Hardening and serrated flow behaviour in the 300–500 K range for Eu-doped alkali halides

Abstract

The hardening behaviour of Eu-doped alkali halides has been investigated in the 300–500 K temperature range. The aggregation/precipitation state of the europium has been monitored by means of optical and electron spin resonance spectroscopy. Main attention has been paid to investigate dynamic effects on the yield-stress as well as the occurrence of serrated flow behaviour. In fact, jerky flow, which increases with impurity concentration, has been found for Eu-doped NaCl, KCl and KBr, although the active temperature range is different for the three systems. This behaviour is characteristic of the impurity in solid solution and disappears or markedly decreases when precipitates are formed. On the other hand, yield-stress σ increases with temperature above RT for NaCl, keeps essentially constant for KCl and decreases for KBr. The enhancement of σ observed in NaCl has been shown to be a dynamic effect, not related to either impurity aggregation or static strain hardening. These results as well as those on serrated flow are, consequently, discussed in terms of available dynamic strain ageing models. It has been concluded after some numerical estimates, that formation of a Cottrell cloud, probably involving diffusion of cation vacancies, may be an appropriate dynamic aging mechanism to account for the observed effects. On the other hand, the Snoek mechanism of dipolar reorientation, has to be ruled out.