On the Atomistic Simulation of Plastic Deformation and Fracture in Crystals
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Tensile stretching of a two-dimensional model crystal was computationally studied using molecular statics simulations. Attention was directed to the atomistics of defect activities throughout the deformation history. It is shown that the incorporation of an initial point defect is able to trigger dislocation slip in a repetitive and controlled manner. The initial defect is also seen to have potential bearing on the formation of voiding damage that leads to ductile fracture of the crystal. Implications to the nanoscale mechanical behavior and its modeling are discussed. Atomistic simulations 1of defect mechanisms in crystals are best manifested by problems such as nanoindentation1–5 and crack tip processes.6–12 The localized nature of deformation field induces nucleation and motion of dislocations that can be correlated with, or at least can shed light on, experimentally observed phenomena. A simpler loading configuration, namely nominal uniaxial stretching of a crystal, however, has generated more uncertainties. The simulated overall tensile load– displacement (stress–strain) curves frequently showed a dramatic load drop upon plastic yielding, and the load remained low thereafter up to final fracture.13–19 Ductile failure resulting from the simulation was often associated with necking of the crystal down to nearly a sharp point.13,15,18 Though dislocations can be observed from the modeling, the corroboration between dislocation slip and crystal plasticity during uniaxial tension has only been vaguely treated. On the basis of comparison studies using the Lennard-Jones or Morse pair potentials and the many-body embedded atom potential,20,21 it has been conceived that, while simulations involving only pair potentials generally yield brittle behavior, ductile materials must be described with a many-body potential.16,17 This communication presents molecular statics simulation results on tensile deformation of a model crystal. The calculation is two-dimensional (2D) with a small sample size. The primary objective is to illustrate clearly the special features associated with the nanoscale simulation for the purpose of bringing up issues worthy of further investigations. In particular, we attempt to highlight the following observations resulting from the simulation. (i) With a simple modeling strategy adopted, the use of pairwise potentials is able to yield ductile response for metallic crystals under the current simulation framework. (ii) Detailed dislocation activities and overall plastic deformation and damage evolution can be correlated in an unambiguous manner. (iii) An existing point DOI: 10.1557/JMR.2004.0126 J. Mater. Res., Vol. 19, No. 4, Apr 2004
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defect in the crystal can act as a source for recurring dislocation generations. (iv) The initial defect source seemingly predetermines the location of damage initiation and final fracture of the crystal. A representative 2D atomic arrangement is shown in Fig. 1. This initial configuration represents a closepacked
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