A study of high temperature creep in LiF single crystals

Publisher Website
Google Scholar
Abstract
The high temperature/low stress creep behaviour of low dislocation density single crystals of lithium fluoride containing less than 1 p.p.m. divalent cation impurities has been studied. The creep behaviour is found to be sigmoidal in character and to conform to the dislocation multiplication model of creep developed by Haasen and co-workers for covalent crystals. Comparison of the data with the Haasen model permits an indirect determination of the stress and temperature dependence of the average dislocation velocity as well as a determination of the parameters which describe dislocation multiplication and dislocation-dislocation interactions. The dislocation densities predicted by the Haasen model are in good agreement with the measured etch-pit densities. Analysis of the creep data leads to the following expression for the average dislocation glide velocity: where τe is the effective shear stress (in g/mm2) acting on the dislocations, m = 7.8 and U = 3.94 ev is the apparent activation energy for dislocation glide. Dislocation multiplication is found to be proportional to the effective stress with a breeding constant, δ, in close agreement with that found for LiF at room temperature. The constant of dislocation interaction, α, as found in the usual formulation for back stress, αGb√N, was determined to be 0.2, in reasonable agreement with the value obtained for LiF through strain-hardening experiments. At present, the very high effective stress dependence and the high apparent activation energy for a glide are not understood.