Observations on Work-Hardening and Fracture of Pure Polycrystalline Zinc in Tension

Abstract

High-purity zinc wires of different grain sizes were subjected to increasing tensile loading at different temperatures, and the associated changes in length, internal friction, and shear modulus were simultaneously measured. The work-hardening coefficient was found to be parabolic and to decrease with increase of grain size and temperature. This was interpreted in terms of the associated variations of the dislocation slip distance. An internal friction peak was observed at low stresses before excessive slip took place, and was attributed to the freeing and subsequent locking of the loosely pinned dislocations under the action of the applied stress. In fine-grained samples, a rise in the shear modulus was also observed and was attributed to the dislocations created by activated Frank—Read sources, which pile up against grain boundaries. The grain-boundary fracture surface energy, calculated from the observed dependence of the fracture and yield stresses on grain size, amounted to 1280 ergs/cm2 at room temperature.