Steady state deformation and dislocation structure of pure and Mg-doped LiF single crystals. I. Analysis of steady state deformation

Abstract

In compressive creep deformation as well as in constant strain rate deformation of LiF between 250 and 800°C a steady state is established. The stress dependence of steady state deformation rate \documentclass{article}\pagestyle{empty}\begin{document}$ \mathop \varepsilon \limits^. _S $\end{document} can be characterized by the stress exponent \documentclass{article}\pagestyle{empty}\begin{document}$n(\sigma) = (\partial \ln \mathop \varepsilon \limits^. /\partial \ln \sigma)_T $\end{document} measured in stress or strain rate change tests. In the low stress range n = 4. At high stresses n rises strongly with σ and is not affected by Mg additions in solid solution. Mg-rich precipitates lower n. The thermal activation analysis supplies the apparent and effective activation area of steady state deformation. Typical values are in the range of several hundred b2. The activation area decreases approximately as 1/σ at low stresses but is nearly stress independent at high stresses. The temperature dependence of \documentclass{article}\pagestyle{empty}\begin{document}$ \mathop \varepsilon \limits^. _S $\end{document} for pure LiF is characterized by “true” activation energy at zero effective stress, Qt = 2.36 eV, which can be interpreted in terms of anion diffusion.