Computational nanoscale plasticity simulations using embedded atom potentials

M. F. Horstemeyer, M. I. Baskes, S. J. Plimpton, Theoretical and Applied Fracture Mechanics, 37, 49-98 (2001).

In determining structure-property relations for plasticity at different size scales, it is desired to bridge concepts from the continuum to the atom. This raises many questions related to volume averaging, appropriate length scales of focus for an analysis, and postulates in continuum mechanics. In a preliminary effort to evaluate bridging size scales and continuum concepts with discrete phenomena, simple shear molecular dynamics simulations using the embedded atom method(EAM) potentials were performed on single crystals. In order to help evaluate the continuum quantities related to the kinematic and thermodynamic force variables, finite element simulations (with different material models) and macroscale experiments were also performed. Various parametric effects on the stress state and kinematics have been quantified. The parameters included the following: crystal orientation (single slip, double slip, quadruple slip, octal slip), temperature (300 and 500 K), applied strain rate (10sup 6-10sup 12 ssup -1), specimen size (10 atoms to 2 mu m), specimen aspect ratio size (1:8-8:1), deformation path (compression, tension, simple shear, and torsion), and material (nickel, aluminum, and copper). The yield stress is a function of a size scale parameter (volume-per-surface area) that was determined from atomistic simulations coupled with experiments. As the size decreases, the yield stress increases. Although the thermodynamic force (stress) varies at different size scales, the kinematics of deformation appears to be very similar based on atomistic simulations, finite element simulations, and physical experiments. Atomistic simulations, that inherently include extreme strain rates and size scales, give results that agree with the phenomenological attributes of plasticity observed in macroscale experiments. These include strain rate dependence of the flow stress into a rate independent regime; approximate Schmid type behavior; size scale dependence on the flow stress, and kinematic behavior of large deformation plasticity.

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