Atomic defects in hexagonal tungsten carbide studied by positron annihilation

Abstract

Vacancies on the two sublattices of hexagonal tungsten carbide were identified by means of positron lifetime studies after irradiation with electron of various energies. In the as-prepared state two predominant positron lifetime components occur indicating positron annihilation in the free state with a lifetime ${\ensuremath{\tau}}_{f}=124\ifmmode\pm\else\textpm\fi{}10$ ps according to the simple trapping model and positron trapping and annihilation in metal vacancies $({\ensuremath{\tau}}_{{V}_{W}}=175\ifmmode\pm\else\textpm\fi{}20$ ps) of low concentration. By means of low-energy electron irradiation at 1 MeV with an electron dose of $1.8\ifmmode\times\else\texttimes\fi{}{10}^{23} {\mathrm{m}}^{\ensuremath{-}2}$ exclusively carbon vacancies are introduced giving rise to saturation trapping and annihilation of positrons at carbon vacancies with a low lifetime of ${\ensuremath{\tau}}_{{V}_{C}}=136\ifmmode\pm\else\textpm\fi{}3$ ps. The isochronal annealing of irradiation induced vacancies reveals two annealing stages at 800 and 1200 K, which are ascribed to the annealing of carbon vacancies and the dissolving of small carbon vacancy clusters.