Vacancy migration enthalpy in tungsten at high temperatures

Abstract

The curvature found in the Arrhenius plot for tungsten self-diffusion has been commonly interpreted in terms of simultaneous diffusion by two different defect processes. An alternative interpretation has been in terms of the temperature dependence of the migration enthalpy H m 1V and/or the formation enthalpy H f 1V for single vacancies. In order to study the magnitude of the temperature dependence of the migration enthalpy H m, the present experiments were done to study the annealing of a supersaturation of vacancies at high temperatures in tungsten. The annealing of vacancies was monitored by interrupting quenches from 2900 K at well-defined anneal temperatures T a for a series of different time periods, followed by a quench to 300 K. The quenched-in resistivity Δρq was subsequently measured at 4 K. The decay of Δρq was observed for approximately ten delay times at each anneal temperature in order to define a characteristic decay time τ1/2. Analysis of the decay curves showed that at all the anneal temperatures the decay was dominated by a single process. The Arrhenius plot of τ−1 1/2 against 1/T a was curved such that the effective migration enthalpy increased from 1·68 eV (T a=1550 K) to 2·02 eV (T a=2600 K). The magnitude of the temperature dependence of H m 1v alone is insufficient to explain the Arrhenius curvature found for self-diffusion in tungsten. However, the curvature could be explained if the formation enthalpy H f 1v had a temperature dependence of similar magnitude.