Methods for Designing Concurrently Strengthened Severely Deformed Age-Hardenable Aluminum Alloys by Ultrafine-Grained an
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TRODUCTION
IN general, strengthening mechanisms of aluminum alloys include strain hardenings, hardenings by grain refinement, solid-solution hardenings, and/or precipitation hardenings because no allotropic or martensitic transformation is available. Severe plastic deformation (SPD) such as equal-channel angular pressing (ECAP), accumulative roll bonding (ARB), and high-pressure torsion (HPT) has been successfully utilized as a method for improving the strength of metallic materials based on the first two strengthening mechanisms.[1] However, the combined processing of SPD and age-hardenings treatment does not necessarily result in a further increase in strength of heat-treatable aluminum alloys, unlike in the case of non-heat-treatable aluminum alloys where solid-solution hardenings is almost additive to strain hardenings or ultrafine-grained hardenings (i.e., hardenings by grain refinement to the submicrometer or nanometer level).[2] This is because if the microstructure of age-hardenable aluminum alloys is controlled for strain hardenings and/or ultrafine-grained hardenings, SHOICHI HIROSAWA, Associate Professor, is with Yokohama National University, Yokohama 240-8501, Japan. Contact e-mail: [email protected] TAKUMI HAMAOKA, formerly with Yokohama National University, is now Assistant Professor with Tohoku University, Sendai 980-8577, Japan. ZENJI HORITA, Professor and SEUNGWON LEE, Postdoctoral Researcher, are with Kyushu University, Fukuoka 819-0395, Japan. KENJI MATSUDA, Professor, is with University of Toyama, Toyama 930-8555, Japan. DAISUKE TERADA, Assistant Professor, is with Kyoto University, Kyoto 606-8501, Japan. Manuscript submitted February 22, 2013. Article published online April 17, 2013 METALLURGICAL AND MATERIALS TRANSACTIONS A
the subsequent precipitation hardenings is often limited due to the competitive precipitation among dislocations, grain boundaries, and the matrix. Table I summarizes previously reported experimental results on concurrent strengthening by SPD and the age-hardenings technique for a number of age-hardenable aluminum alloys.[3–20] Although the specimens under no SPD condition (i.e., equivalent strain less than 4) possess relatively good agehardenability, most of the severely deformed and then artificially aged specimens exhibit either suppressed age hardenings or age softening as denoted by smaller D or large .. The following are the main features extracted from Table I, where attained values of hardness/ strength, the degree of age hardenings/softening, and the corresponding microstructures are reviewed according to the alloy system, equivalent strain, and aging temperature. (i) The higher the aging temperature applied, the lower the attained hardness/strength and the degree of age hardenings. This is of course not unusual regardless of the application of SPD, but the degree appears to be significant (Indeed, in some cases, only age softening occurs). This can be explained not only by the more predominant recovery of dislocations and growth of ultrafine grains but also by coarsened
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