Cyclic deformation of pure aluminum bicrystals
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I. INTRODUCTION
MUCH fundamental work has been carried out on the cyclic deformation and fatigue behavior of bicrystals, tricrystals, and polycrystals in strain-controlled experiments.[1–7] Most of the investigations were carried out to obtain a thorough understanding of the technological importance of the cyclic deformation and fatigue behavior of polycrystalline materials, rather than to analyze the GB motion and microstructure evolution of materials. Less work has been published on the cyclic stress response of aluminium bicrystals at higher temperatures. However, the high-temperature, low-cycle fatigue (HT-LCF) behavior of aluminium bicrystals has been reported[8] with some special grain boundaries, under strain-controlled conditions. In many investigations, it was observed that the migrating grain boundaries exhibit, on the sample surface, one faint trace plus (N 1) sharp markings, which were formed when the specimen experienced a total of N cycles of loading, with or without an interruption in the test.[9-11] Furthermore, a model for cyclic sliding and migration and its experimental consistency was presented by Langdon and Gifkins;[11,12] this involved reverse bending and torsion fatigue at low-cycle frequencies. It was noted that there was clear evidence for one-to-one correspondence between the markings from GB migration and the numbers and patterns of cyclic loading. But the source of the driving force for the GB motion of these particular grain boundaries was not clarified properly. The present study focuses on the high-temperature cyclic stress response of high-purity aluminium bicrystals, which were investigated by means of quasi-in-situ measurements of the GB, as well as of the respective grains. In particular, the behavior of low- and high-angle grain boundaries was analyzed and was compared to the results of the experiments on the same grain boundaries, but under the influence of a constant shear stress.[13,14] S. BADIRUJJAMAN, Postdoctoral Student, is with the Institut für Materialforschung, Forschungszentrum Karlsruhe, 76021 Karlsruhe, Germany. M. WINNING, Research Associate, is with the Institut für Metallkunde und Metallphysik, RWTH Aachen, 52056 Aachen, Germany. Contact e-mail: [email protected] Manuscript submitted March 10, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS A
II. EXPERIMENTAL PROCEDURE A. Fabrication of Bicrystals Single crystals of highly pure aluminium (a total impurity content of 7.7 ppm) were grown from the melt. Crystals of corresponding orientations and directions of plane were cut, reoriented, and joined by the standard Bridgman process, to generate 112-symmetric-tilt grain boundaries (Figure 1). The grown bicrystals were cut to a special shape and size: about 18 mm in total length, 12 mm in gage length, 5 mm in width, and 5 mm in thickness, by spark erosion (Figure 2). In all experiments, the GB plane was perpendicular to the stress axis. B. Deformation Methods and Parameters After the characterization of the initial state of the microstructure and the orien
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