Hardening of Eu-doped KBr single crystals

Abstract

The yield stress of Eu-doped KBr crystals has been measured as a function of the testing temperature and impurity concentration after several types of heat treatment, in order to obtain various aggregation states of the impurity which are characterized by means of optical emission bands and ESR spectra of Eu²⁺ The hardening arising from free dipoles, small aggregates and precipitates has also been studied. For free dipoles, the hardening is temperature-dependent in the range 77-200 K and insensitive to the testing temperature above 200 K. The first stage is associated with an interaction between <110> dipoles and edge dislocations whereas, in the second stage, the Snoeck mechanism operates. The formation of small aggregates does not alter the temperature-dependence and concentration-dependence of hardening. In contrast to the NaCl: Eu system, two temperature ranges of precipitation are found in Eu-doped KBr samples. If the annealing temperature is less than 200°C, a precipitated phase forms, which may be ordered. For temperatures greater than 200°C, EuBr₂-type precipitates nucleate. In both cases, a softening is observed when the precipitate size increases and interface creation seems to be the mechanism responsible for hardening. For each type of obstacle, relevant parameters of theoretical models are determined. However, the success of a comparison between experiment and theory is found to depend strongly on the superposition law used to determine the appropriate hardening in each case.