Hysteresis Loop Shape of a Cyclically-Deformed Copper Tricrystal Having Two Longitudinal Grain Boundaries

Abstract

The nature of the Bauschinger effect in metallic materials is explained mainly by two different approaches. One takes account of the internal stresses related to macroscopic residual stresses between the neighboring grain which shows different hysteresis loops. The other approach is based on the long-range elastic interactions between dislocations creating microscopic stress field (see the review (1) for details). Obviously, these two mechanisms operate simultaneously in a common polycrystalline material. Discrimination between these two mechanisms is not quite straightforward, and the magnitudes of individual contributions to the Bauschinger effect are still ambiguous. In order to separate these two contributions, it might be convenient to compare the results obtained on a bicrystal having a longitudinal grain boundary with those of constituent single crystals. However, a technical problem arises in experiment with bicrystals. If the flow stresses of constituent grains are different, the deviation in the axial stress gives rise to a certain bending moment leading to inhomogeneous stress and strain distribution in the specimen. In order to diminish this bending moment, we employed a tricrystal with two longitudinal parallel boundaries as shown schematically in Fig. 1. The grains at both ends of the specimens are of the same orientation and dimension. In the present study, the cyclic deformation tests were carried out on the tricrystal and its constituent single crystals. The Bauschinger effect during the cyclic deformation was compared between the tricrystal and the constituent single crystals by means of the Bauschinger energy parameter bE (2) and the Bauschinger stress parameter bs (3). The authors attempted to understand the Bauschinger effect in the cyclically-deformed tricrystal by mixing the corresponding pairs of hysteresis loops of the constituent grains.