The Effect of Humidity on Recrystallization and Aging of Plastically Deformed Alkali Halide Crystals

Abstract

The effect of water and temperature on grain growth of plastically deformed doped potassium chloride has been studied. The results of experiments show that it is crucial to avoid contact with saturated water vapor in order to prevent a deformed crystal from decomposing. New grains oriented as twins with respect to the initial single crystals, as well as to the deformed matrix, grow fast in the presence of water vapor. The reasons why the properties of the deformed crystals in contact with water vapor deteriorate with time can conceivably be as follows: at first, strontium chloride hexahydrate forms on grain boundaries, then its particles are dissolved by the salt water solution, forming a liquid layer which affects grain boundaries, making a crystal fall to pieces in the end. A deformed matrix can be preserved for more than a year, given that the sample is placed in vacuum for about a month immediately after deformation and then stored at room temperature in the open air. Though a temperature of storage is not that essential in the range from minus -13 to +25°, some special heat treatment after deformation may be useful to dry a sample and, as a result, to defer recrystallization and, therefore, keep mechanical strength at a high level.