Dislocation structures of monocrystalline copper during corrosion fatigue in 0.1 M perchloric acid
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I.
INTRODUCTION
THE dislocation
structures of cyclically deformed specimens, especially for single crystals of copper, have been extensively studied. ~=s It is well known that cycled fcc metals usually contain two phases of dislocation structure: a soft phase, persistent slip bands (PSB's), and a hard phase (loop patch or matrix structure), provided the applied plastic strain amplitude lies in the plateau region of the cyclic stress-strain curve. (The "plateau" means that the saturation stress is independent of shear strain amplitude over the range: 6 x 10 5 to 7.5 x 10 3.) The dislocation arrangement in the PSB's consists of dipolar walls aligned normal to the primary Burgers vector and separated by dislocation-free channels. The spacing between the walls is 1.3 = 1.5 /xm for room temperature cycling. The dipolar walls occupy about 0.1 volume fraction. The matrix contains loop patch structure which also consists of regions having higher dislocation density separated by dislocation-free channels, but the volume fraction of the loop patches is much higher (-O.5). Generally, the dislocation structure of a material is directly associated with its mechanical behavior. For example, it has been demonstrated that single crystals of copper show much higher saturation stress at low temperature and reduced spacing of the dipolar walls. 6 It is also widely recognized that cell sizes decrease with increasing applied strain. 7 The effect of an aggressive aqueous environment on reducing fatigue life has bee n convincingly demonstrated in the early literature. In a recent study of single crystals of copper in 0.1 M HC104 under different polarization potentials, we demonstrated that the life-degrading effect of the solution under anodic potentials was due to enhanced strain BENDA YAN, formerly with the University of Pennsylvania, is now with the Department of Metallurgical Engineering and Materials Science, Carnegie-Mellon University, Pittsburgh, PA 15213. GREGORY C. FARRINGTON and CAMPBELL LAIRD are with the Department of Materials Science and Engineering, University of Pennsylvania, 3231 Walnut Street, Philadelphia, PA 19104. Manuscript submitted September 24, 1984. METALLURGICAL TRANSACTIONS A
localization at the PSB's associated with preferential dissolution; 8 the maximum strain localization factor (ratio of localized strain to overall applied strain) at the PSB's could be as high as 53, which is almost three times higher than that observed in air. 9 As is well known, the ability of a material to carry a certain amount of plastic deformation (e.g., 1 pct for the PSB's on average) is determined by the particular dislocation arrangement, and we would therefore expect that enhanced strain localization should have some effect on the dislocation structure. Contrary to the abundant availability of substructure studies for specimens cycled in air, TEM observations of specimens fatigued in corrosive environments are still very limited. In the present work we report such observations in an attempt to improve our understanding of c
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