MD Simulations of Compression of Nanoscale Iron Pillars
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MD Simulations of Compression of Nanoscale Iron Pillars Con J. Healy and Graeme J. Ackland School of Physics and Astronomy, The University of Edinburgh, James Clerk Maxwell Building, Mayfield Road, Edinburgh EH9 3JZ, UK ABSTRACT It is now possible to create perfect crystal nanowires of many metals. The deformation of such objects requires a good understanding of the processes involved in plasticity at the nanoscale. Isotropic compression of such nanometre scale micropillars is a good model system to understand the plasticity. Here we investigate these phenomena using Molecular Dynamics (MD) simulations of nanometre scale single crystal BCC iron pillars in compression. We find that pillars with large length to width ratio may buckle under high strain rates. The type of buckling behaviour depends sensitively on the boundary conditions used: periodic boundary conditions allow for rotation at top and bottom of the pillar, and result in an S shaped buckle, by contrast fixed boundaries enforce a C shape. Pillars with a length to width ratio closer to that used in experimental micropillar compression studies show deformation behaviour dominated by slip, in agreement with the experiments. For micropillars oriented along , slip occurs on planes and localized slip bands are formed. Pillars of this size experience higher stresses than bulk materials before yielding takes place. One might expect that this may be in part due to the lack of nucleation sites needed to induce slip. However, further simulations with possible dislocation sources: a shorter iron pillar containing a spherical grain boundary, and a similar pillar containing jagged edges did not show a decreased yield strength. INTRODUCTION Despite advances made over the course of the last century on microscopic behavior of solids, the microstructure of solids remains difficult to represent in models of macroscopic plasticity. New experimental techniques are now becoming available to analyze deformation at smaller length scales. The micropillar compression test is an experimental technique which has been developed to analyze plastic deformation at the micron scale [1]. In this technique, pillars of single crystal metal are created and then compressed using an indenter. These pillars then usually deform by slip. Slip is a deformation mechanism characterized by closely packed planes in a crystalline material slide across each other as dislocations pass through the material. Results from pillar compression tests of FCC micropillars show significant size scale dependence on the strength of the micropillar, with smaller micropillars being stronger [1]. Since a micropillar is likely to present less of an obstacle to the movement of dislocations, the implication is that the strength comes from the difficulty in nucleation of sufficient dislocations, combined with their easy destruction at the surface. Molecular Dynamics simulations of FCC nanopillars by Li & Yang [2] show further size scale dependence on strength of pillars on the nanometre scale. This affirms size scale effect
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