Simultaneous Grain Growth and Grain Refinement in Bulk Ultrafine-Grained Copper under Tensile Deformation at Room Temper
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st materials, strength and ductility are conflicting properties. However, with the improvement in the strength of ultrafine-grained (UFG)/nano-grained (NG) materials, there is a widespread interest in attempts to simultaneously improve the ductility.[1] A number of techniques to develop UFG materials with both high strength and high ductility have been proposed.[2–6] Recently, Lu[7] reported a gradient copper with high strength and high ductility. He suggested that the deformation of nano-gradient copper is dominated by a mechanically driven grain boundary migration with
HAILIANG YU, Research Fellow, CHENG LU, Associate Professor, ANH KIET TIEU and HUIJUN LI, Professors, and AJIT GODBOLE, Senior Research Fellow, are with the School of Mechanical, Materials & Mechatronics Engineering, University of Wollongong, NSW 2500, Australia. Contact e-mail: hailiang@uow. edu.au CHARLIE KONG, Senior Research Officer, Electron Microscope Unit, is with the University of New South Wales, Sydney, NSW 2052, Australia. XING ZHAO, Ph.D. Student, is with the State Key Laboratory of High Performance Complex Manufacturing, Central South University, Changsha 410083, China, and also with the School of Mechanical, Materials & Mechatronics Engineering, University of Wollongong. Manuscript submitted November 6, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS A
concomitant grain coarsening and softening. He proposed that deformed coarse grains are hardened by dislocation slip and accumulations, providing work hardening of the global sample. He proposed theoretically that in a critical sub-micro-sized region, neither hardening nor softening is induced as the two mechanisms are balanced. The aforementioned studies refer to theoretical prediction. However, to date, there is no experimental verification of the simultaneous occurrence of grain growth and grain refinement in bulk UFG materials under tensile deformation. Coarse grains can be refined under increased strain at room temperature and reach a stable size due to dynamic recovery, if the strain exceeds a certain threshold value.[8] At the same time, grain growth has also been observed in thin films.[9–11] Apart from the grain growth driven by temperature, shear stress could bring about grain boundary migration and grain rotation, resulting in grain growth. Legros et al.[9] observed that grain growth occurs preferentially under the applied load in the highly stressed regions ahead of crack tips in NG aluminum films. They also reported very large grain boundary velocities, exceeding 200 nm s1 for collapsing small grains and exceeding 30 nm s1 for growing grains. Jin et al.[10] studied the in-situ deformation-induced grain growth of UFG Al films during nano-indentation at room temperature. Mompiou et al.[11] studied the grain boundary migration due to shear in Al polycrystals using a transmission electron microscope (TEM). The deformation associated with grain boundary migration was measured using the superposition of surface markers, which shows a coupling between shear and migration. Using atomistic s
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