High-temperature dislocation mobilities in doped LiF

Abstract

The stress and temperature dependences of the dislocation velocity in LiF doped with 80 p.p.m. Mg have been indirectly measured in the temperature range 773° to 973°K (0.7 to 0.8 T _m). These measurements are similar to those made in a previous study of dislocation mobilities in ’pure’ LiF in that they are based on a study of the sigmoidal creep behaviour of single crystals. This technique for obtaining dislocation mobilities at high temperatures, which was validated in our previous study, is based on Haasen's theory of sigmoidal creep.

The stress dependence of the dislocation velocity as characterized by a stress exponent, m, is large, as in the case of ‘pure’ LiF, and varies significantly with temperature. The stress exponents range between 6 and 15 and reach a maximum near 893°K. The temperature dependence of the dislocation velocity is unusual and cannot be characterized by a single activation energy within the experimental range. The dislocation velocity depends very sensitively on temperature near 893°K.

A qualitative interpretation of the unusual dislocation mobilities of both ‘pure’ and doped LiF, based on the effects of electric charges on dislocations in ionic crystals, is presented. The rate-limiting mechanism for dislocation motion is assumed to be the diffusional drag of the space-charge clouds of point defects which surround the dislocations in these crystals.