Microstructure and texture evolution of Al during hot and cold rolling
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INTRODUCTION
THE evolution of the microstructures and textures of a material during hot deformation is directly associated with the dynamic restoration mechanisms involved. It has been generally accepted that the mechanism of dynamic restoration in A1 during hot working is essentially dynamic recovery (DRV), since the relative rates of dislocation climb and the tendency for cross slip are high due to the high stacking fault energy (SFE) of A1. I~] Many results from investigations of the dynamic softening behavior of A112-141 are in favor of this mechanism. An elongated deformation structure is produced during hot working, even at elevated temperatures well above the static recrystallization (SRX) temperature. ]~] Therefore, it has been indicated [~3j that the tendency for DRV in pure A1 is so high that dynamic recrystallization (DRX) does not occur. However, it has been reported by several investigators[~5 221 that DRX has been observed in pure A1 deformed to large strains at high temperatures. It has been reported t19] that DRX may occur in A1 during hot deformation under certain conditions that hinder DRV. In the work of Belyayev e t al., tlgl DRV occurs in A1 deformed by twisting to large strains in the temperature range from 150 ~ to 350 ~ In work on thermal softening processes in A1 during hydrostatic extrusion Styczynski e t a l . IjTl have shown that for low extrusion ratios, DRV plays a dominant role in softening, whereas for extrusion ratios above R = 500, a DRX process is activated. Studying dynamic restoration of A1 and its alloys during hot rolling, Usui e t a l . 1~51 reported that DRX B A O L U T E REN, Ph.D. Candidate, and JAMES G. MORRIS, Professor of Metallurgical Engineering and Director of the Light Metals Research Labs, are with the Department of Materials Science and Engineering, University of Kentucky, Lexington, KY 40506-0046. Manuscript submitted October 4, 1993. METALLURGICAL AND MATERIALS TRANSACTIONS A
occurs in A1, A1-Si, and A1-Mg alloys during hot rolling. In their work, it was also indicated that high strain rates restrain DRV and thus favor DRX, which is different from the influence of strain rate on Cu and Ni. Recently, Yamagata t2~ reported, during the hot compression of 99.999 pct purity A1 at temperatures from 238 ~ to 547 ~ and strains up to 2, that multipeak stress oscillations are observed that are typical of DRX in metals. He has also demonstrated that DRX occurs in both polycrystal and single-crystal A1 during hot compression) 21,22] Using processing maps, which were developed b y Raj 1231 on the basis of atomistic mechanisms to describe a regime of DRX at elevated temperatures, and a Dynamic Materials Model, which was presented by Prasad e t al. 1241 to account for the dynamic metallurgical processes occurring during hot deformation, Ravichandran and Prasad t~81 suggested that, if certain conditions regarding the deformation temperature and strain rate are satisfied, the efficiency of power dissipation reaches a maximum value of about 55 pct and that DRX occurs in A1.
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