Abstract
The hardening of NaCl induced by the presence of europium ions has been systematically investigated as a function of its aggregation-precipitation state. The latter was characterized by means of optical and EPR techniques. The contribution to the hardening of free dipoles, aggregates and precipitates could be separated by using samples subjected to different suitable thermal treatments. For free dipoles a thermally activated hardening stage is found between 77 and 235 K, followed by a region where the hardening is essentially athermal. Above room temperature a slight increase in the value for the yield stress (σ) is observed. The thermally activated hardening has been analysed in terms of the elastic interaction between edge dislocations and dipoles, which leads to a linear relationship between σ2/3 and T 1/2. The thermal and athermal contributions to the yield stress of dipole aggregates were found to be similar to those of the individual dipoles; therefore, the dipoles forming part of the small aggregates keep their individuality with respect to their interaction with dislocations. Precipitation hardening results from the interaction between dislocations and the {111} and {310} platelets. The thermally activated hardening associated with those obstacles has been analysed in terms of the Kelly-Nicholson model, the activation energy being about 0·37 eV It has been determined that the height of the energy barrier associated with the {111} platelets is higher than that of the {310} platelets.