Nucleation and growth of twins in dislocation-free zinc crystals

Abstract

Zinc whiskers and platelets with their largest faces in the basal plane and thin enough ($\sim \frac{1}{2} \mu$) to be transparent to 100 kV electrons were strained in tension inside the electron microscope and the nucleation and growth of twins observed by the transmission technique. These observations were then related to the various stages of stress-strain curves obtained in a high-sensitivity tensile machine. All of the crystals were found to be initially completely free of dislocations. Twin nucleation occurred in a region where the applied stress was concentrated, namely at the specimen grips, at a corrosion pit or at a growth step. A critical resolved shear stress for twinning of $\sim$ 50 Kg/mm$^2$ was found, which is a reasonable value for the theoretical twinning stress. Twins propagated at a rate controlled by the strain rate, at a stress of 2 to 20 Kg/mm$^2$ and by the nucleation and movement of twinning dislocations. Usually a single twin was formed and grew until it transformed the entire crystal. Basal glide often occurred in a twinned region, and fracture took place in the twin by the propagation of a crack along a basal plane on which a large amount of glide had occurred. The transformation of a matrix containing dislocations with various possible Burgers vectors is considered and the differences between twinning in bulk crystals and in whiskers and plates are discussed.