The influence of tensile strain on the growth of crystals of potash alum and sodium nitrate

Using a specially designed growth cell in which the crystal could be subjected to tensile stain during growth, studies have been made of the influence of strain on the growth rates of the habit faces of potash alum and sodium nitrate. These materials are, respectively, typical of brittle and ductile materials. In both cases, the application of strain led to a reduction in growth rate, In the case of brittle potash alum the reduction in growth fate of the {1 0 0} faces was directly proportional to the applied stress. On removal of the constraint, the growth rate returned to its initial value. The growth rate of the {1 1 1} faces was influenced by strain, but to a much lesser extent. In contrast, sodium nitrate showed a time dependent decrease in growth rate consistent with the gradual partial release of strain by plastic deformation. On release of the strain, the growth rate of the unconstrained crystal was higher than that observed initially. X-ray topographic analysis of the crystals showed that potash alum had undergone only limited plastic deformation during straining to produce few dislocations the orientation of which could not influence growth of the {1 0 0} and {1 1 0} faces. In contrast, sodium nitrate had undergone massive plastic deformation to form large numbers of dislocations many of which intersected the growing interface. Where this had occurred some of these dislocations converted to growth dislocations in the subsequent growth, thus contributing to the noted increase in growth rate, The results are extended to consider the equivalent behaviour of micro crystals and are discussed in the context of growth rate dispersion in industrial crystallisation.