Length scale and time scale effects on the plastic flow of FCC metals
M. F. Horstemeyer, M. I. Baskes, S. J. Plimpton, Acta Materialia, 49, 4363-4374 (2001).
We examine size scale and strain rate effects on single-crystal face-centered cubic (fee) metals. To study yield and work hardening, we perform simple shear molecular dynamics simulations using the embedded atom method (EAM) on single-crystal nickel ranging from 100 atoms to 100 million atoms and at strain rates ranging from 10sup 7 to 10sup 12 ssup -1. We compare our atomistic simulation results with experimental data obtained from interfacial force microscopy (IFM), nano-indentation, micro-indentation and small-scale torsion. The data are found to scale with a geometric length scale parameter defined by the ratio of volume to surface area of the samples. The atomistic simulations reveal that dislocations nucleating at free surfaces are critical to causing micro-yield and macro-yield in pristine material. The increase of flow stress at increasing strain rates results from phonon drag, and a simple model is developed to demonstrate this effect. Another important aspect of this study reveals that plasticity as reflected by the global averaged stress-strain behavior is characterized by four different length scales: (1) below 10sup 4 atoms, (2) between 10sup 4 and 10sup 6 atoms (2 mu m), (3) between 2 mu m and 300 mu m, and (4) above 300 mu m.
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