Cyclic Deformation Behaviors of $$ [\bar{5}79] $$ -Oriented Al Single Crystals
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SINCE Mughrabi established the famous cyclic stress–strain (CSS) curve with three regions for singleslip-oriented Cu single crystal, the cyclic deformation behaviors of various face-centered cubic (fcc) single crystals have been investigated widely. Until now, it is well known that Cu, Ni, and Ag single crystals at room temperature show similar cyclic deformation behaviors, including the formation of a persistent slip bands (PSBs) ladder structure and the appearance of a plateau region.[2–7] Compared with the three typical fcc metals, the cyclic deformation behavior of Al single crystals with a high stacking fault energy (SFE) exhibits more differences, including both the cyclic hardening behavior and dislocation patterns. The related investigations can be retrospected to the 1960s. Snowden[8] and Mitchell and Teer[9,10] successively explored the cyclic hardening curve, surface slip morphology, and microscopic dislocation structure of Al single crystals oriented for single slip. They found that in the earliest stages of fatigue, both striations and matrix formed simultaneously. The matrix in this stage consists of irregular cell boundaries, and the striations crossed these boundaries without apparent interaction. The striations exist prior to the formation of the dipole cluster. At intermediate stages of fatigue, the matrix contained two distinct structures, namely banded loop clusters and cellular regions. With subsequent cyclic deformation, the banded loop structures gradually disappeared, and until the end, the matrix consisted solely of the cellular structure. [1]
PENG LI, Assistant Professor, and SHOUXIN LI, ZHONGGUANG WANG, and ZHEFENG ZHANG, Professors, are with the Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, 110016, Shenyang, P.R. China. Contact e-mail: [email protected] Manuscript submitted March 5, 2010. Article published online June 26, 2010 2532—VOLUME 41A, OCTOBER 2010
The evolution of the dislocation arrangements in fcc single crystals determines the differences in the macroscopic deformation behavior. For cyclic deformation of Al single crystals, a hardening–softening–secondary hardening sequence generally was found. Vorren and Ryum[11] first observed this phenomenon and regarded it as one of the typical features in fatigued Al single crystals. In addition, Vorren and Ryum[12] studied systematically the temperature effect on the cyclic deformation behavior of Al single crystals. They found that at 77 K (–196 C), mechanical saturation was reached, and the CSS curve had a plateau region in the plastic shear strain amplitude range of 4.2 9 103 < cpl < 1.2 9 102. Furthermore, the dislocation structure formed at 77 K (–196 C) was rather similar to the two-phase structure developed in Cu single crystals oriented for single slip. Dense bundles of dislocation loops are separated by channels with a much lower dislocation density. But at both room temperature and 77 K (–196 C), the ladder structure was not observed. However, at 77 K (–196
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