Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

Fast parallel algorithms for short-range molecular dynamics

Mechanical properties of nanocrystalline materials

http://nanomechanics.mit.edu/sites/default/files/documents/Kumar_03.pdf

Mechanical behavior of nanocrystalline metals and alloys

Plasticity in small-sized metallic systems: Intrinsic versus extrinsic size effect

Nano-grained (NG) metals are believed to be strong but intrinsically brittle: Free-standing NG metals usually exhibit a tensile uniform elongation of a few percent. When a NG copper film is confined by a coarse-grained (CG) copper substrate with a gradient grain-size transition, tensile plasticity can be achieved in the NG film where strain localization is suppressed. The gradient NG film exhibits a 10 times higher yield strength and a tensile plasticity comparable to that of the CG substrate and can sustain a tensile true strain exceeding 100% without cracking. A mechanically driven grain boundary migration process with a substantial concomitant grain growth dominates plastic deformation of the gradient NG structure. The extraordinary intrinsic plasticity of gradient NG structures offers their potential for use as advanced coatings of bulk materials.

https://science.sciencemag.org/content/sci/early/2011/02/16/science.1200177.full.pdf

Revealing Extraordinary Intrinsic Tensile Plasticity in Gradient Nano-Grained Copper

The present work deals with the atomic mechanism responsible for the emission of partial dislocations from grain boundaries (GB's) in nanocrystalline metals. It is shown that in 12 and 20 nm grain size samples GB's containing GB dislocations can emit a partial dislocation during deformation by local atomic shuffling and stress-assisted free volume migration. The free volume is often emitted or absorbed in a neighboring triple junction. It is further suggested that the degree of delocalization surrounding the grain boundary dislocation determines whether atomic shuffling can associate displacements into the Burgers vector necessary to emit a partial dislocation. Temporal analysis of atomic configurations during dislocation emission indicates that creation and propagation of the partial might be separate processes.

Atomic mechanism for dislocation emission from nanosized grain boundaries

Tensile experiments of fully dense nanocrystalline structures with a mean grain size of less than 100 nanometers demonstrate a considerable increase in hardness but a remarkable drop in elongation-to-failure, indicating brittle behavior. However, dimple structures are often observed at the fracture surface, indicating some type of ductile fracture mechanism. Guided by large-scale atomistic simulations, we propose that these dimple structures result from local shear planes formed around clustered grains that, because of their particular misorientation, cannot participate in the grain boundary accommodation processes necessary to sustain plastic deformation. This raises the expectation that general high-angle grain boundaries are necessary for good ductility.

https://science.sciencemag.org/content/sci/300/5625/1550.full.pdf

Dimples on nanocrystalline fracture surfaces as evidence for shear plane formation.

On non-equilibrium grain boundaries and their effect on thermal and mechanical behaviour: a molecular dynamics computer simulation

Interaction between dislocations and grain boundaries under an indenter - a molecular dynamics simulation

Large scale molecular dynamics (MD) simulations are used to simulate the plastic deformation of a nanocrystalline model Ni sample with an average grain size of 5 nm containing 125 grains at 800 K up to 4.0% plastic strain. For the first time in MD simulation, emerging shear planes involving several grain boundaries have been observed indicating cooperative plastic deformation activity between grains. It is found that three mechanisms have been involved in the development of such shear planes: grain-boundary migration, continuity of shear plane via intragranular slip, and rotation and coalescence of grains.

Abdellatif Hasnaoui

Papers

Atomic mechanism for dislocation emission from nanosized grain boundaries

Dimples on nanocrystalline fracture surfaces as evidence for shear plane formation.

On non-equilibrium grain boundaries and their effect on thermal and mechanical behaviour: a molecular dynamics computer simulation

Interaction between dislocations and grain boundaries under an indenter - a molecular dynamics simulation

Cooperative processes during plastic deformation in nanocrystalline fcc metals: A molecular dynamics simulation