Multiscale simulation of onset plasticity during nanoindentation of Al (001) surface

School of Engineering and Materials Science, Queen Mary, University of London, London E1 4NS, UK
Acta Materialia (Impact Factor: 4.47). 09/2008; 56(16):4358-4368. DOI: 10.1016/j.actamat.2008.04.064


The onset of plasticity in crystalline materials is important to the fundamental understanding of plastic deformation and the development of precision devices. Dislocation nucleation and interactions at the onset of plasticity are investigated here using a multiscale quasi-continuum (QC) method for the nanoindentation of the (0 0 1) surface of a single crystal aluminium (Al) of 200 × 100 nm2 with infinite thickness. Deformation twinning was noted to occur during the nanoindentation of Al. We used unrelaxed and relaxed QC simulations with three embedded atom potentials of Al to evaluate the generalized planar fault (GPF) energies. The energy barrier for initial dislocation nucleation is much higher than that for subsequent nucleation events adjacent to the pre-existing defect. This mechanism promotes deformation twinning when some of the available slip systems are constrained. Dislocation initiation causes a minor load drop in the load–displacement curve, whereas major displacement excursion from experimental observations is the result of collective dislocation activities. (Some figures in this article are in color only in the on-line version.)

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Available from: Z. Xiao Guo, Nov 09, 2014
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    • "[2] For example, deformation twinning has been frequently observed even in NC face-centered cubic (fcc) materials that do not deform by twinning in their coarse-grained counterparts.[2] [3] Formation of twins in NC metals [2] [4] [5] plays a critical role in their physical and mechanical properties, such as good electrical conductivity, and excellent resistance to currentinduced diffusion.[6] [7] The interactions between twins and gliding dislocations at twin boundaries (TBs) have been observed both experimentally [7] [8] [9] [10] [11] [12] [13] [14] and by molecular dynamics (MD) simulations [15] [16] [17] [18] to result in an unusual combination of ultrahigh strength and high ductility.[7,9,19–24] In NC materials, MD simulations [25] [26] [27] have predicted that single or multiple deformation twins can be formed by emission of Shockley partial dislocations on adjacent {111} planes from grain boundaries . "
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    ABSTRACT: Atomistic simulations are used to investigate how the nanoindentation mechanism influences dislocation nucleation under molecular dynamic behavior on the aluminum (0 0 1) surface. The characteristics of molecular dynamics in terms of various nucleation criteria are explored, including various molecular models, a multi-step load/unload cycle, deformation mechanism of atoms, tilt angle of the inden-ter, and slip vectors. Simulation results show that both the plastic energy and the adhesive force increase with increasing nanoindentation depths. The maximum forces for all indentation depths decrease with increasing multi-step load/unload cycle time. Dislocation nucleation, gliding, and interaction occur along Shockley partials on (1 1 1) slip planes. The indentation force applied along the normal direction, a tilt angle of 0°, is smaller than the force component that acts on the surface atoms. The corresponding slip vector of the atoms in the (1 1 1) plane has low-energy sessile stair-rod dislocations in the pyramid of intrinsic stacking faults.
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