Length scale and time scale effects on the plastic flow of fcc metals

We examine size scale and strain rate effects on single-crystal face-center ed cubic (fee) metals. To study yield and work hardening, we per-form simpl e shear molecular dynamics simulations using the embedded atom method (EAM) on single-crystal nickel ranging from 100 atoms to 100 million atoms and a t strain rate., ranging from 10(7) to 10(12) s(-1). We compare our atomisti c simulation results with experimental data obtained from interfacial force microscopy (IFM). nano-indentation, micro-indentation and small-scale tors ion. The data are found to scale with a geometric length scale parameter de fined by the ratio of volume to surface area of the samples. The atomistic simulations reveal that dislocations nucleating at free surfaces are critic al to causing micro-yield and macro-yield in pristine material. The increas e of flow stress at increasing strain rates results from phonon drag, and a simple model is developed to demonstrate this effect. Another important as pect of this study reveals that plasticity as reflected by the global avera ged stress-strain behavior is characterized by four different length scales : (1) below 10(4) atoms, (2) between 10(4) and 10(6) atoms (2 mum), (3) bet ween 2 mum and 300 mum, and (4) above 300 mum. (C) 2001 Acta Materialia Inc . published by Elsevier Science Ltd. All rights reserved.