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

Nanocrystals are fundamental to modern science and technology. Mastery over the shape of a nanocrystal enables control of its properties and enhancement of its usefulness for a given application. Our aim is to present a comprehensive review of current research activities that center on the shape-controlled synthesis of metal nanocrystals. We begin with a brief introduction to nucleation and growth within the context of metal nanocrystal synthesis, followed by a discussion of the possible shapes that a metal nanocrystal might take under different conditions. We then focus on a variety of experimental parameters that have been explored to manipulate the nucleation and growth of metal nanocrystals in solution-phase syntheses in an effort to generate specific shapes. We then elaborate on these approaches by selecting examples in which there is already reasonable understanding for the observed shape control or at least the protocols have proven to be reproducible and controllable. Finally, we highlight a number of applications that have been enabled and/or enhanced by the shape-controlled synthesis of metal nanocrystals. We conclude this article with personal perspectives on the directions toward which future research in this field might take.

Shape‐Controlled Synthesis of Metal Nanocrystals: Simple Chemistry Meets Complex Physics?

Mechanical properties of nanocrystalline materials

Principles of equal-channel angular pressing as a processing tool for grain refinement

인공고관절의 세라믹 볼 헤드의 기계적 안정성 평가를 위한 3 차원 유한요소 해석

The search for deformation mechanisms in nanocrystalline metals has profited from the use of molecular dynamics calculations. These simulations have revealed two possible mechanisms; grain boundary accommodation, and intragranular slip involving dislocation emission and absorption at grain boundaries. But the precise nature of the slip mechanism is the subject of considerable debate, and the limitations of the simulation technique need to be taken into consideration. Here we show, using molecular dynamics simulations, that the nature of slip in nanocrystalline metals cannot be described in terms of the absolute value of the stacking fault energy-a correct interpretation requires the generalized stacking fault energy curve, involving both stable and unstable stacking fault energies. The molecular dynamics technique does not at present allow for the determination of rate-limiting processes, so the use of our calculations in the interpretation of experiments has to be undertaken with care.

Stacking fault energies and slip in nanocrystalline metals

Molecular-dynamics computer simulations of a model Ni nanocrystalline sample with a mean grain size of 12 nm under uniaxial tension is reported. The microscopic view of grain-boundary sliding is addressed. Two atomic processes are distinguished in the interfaces during sliding: atomic shuffling and stress-assisted free-volume migration. The activated accommodation processes under high-stress and room-temperature conditions are grain-boundary and triple-junction migration, and dislocation activity.

Grain-boundary sliding in nanocrystalline fcc metals

Plastic deformation in coarse-grained metals is governed by dislocation-mediated processes. These processes lead to the accumulation of a residual dislocation network, producing inhomogeneous strain and an irreversible broadening of the Bragg peaks in x-ray diffraction. We show that during plastic deformation of electrodeposited nanocrystalline nickel, the peak broadening is reversible upon unloading; hence, the deformation process does not build up a residual dislocation network. The results were obtained during in situ peak profile analysis using the Swiss Light Source. This in situ technique, based on well-known peak profile analysis methods, can be used to address the relationship between microstructure and mechanical properties in nanostructured materials.

https://science.sciencemag.org/content/sci/304/5668/273.full.pdf

Plastic Deformation with Reversible Peak Broadening in Nanocrystalline Nickel

We investigate the structure and mobility of single self-interstitial atom and vacancy defects in body-centered-cubic transition metals forming groups 5B (vanadium, niobium, and tantalum) and 6B (chromium, molybdenum, and tungsten) of the Periodic Table. Density-functional calculations show that in all these metals the axially symmetric self-interstitial atom configuration has the lowest formation energy. In chromium, the difference between the energies of the and the self-interstitial configurations is very small, making the two structures almost degenerate. Local densities of states for the atoms forming the core of crowdion configurations exhibit systematic widening of the "local" d band and an upward shift of the antibonding peak. Using the information provided by electronic structure calculations, we derive a family of Finnis-Sinclair-type interatomic potentials for vanadium, niobium, tantalum, molybdenum, and tungsten. Using these potentials, we investigate the thermally activated migration of self-interstitial atom defects in tungsten. We rationalize the results of simulations using analytical solutions of the multistring Frenkel-Kontorova model describing nonlinear elastic interactions between a defect and phonon excitations. We find that the discreteness of the crystal lattice plays a dominant part in the picture of mobility of defects. We are also able to explain the origin of the non-Arrhenius diffusion of crowdions and to show that at elevated temperatures the diffusion coefficient varies linearly as a function of absolute temperature.

/pdf/multiscale-modeling-of-crowdion-and-vacancy-defects-in-body-27qiktmh1j.pdf

Multiscale modeling of crowdion and vacancy defects in body-centered-cubic transition metals

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.

Peter M. Derlet

Papers

Stacking fault energies and slip in nanocrystalline metals

Grain-boundary sliding in nanocrystalline fcc metals

Plastic Deformation with Reversible Peak Broadening in Nanocrystalline Nickel

Multiscale modeling of crowdion and vacancy defects in body-centered-cubic transition metals

Atomic mechanism for dislocation emission from nanosized grain boundaries