The influence of grain size on the yield phenomenon in steel

Abstract

This paper presents the results of experimental measurements of the influence of grain size upon the static yield point and the delay time for yielding in a very low carbon steel. These are interpreted in terms of a theory based upon the assumption that macroscopic yielding begins after slip bands have formed in a certain fraction of all grains and have penetrated the grain boundaries. Comparison of the theory with the measurements of static upper yield point as a function of grain size indicates that plastic shear deformation of a grain boundary requires an energy of about 580 ergs/cm 2 of grain boundary area. This result is consistent with the concept that the grain boundaries are saturated with carbon atoms and that the binding energy of a carbon atom in a grain boundary is about equal to the binding energy between a carbon atom and a dislocation. The delay time for yielding is the time required, at constant applied stress, for the density of slip bands to increase to the value necessary for macroscopic yielding. This time is governed by the thermally activated release of dislocations within grains from their Cottrell atmospheres. The theory predicts that the delay time should be inversely proportional to the cube of the grain diameter. This prediction is in reasonable agreement with the experimental measurements.