The low-temperature flow stress of europium-doped sodium chloride

Abstract

The flow stress of single-crystal NaCl: Eu has been measured as a function of europium concentration and heat treatment at 80, 196, 273 and 300 K. Samples that were fast-cooled from temperatures above 673 K contain isolated Eu²⁺-Na vacancy (IV) dipoles. These samples are considerably softer than samples which had been aged at 423 K for 24 hours. The aged samples contain an aggregated phase. A linear dependence of flow stress on concentration was observed at all temperatures for the feat-cooled samples. At 80 K the curve of the flow stress versus concentration had a line gradient approximately twice the value observed at the higher temperatures. If this linear dependence is caused by a Snook relaxation of the isolated IV dipoles, their relaxation time must be considerably modified by the presence of the moving dislocations. The aged samples showed a square-root dependence of flow stress on concentration for temperatures above 196 K.