Critical Strengths for Slip Events in Nanocrystalline Metals: Predictions of Quantized Crystal Plasticity Simulations
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A quantized crystal plasticity (QCP) model has been shown to qualitatively reproduce several distinct features of nanocrystalline (NC) metals, including enhanced flow stress, an extended elasto-plastic transition strain, and a propensity for strain localization.[1] The model is motivated by molecular dynamics simulations[2] that show dramatic jumps in grain-averaged plastic strain and violent oscillations in local stress due to discrete slip events. Prior application of the QCP model suggests that an asymmetric grain-to-grain variation in the critical resolved shear stress sc is consistent with these unique stress-strain features. NC metals exhibit other distinctive mechanical features compared to their coarse-grained counterparts.[3,4] Recent reports for NC Al and Au thin films show that more than 40 pct of plastic deformation is recoverable upon unloading.[5,6] The extraordinary plastic recovery in NC metals is attributed to large residual stress, which is enhanced in principal by the quantized nature of slip and a typically large variation in microstructure, e.g., grain size.[7] Moreover, in-situ X-ray diffraction studies reveal that peak broadening observed during room temperature deformation of electrodeposited (ED) NC Ni is fully reversible upon unloading.[8–10] Thus, the inhomogeneous strain induced during deformation appears to be LIN LI, PhD Student, and PETER M. ANDERSON, Professor, are with the Department of Materials Science and Engineering, The Ohio State University, Columbus, OH 43210. Contact e-mail: [email protected] MYOUNG-GYU LEE, Assistant Professor, is with the Graduate Institute of Ferrous Technology, Pohang University of Science and Technology, Pohang, Gyeongbuk 790-784, Korea. Manuscript submitted March 21, 2010. Article published online September 8, 2010 METALLURGICAL AND MATERIALS TRANSACTIONS A
recoverable, suggesting that no permanent dislocation network forms during deformation. Also, strain-dip testing of such samples shows larger values (~GPa) for effective and internal stress compared to coarse-grained metals (~MPa).[11] These tests also reveal negative creep, which is interpreted in terms of dislocation interaction with GB ledges.[11] Underlying deformation mechanisms in NC metals have been studied in detail using MD simulations.[12–16] The observations show formation of dislocation loops from GBs. Initially, they exist in an incipient state in which grain boundary pinning prevents expansion. Ultimately, they can unpin, expand through a relatively clean grain interior, and absorb into the opposite GB. As grain size decreases, the stress to unpin tends to increase and interactions between dislocations and GBs can become more important. Such a description suggests that as grain size decreases, stress states can become more inhomogeneous and strain relaxation processes may depend on the kinetics of unpinning.[5–7,10,11] The present work incorporates reverse slip into QCP simulations.[1] This enables investigation of the combined effects of plastic predeformation (eppre), grain-tograi
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