Improvement in Strength and Ductility of Asymmetric-Cryorolled Copper Sheets Under Low-Temperature Annealing
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Miniaturization facilitates product usage to a greater degree, enabling product functions to be implemented in microscale geometries, as well as reducing product weight, volume, and cost.[1] A number of miniature product parts are in the form of sheets, foils, or films.[2] The smaller the microparts are, the thinner the foils need to be. At the same time, these foils should be strong enough to maintain the structural stability of the microparts. Ultrafine-grained/nano-grained (UFG/ NG) copper has been demonstrated to have good mechanical strength while retaining its good electrical conductivity.[3] Takata et al.[3] found that UFG copper
HAILIANG YU and QINGLIN DU are with the State Key Laboratory of High Performance Complex Manufacturing, Central South University, Changsha 410083, China and also with the College of Mechanical and Electrical Engineering, Central South University, Changsha 410083, China. Contact emails: [email protected], [email protected] AJIT GODBOLE and CHENG LU are with the School of Mechanical, Materials & Mechatronics Engineering, University of Wollongong, Wollongong, NSW 2500, Australia. CHARLIE KONG is with the Electron Microscope Unit, University of New South Wales, Sydney, NSW 2052, Australia. Manuscript submitted February 27, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
which was fabricated using accumulative roll bonding (ARB) had a sub-micron grain size, and could achieve both superior mechanical properties and high electric conductivity. Champion and Bre´chet[4] pointed out that UFG copper sheets are ideal for micro-electromechanical applications. Bulk UFG/NG materials have excellent properties such as high strength, high fatigue service life, and extreme creep resistance. These materials have been attracting a great deal of attention during recent years.[5–8] Bulk fabrication of metallic sheets using severe plastic deformation techniques such as ARB,[9,10] asymmetric rolling (AR),[11,12] cryorolling (CR),[13,14] and asymmetric cryorolling (ACR)[15,16] has probably brought us closer to enabling use of UFG materials for structural and functional applications. AR generates additional shear strain that contributes to grain rotation and subdivision, producing grain refinement and modification of the crystallographic texture. This makes it possible to manufacture thinner products. In the case of CR, the suppression of dynamic recovery during deformation at extremely low temperatures (achieved by liquid nitrogen) preserves the high density of dislocations generated by deformation. ACR is a technique that combines the features of AR and CR. UFG/NG materials possess high amounts of interfaces and nano-sized grains. Therefore, it is expected that their grain growth will be extremely rapid during consolidation or during service, even at room temperature. The initially small grain size in UFG/NG materials may coarsen during service due to homogeneous coarsening of the sub-grain structure while attaining the new steady state.[17] Research has demonstrated that the growth of nano-size grains
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