Microstructure evolution of accumulative roll bonding processed pure aluminum during cryorolling
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Charlie Kong Electron Microscope Unit, University of New South Wales, Sydney, New South Wales 2052, Australia
Xing Zhao Key Laboratory of High Performance Complex Manufacturing, Central South University, Changsha 410083, Hunan, China; and School of Mechanical, Materials & Mechatronics Engineering, University of Wollongong, New South Wales 2500, Australia (Received 27 August 2015; accepted 8 February 2016)
The microstructure evolution and mechanical properties of ultrafine-grained (UFG) Al sheets subjected to accumulative roll bonding (ARB) and subsequent cryorolling was studied. Cryorolling can suppress the dynamic softening of UFG Al sheets subjected to ARB at room temperature. After the third ARB pass, the grains are slightly refined as the number of ARB passes increases. However, the grains are significantly refined further during cryorolling. The grain size of 460 nm achieved after the third ARB pass is reduced to 290 nm after two cryorolling passes with total reduction ratio 80%. Sheets subjected to ARB 1 cryorolling show improved mechanical properties compared to only ARB-processed sheets due to a change in the fraction of high-angle boundaries and elongated grains. The deformation mechanism for ultrafine grains at room temperature is determined by grain boundary sliding or dislocation-based recovery, while it is governed by dislocation glide at cryogenic temperature. I. INTRODUCTION
Globally, more than 20% of metal products are in the form of sheets, which are widely used in the automobile, aerospace, and other industries. Sheets made of ultrafinegrained/nano-grained (UFG/NG) materials have many desirable properties. A number of severe plastic deformation (SPD) techniques have been developed for manufacturing UFG materials: equal channel angular press (ECAP),1–4 high pressure torsion (HPT),5–7 accumulative roll bonding (ARB),8–10 improved ARB (e.g., cross-ARB and four-layer ARB),11–13 asymmetric rolling (AR),14–17 cryorolling,18–20 asymmetric cryorolling (ACR),21–23 twist extrusion24 and multiaxial incremental forging and shearing (MIFS).25 Among these SPD techniques, ARB, AR, cryorolling, and ACR can be used to fabricate UFG metal sheets. Aluminum alloys have been widely used to study the mechanism of grain refinement brought about by SPD, during which dynamic softening occurs with a higher equivalent strain when the strain exceeds a certain threshold value at room temperature. Ito and Horita7 Contributing Editor: George M. Pharr Address all correspondence to these authors. a) e-mail: [email protected], [email protected] b) e-mail: [email protected] DOI: 10.1557/jmr.2016.70 J. Mater. Res., Vol. 31, No. 6, Mar 28, 2016
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studied the microstructural evolution in pure Al processed by HPT. They found that the maximum Vickers microhardness of the samples reaches 45 Hv when the equivalent strain is 2.5, and gradually reduces with further increase in the equivalent strain. Montazeri-Pour et al.25 studied the effect of MIFS on an AA1100 alloy sample at room
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