Dislocation density: stress relationships in natural and synthetic sodium chloride

To determine the relationship between applied stress and dislocation density, creep experiments have been carried out on single crystals of natural and synthetic sodium chloride with differential stresses between 2.0 and 20.0 MPa at temperatures in the range 20 to 600° C. Dislocation densities were measured on etched cleavage surfaces using a scanning electron microscope. The dislocation density is proportional to the differential stress to approximately the 1.3 power. At a given differential stress, the density of edge dislocations is larger and the density of screw dislocations smaller in natural crystals than in synthetic samples; the mean dislocation density was unaffected. This difference in the dislocation densities between natural and synthetic crystals is related to differences in the concentrations of impurities. Natural salt samples from a Lansing N.Y., salt mine were collected from locations in close proximity to excavation openings. Dislocation densities in the natural samples, when compared to our experimentally derived stress-dislocation density curve, indicate that the samples were deformed at in-situ stresses of 10 to 30 MPa.