XXV. The direct observation of dislocation nets in rock salt single crystals

Abstract

A method for decorating dislocation lines in rocksalt single crystals is described. The method consists in colouring the crystals additively by means of sodium metal. It is found that specks are formed along the dislocation lines, which are, as a consequence, visible in ultra microscopic observation. It is thought that the specks are colloidal sodium particles, and an explanation is given for the decorating mechanism. The method is used for the study of the geometry of dislocations in deformed and annealed single crystals (i.e., polygonized). Hexagonal, square and lozenge shaped networks, as well as sets of parallel lines are found. Deviations from the ideal pattern are discussed from a theoretical point of view and compared with observations. It is found that the density of decorating particles is a function of the orientation of the dislocation line with respect to its Burgers vector. Evidence is found that screw dislocations are not decorated; it is further concluded that the nodes of a network are decorated preferentially. The emergence points of the dislocation lines correspond to the centres of the etchpits produced by the methyl alcohol etch.