Impurity Nucleation of Dislocations in Zinc

Abstract

Single crystals of Zn containing 0.1% Bi were grown at different rates by Bridgman's method. The as-grown crystals were cut chemically with 10% solution of nitric acid to avoid plastic deformation. Chemical polishing of the crystals revealed cellular segregation and pox distribution of impurities. Crystals containing impurity substructures were chemically etched to investigate dislocation correlation with impurity substructures. Etching with Méléka's reagent gave oriented precipitates along cell boundaries. The formation of these precipitates is envisaged to be due to high local strains. This has been confirmed by etching with 5% solution of bromine in methanol. Individual etch pits were revealed at cell caps and cell boundaries. Dislocation density along the cell boundaries was more than at cell caps. Etching of the (0001) surface containing pox distribution of impurities revealed preferential clustering of dislocation etch pits around impurity-segregation sites. Dislocation content of inhomogeneous alloy crystals was 102–103 times larger than the dislocation content of pure Zn crystals. The role of impurities in dislocation nucleation is discussed along with other competitive dislocation-nucleation sources. Sufficient evidence of dislocation nucleation to relieve stresses set up at the composition-fluctuation boundaries has been obtained. Prolonged annealing of the crystals at higher temperatures produced partial homogenization of impurities. Chemical etching on the partially homogenized specimens revealed the dislocation arrays in homogenized regions and dislocation clustering at segregated boundaries. The array formation has been discussed on the basis of low strain energy of this configuration.