Analysis of a one-billion atom simulation of work-hardening in ductile materials
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Analysis of a one-billion atom simulation of work-hardening in ductile materials Markus J. Buehler1, Alexander Hartmaier1, Mark Duchaineau2, Farid F. Abraham3 and Huajian Gao1 1 Max Planck Institute for Metals Research, 70569 Stuttgart, Germany 2 Lawrence Livermore National Laboratory, Livermore, CA 94550-9234, USA. 3 IBM Almaden Research Center, San Jose, CA 95120-6099, USA ABSTRACT We analyze a large-scale molecular dynamics simulation of work hardening in a ductile model material comprising of 500 million atoms interacting with a Lennard-Jones pair potential within a classical molecular dynamics scheme. With tensile loading, we observe emission of thousands of dislocations from two sharp cracks. The dislocations interact in a complex way, revealing three fundamental mechanisms of work-hardening. These are (1) dislocation cutting processes, jog formation and generation of point defects; (2) activation of secondary slip systems by cross-slip; and (3) formation of sessile Lomer-Cottrell locks. The dislocations self-organize into a complex sessile defect topology. Our analysis illustrates mechanisms formerly only known from textbooks and observed indirectly in experiment. It is the first time that such a rich set of fundamental phenomena has been seen in a single computer simulation. INTRODUCTION The plastic or non-reversible deformation of materials occurs immediately after a regime of recoverable elastic deformation and is governed by the nucleation and motion of defects in the crystal lattice [1-3]. In experiment, scientists often rely on indirect techniques to investigate the crystallographic structure of defects. In theory, predictions are primarily based on continuum theory with phenomenological assumptions. While the continuum description has been very successful in the past, some key features of plasticity can only be understood when the atomistic viewpoint is adapted. This may be achieved using molecular dynamics (MD) simulation [4-9]. However, most MD studies consider a small number of dislocations and are limited to specific dislocation mechanisms [9, 10]. Here we present the analysis of ultra-large scale simulations of work hardening in system sizes of 500,000,000 and 1,000,000,000 atoms in a rare-gas atom FCC crystal modelled by a LJ potential fitted to copper within a classical MD framework. Figure 1: Simulation geometry (a), lattice orientation (b) and timesequence of the workhardening simulation (c). The snapshots present an overview over the total simulation and show build-up of a complex, entangled dislocation network.
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MODELING AND ANALYSIS TECHNIQUES The simulation geometry, lattice orientation and coordinate system are presented in Fig. 1a and b. We apply tensile mode I loading in the x-direction as indicated in the figure. Dislocations are visualized based on the energy method, that is, only atoms with energy a few percent higher than the bulk are displayed. An alternative method we use is the centrosymmetry method that allows distinguishing stacking faults in addition. Both tec
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