Large-Strain Softening of Aluminum in Shear at Elevated Temperature: Influence of Dislocation Climb
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his article complements an earlier publication in this journal by the authors[1] describing the basis for large-strain softening in aluminum under pure shear at elevated temperature within the five-power-law creep regime. An example of this softening is illustrated in Figure 1. In particular, this article discusses the basis for the softening, including a new basis, dislocation climb, not considered in any earlier work by these or any other authors.[1–15] There has been work[16] in which the dislocation force and its resulting climb stress were related to the experienced tensile stress on the material during testing. The Al deformed in torsion hardens to a peak stress at strains generally less than 0.5 followed by a decrease in the flow stress of about 17 pct over a strain range of 1 to 2 to an approximately constant flow stress. Dynamic recovery (DRV) occurs exclusively over the entire strain range. The most common and perhaps most widely
M.E. KASSNER is with the Aerospace and Mechanical Engineering Department, University of Southern California, Los Angeles, CA 90089, and also with the Chemical Engineering and Materials Science Department, University of Southern California. Contact e-mail: [email protected] C.S. CAMPBELL is with the Aerospace and Mechanical Engineering Department, University of Southern California. R. ERMAGAN is with the Chemical Engineering and Materials Science Department, University of Southern California. Manuscript submitted March 24, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
accepted explanation for the softening is its variance from decreases in the average Taylor factor (textural softening leading to a decrease in the stress for dislocation glide). Dislocation climb will be more pronounced at higher temperatures in aluminum than in most other materials due to the relatively high stacking fault energy. One group suggests that changes in the dislocation substructure (e.g., increase in the average subgrain size) through increased DRV are a partial basis of the softening,[15] in addition to a decrease in the dislocation glide stress. However, earlier work by the authors[18] found that both the subgrain size and the dislocation density (not associated with subgrain boundaries) are approximately constant throughout deformation to large strains of 16. However, we found that there is a dramatic increase in high-angle boundary area through geometric dynamic recrystallization that can accompany DRV with large-strain deformation. Table I reports the observed textures (A, C, and B1[1–15,17–22]) with the large-strain deformation in aluminum at elevated temperature. The first index is the crystallographic plane in the shear plane and the second is the shear direction. McQueen[13] and Kocks et al.[24] found that the B1 texture may be the most pronounced. Shrivastava et al.[25] calculated the torsional Taylor factors using the Bishop and Hill (for traditional slip systems) method for A, C, and B1 textures, which are also listed in Table I. The average Taylor factor for the three textures is 2.34, or abo
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