\ifmmode^\circ\else\textdegree\fi{}K process. A discussion of internal-friction background increase with heavy irradiation is also presented.