The effect of the change of the state of impurities resulting from heat treatment on the microhardness of NaCl crystals

Abstract

The interaction among dislocations in alkali halides is according to experience much weaker than the interaction between dislocations and point of point like defects, of interactions between dislocations and some otheT defects of vai type. Consequently, the strength and plastical properties of these crystals ate mainly determined by the latter interactions. [1]. Pointlike defects exist also in "pure" crystals (e.g. vacancy clusters) though their effect on hardness and plastical properties is considerably smaller than the effects due to impurities in doped crystals. Ir is a well known fact that impuritles, especially divalent cations, introduced into sodium chloride type crystals result in considerable hardening, which depends upon the type of impurity, its concentration and the way of impuri ty arrangement in the crystal [1, 2, 3]. In the experiments to be discussed the various impurity states of pure and doped NaC1 crystals were realized by heat treatment, the resulting effect was investigated by measuring the microhardness of the quenched crystals. In order to avoid any contamination due to various contaminants diffusing at high temperatuTe from the air into the crystals the quenching has been carried out in dry nitrogen atmosphere. Because of the requirement of fast heating and cooling a furnace of low heat capacity was used. The mechanical stresses and plastic deformation formed during the quenching process were investigated by a polarization microscope. In the experiments the heating time up to Tq as well as the cooling time from Tq was negligibly small compared to the time of heat treatment at Tq. With NaC1 9 Ca z + crystals as well as with NaCI" K + crystals two types of samples were quenched: aged crystals in which at room temperature the contaminant was thermodynamically stable and pre-quenched crystals, in which ir was in a thermodynamically unstable state. The O H free pure crystals and samples doped with Ca 2+ or K + impurities were grown by the Research Group for Crystal Physics of the Hunga-