Dynamic shear deformation in zinc crystals

Abstract

The initial stages of yielding in single crystals of zinc have been investigated at temperatures of -292°F (-180°C), -93°F (-70°C) and +75°F (24°C), by means of rapidly applied constant stress pulses to produce slip along the basal planes of these crystals. The experimental results are interpreted in terms of a dislocation mechanism based upon the assumptions that the average velocity of dislocations remains constant during the period of constant applied stress and that the density of moving dislocations increases in linear proportion to the plastic strain. The experimental curves of plastic strain vs. time exhibit the form predicted by the assumed dislocation mechanism. The average dislocation velocity is found to be proportional to the 2.5 power of the excess of the applied stress over the static yield stress. The purity of the specimen and the test temperature are found to influence the average dislocation velocity only through their influence upon the static yield stress. These results show that the behavior of basal slip dislocations in zinc is qualitatively the same as the behavior of slip dislocations in lithium fluoride and silicon-iron, as reported by Johnston and Gilman and by Stein and Low, respectively. Hence it is concluded that the concept of a lattice resistance to dislocation motion proposed by Gilman to explain the observed behavior of lithium fluoride and silicon-iron can be applied equally well to explain the observed behavior of zinc, and in fact that this is the only explanation consistent with the observed results.