Microstructure and mechanisms of cyclic deformation in aluminum single crystals at 77 K: Part II. Edge dislocation dipol
- PDF / 625,387 Bytes
- 3 Pages / 612 x 792 pts (letter) Page_size
- 94 Downloads / 197 Views
I.
INTRODUCTION
THE purpose of this research is to accurately determine the spacing of edge dipoles in aluminum single crystals oriented for single slip at 77 K. This extends some of the authors’ earlier experiments on the cyclic deformation of A1 single crystals, reported recently in this journal;[1] however, this article provides a more complete background and referencing than the previous manuscript. In particular, this work, as an extension of the earlier work, characterizes the dipole height of a large number of dipoles, both within the dipole bundles or veins as well in channels. The channels typically contain an order-of-magnitude lower dislocation density (in the form of, particularly, dipoles) than that of the veins. This analysis was initiated by the fact that there are remarkably few investigations of the dipole height. One was the early work of Antonopoulos et al.,[2] which is widely referenced, as well as some very recent work by Tippelt et al.,[3] and Bretschneider et al.,[4] on Ni. A particularly interesting observation by the latter group was that the average (as well as the maximum and minimum) dipole heights in veins, the dense dipole walls of persistent slip bands (PSBs), and the channels are nearly equal, as indicated in Table I, which is taken from Reference 3. This is interesting in that, according to the classic dipole equation[5] t5
Gb 8p (1 2 v)h
[1]
the stress in the region of the subject dipole should be calculable through Eq.[1], albeit it is not clear whether the average or maximum dipole height should be considered. The stress calculations suggest a uniform but ‘‘elevated’’ stress, approximately a factor of 4 to 5 higher than the applied stress. Although one interpretation is that the stress state may be uniform, Tippelt et al.[3] suggest a stress con-
M.E. KASSNER, Northwest Aluminum Professor, is with the Department of Mechanical Engineering, Oregon State University, Corvallis, OR 97331. M.A. WALL, President, is with ML Tech, Stockton, CA 95267. Manuscript submitted May 21, 1998.
METALLURGICAL AND MATERIALS TRANSACTIONS A
centration at PSB walls leading to a uniform dipole separation across the microstructure. The present work attempts to verify the Ni trends (identical dipole heights within veins and channels) in a higher stacking fault energy metal. II.
EXPERIMENTAL PROCEDURE
Aluminum single crystals of 99.999 pct purity were cyclically deformed in single slip, with maximum and minimum Schmid factors of 50.5 at 77 K, to 560 cycles at a plastic strain amplitude of 1.2 3 1023. This deformed the crystal to presaturation to about half of the expected saturation stress. As discussed in detail in Reference 1 (Part I of this article) by the authors, the microstructure is composed exclusively of vein dipole bundles and channels without PSBs. The vein spacing is about 1 mm and the volume fraction occupied by the veins is about 20 pct. The dislocation density in the veins is about 3.2 3 1010 cm22, and the density in the channels is about 2.4 3 109 cm22. A typical micrograph that co
Data Loading...
