Creep of lithium fluoride single crystals at elevated temperatures

Abstract

Abstract The creep deformation of lithium fluoride single crystals was studied in compression over the temperature range 650°-750°C (0.8–0.9 Tm). Extended primary creep was observed for crystals deformed with the compression axis in ⟨100⟩ orientation; strains of 0.20 or more were required for the establishment of steady-state conditions. Crystals with compression axes in the ⟨111⟩ direction reached steady state quickly, at strains of 0.05 or less. The steady-state strain rate was found to be proportional to the stress raised to the power n, where n ranged from 3.1 for ⟨111⟩ crystals to 4.1 for ⟨100⟩ crystals. The activation energy for creep was determined to be 53 ± 7 kcal/mol over the temperature range considered, regardless of crystal orientation or impurity content up to 300 p.p.m. total impurities; this compares favourably with the activation energy for lattice diffusion of the fluorine ion in LiF. Well developed substructures were observed in the deformed crystals, the subgrain diameter varying inversely with the applied stress. These results suggest that the creep deformation of lithium fluoride single crystals may be similar to that for metals and is probably dependent upon the dislocation climb process.