Comparative Study on Plastic Deformation of Nanocrystalline Al and Ni
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INTRODUCTION
DUE to the large volume fraction of grain boundaries and the extremely small size of grains, the deformation of nanocrystalline (nc) metals and alloys is quite distinct from their coarse-grained counterparts: conventional plastic deformation of coarse-grained materials carried by dislocation activities inside grains is taken over by grain boundary (GB)-related dislocation processes in nc metals and alloys, where GBs serve as both sources and sinks for dislocations.[1–5] In particular, with the decrease of grain size, GB-mediated processes, including GB sliding and migration, gradually dominate the plastic deformation of nc materials.[5–15] This crossover from dislocation-mediated plasticity to GB-mediated plasticity takes place at a critical grain size of 10 to 20 nm, where the strength of a nc material reaches a maximum (the strongest size).[5,11–16] It has been shown by both experiments and molecular dynamics (MD) simulations that the plastic deformation inside the grains of nc face-centered cubic (fcc) materials is carried by full dislocations, partial dislocations, and twins.[1–3,7–9,14,17] The actual operative mechanism has been considered as being controlled by the grain size d, the stacking fault energy csf, and the elastic properties of the material.[2,5,9,18] A more precise prediction of slip nature of a nc material requires the generalized stacking
MAO WEN, Professor, is with the State Key Laboratory of Metal Matrix Composites, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, P.R. China. Contact e-mail: [email protected] MINGWEI CHEN, Professor, is with the State Key Laboratory of Metal Matrix Composites, School of Materials Science and Engineering, Shanghai Jiao Tong University, and also with the WPI-Advanced Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan. Manuscript submitted March 5, 2013. Article published online October 31, 2013 METALLURGICAL AND MATERIALS TRANSACTIONS A
fault energy curve, i.e., not only csf, but also the unstable stacking fault energy cusf and the unstable twin fault energy cutf have to be taken into consideration.[3,19] In the present study, we focus on the plastic deformation of nc Al and Ni. It has been shown by MD simulations that nc Al and Ni are very different in deformation behavior,[3] even though they have similar stacking fault energies. For example, full dislocations dominate in the process of plastic deformation in nc Al,[3,20,21] while partial dislocation activities are observed for Ni.[17,22,23] There is also a discrepancy between simulations and experiments: deformation twins have been seldom observed in three-dimensional MD simulations of both nc Al and Ni,[3] but they have been usually observed by transmission electron microcopy (TEM).[2,24–29] There have been extensive MD simulations aimed at understanding the deformation of nc materials. However, almost all MD simulations of nc materials have only analyzed dislocation structures and activities by identifying defects using comm
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