Dislocation structures of monocrystalline copper in cyclic latent hardening
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INTRODUCTION
IN a previous paper, m we reported the results of mechanical tests in a study of cyclic latent hardening. It was found that during cyclic deformation of monocrystalline copper, the latent hardening ratio (LHR) of a coplanar system is close to 1, and of non-coplanar systems, 1.25 to 1.30. The latent hardening ratio is defined as LHR =
the yield stress of the secondary test the final flow stress of the parent crystal in the first test
where, in the present work, the first test refers to the cycling in a single crystal which establishes a primarydominated dislocation structure, and the secondary test refers to the monotonic test carried out on a sample cut from the first test sample. The cutting may or may not involve reorienting the crystal to excite a different slip system, depending on whether one desires to investigate a non-coplanar or coplanar system. Also reported in Reference 1 was the isotropic behavior of the latent hardening effect in non-coplanar systems. In the present paper, we report the dislocation structures obtained by TEM studies of the monocrystalline specimens used for the cyclic latent hardening study. As is well known, dislocation structures of monocrystalline copper have already been thoroughly investigated for both monotonic and cyclic deformation. At different stages of deformation, dislocation structures in
ZHIRUI WANG, formerly Research Associate, Department of Materials Science and Engineering, University of Pennsylvania, is now Assistant Professor with the Department of Metallurgy and Materials Science, University of Toronto, Toronto, ON M55 IA4, Canada. CAMPBELL LAIRD, Professor, is with the Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, PA 19104-6272. Manuscript submitted August 12, 1988. METALLURGICAL TRANSACTIONS A
both types of deformation have some corresponding features and, naturally, different ones as well. In Stage I monotonic deformation, dislocations pile up on the primary planes and tangles occur periodically. In addition, there is much evidence for the existence of dislocation dipoles between the tangles. Also, secondary dislocations, especially associated with the cross slip system, may be detected. In Stage II, the dislocations form cell boundaries more definitely. On thin foils sliced parallel to the primary slip planes, it is observed that regions clear of dislocations are usually surrounded by dislocation tangles, which tend to be aligned along (110) directions. Primary dipoles and edge dislocations are predominant in the cell boundaries. However, secondary dislocations are also found to contribute to cell boundary formation. In Stage III, cross slip, in addition to multislip in Stage II, becomes a major phenomenon. The dislocation structures, therefore, are much more complicated. All of the above-mentioned results are reported in papers and textbooks almost too numerous to mention. I2] In cyclic deformation of copper single crystals, the dislocation structures have usually been investigated with respect to t
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