Characterizing Hardening on Annealing of Cold-Rolled Aluminum AA3103 Strips
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AA3103 aluminum alloy belongs to the group of nonheat-treatable aluminum alloys where the strength is obtained mainly by cold rolling (CR). Some additional strength can be obtained from the alloying elements, Mn and Si, either present in solid solution[1] or by forming dispersoids during homogenization treatment, subsequent warm rolling, and annealing process steps. In heat exchanger applications, every strength contribution including hardening on annealing (HOA) is valuable since the use of stronger alloys is limited by brazeability requirements and additions of Mn decrease the thermal conductivity. A conventional annealing treatment would reduce the strength and increase the ductility. However, in sharp contrast to the conventional property changes associated with annealing, it has been reported that commercial-purity Al subjected to accumulated roll bonding (ARB) exhibits an increase in strength and a loss of ductility after a low-temperature annealing treatment. This was reported to be due to the HOA behavior exhibited by nanostructured materials.[2] In the current article, a similar phenomenon is investigated for an NAGARAJ VINAYAGAM GOVINDARAJ, Ph.D. Candidate, and BJØRN HOLMEDAL, Professor, are with the Department of Materials Science & Engineering, Norwegian University of Science & Technology, 7491 Trondheim, Norway. Contact e-mails: nagaraj. [email protected]; [email protected] RUBEN BJØRGE, Postdoctoral Fellow, is with the Department of Physics, Norwegian University of Science & Technology, 7491 Trondheim, Norway. Manuscript submitted March 17, 2013. Article published online October 16, 2013 METALLURGICAL AND MATERIALS TRANSACTIONS A
AA3103 alloy, not only in the severely deformed condition but also in the cold-rolled condition that is not nanostructured. The HOA behavior in commercial-purity nanostructured Al was attributed to the limitation of dislocation sources.[2] It is recognized in the tensile curves as a yield point. A 15 pct thickness reduction by cold rolling subsequent to the annealing approximately restores the as-deformed behavior.[2] This phenomenon has been observed in interstitial-free steels, nickel, and aluminum alloys of different purity levels,[3] and it is generally obtained by a low-temperature annealing treatment subsequent to severe plastic deformation by ARB. However, the full range of metals and alloys exhibiting this phenomenon has not yet been explored. Investigations of HOA so far have mainly considered severe plastic deformation obtained by ARB, equal channel angular pressing, and heavy cold rolling.[4] However, since the HOA mechanism is obtained for ARB of high-purity aluminum with grain sizes of at least a few micrometers,[5] our attention should not be limited only to the nanostructured materials. Most investigations of HOA have considered low-temperature annealing, where the temperature that gives the peak hardness after about half an hour is applied. This is the practical limit for the time required in air furnace annealing. At longer holding times, the competi
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