Aging Effect on Texture Evolution during Warm Rolling of ZK60 Alloys Fabricated by Twin-Roll Casting
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ZK60 is a precipitation-hardening alloy, and its hardness value varies with aging. The Mg-Zn system typically exhibited precipitate aging behaviors; the microhardness and strength of such systems changed with aging.[1–3] The temporal changes in the average diameters and number densities of the metastable phases of b1 and b2 were investigated. The ZK60 (Mg-Zn-Zr) alloys exhibit precipitation-hardening behaviors similar to those of the Mg-Zn system.[4–7] Rod- or lath-shaped b1 particles were formed aligned with the long axis parallel to the hexagonal close-packed (hcp) c-axis. The Disk-shaped b2 particles were located on the {0002} plane of the a-matrix. Frequently, bulky particles formed at a casting stage were also found. The b2 phase has been known to have a hexagonal crystal structure similar to that of MgZn2. The crystal structure of the b1 phase, however, has not yet been clarified. Variations of the Zn content from 3 wt pct to 6 wt pct affected the age hardening responses.[8] The higher the Zn content, the more precipitates are formed. After overaging, the hardness decreased. The rod- and disk-shaped precipitates b1 and b2 hinder dislocation movements. In particular, the b1 precipitates vertically pinned on the basal planes can block basal slip deformation effectively. The Mg alloys have limited slip systems. The critical resolved shear stresses (CRSS) of nonbasal slip systems JAE-HYUNG CHO, Senior Researcher, HYOUNG-WOOK KIM, Principal Researcher, and SUK-BONG KANG, Principal Researcher, are with Korea Institute of Materials Science, Kyungnam 641-831, South Korea. Contact e-mail: [email protected] HONG-MEI CHEN, Ph.D., is with the School of Materials Science and Engineering, Shandong University, Jinan 250061, P.R. China. SHI-HOON CHOI, Professor, is with the Department of Materials Science and Metallurgical Engineering, Sunchon National University, Sunchon, Jeonnam 540-742, South Korea. Manuscript submitted October 5, 2009. Article published online July 1, 2010 METALLURGICAL AND MATERIALS TRANSACTIONS A
are much higher than that of basal slip system at room temperature, and thus, basal slip is dominant. The limitation of operating slip systems results in twinning activation to accommodate plastic deformation at low temperature. As the temperature increases, nonbasal slip systems start to operate as a result of the lowered CRSS. The deformation mechanisms of ZK60 alloys were investigated under compression, tension, and rolling at various temperatures and strain rates.[9–12] Dynamic recrystallization (DRX) was also observed during hot deformation of ZK60. The dislocation motion is inhibited by the limited slip systems in Mg alloys, and continuous DRX by progressive lattice rotation was suggested to occur during straining.[13] During conventional rolling process of wrought Mg alloys, a strong basal texture usually develops. Basal planes are aligned with the rolling plane, and thus the c-axis is parallel to the normal direction of rolled sheets. A basal texture with a low Schmid factor results in poor formabili
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