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

The aim of this review article on recent developments of mechanochemistry (nowadays established as a part of chemistry) is to provide a comprehensive overview of advances achieved in the field of atomistic processes, phase transformations, simple and multicomponent nanosystems and peculiarities of mechanochemical reactions. Industrial aspects with successful penetration into fields like materials engineering, heterogeneous catalysis and extractive metallurgy are also reviewed. The hallmarks of mechanochemistry include influencing reactivity of solids by the presence of solid-state defects, interphases and relaxation phenomena, enabling processes to take place under non-equilibrium conditions, creating a well-crystallized core of nanoparticles with disordered near-surface shell regions and performing simple dry time-convenient one-step syntheses. Underlying these hallmarks are technological consequences like preparing new nanomaterials with the desired properties or producing these materials in a reproducible way with high yield and under simple and easy operating conditions. The last but not least hallmark is enabling work under environmentally friendly and essentially waste-free conditions (822 references).

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Hallmarks of mechanochemistry: From nanoparticles to technology

3D printing of Aluminium alloys: Additive Manufacturing of Aluminium alloys using selective laser melting

A review on fundamental of high entropy alloys with promising high–temperature properties

Defect-interface interactions

The precipitate nanostructure and the strength of an Al-0.055Sc-0.005Er-0.02Zr at% alloy with Si additions, in the range 0–0.18 at%, were investigated utilizing micro-hardness, electrical conductivity, scanning electron microscopy and atom-probe tomography techniques. Si-containing alloys are cost-effective due to the existence of Si in commercial purity Al. In all studied alloys, homogenization for at least 0.5 h at 640 °C is needed to eliminate Al 3 Er primary precipitates. Alloys containing the higher Si concentrations achieve higher microhardness by increasing the heterogeneous nucleation current of (Al, Si) 3 (Sc, Zr, Er) precipitates. The alloy containing 0.18 at% Si achieves an 60% improvement in peak-microhardness compared to the Si-free alloy, during isothermal aging at 400 °C. Silicon additions reduce the peak-aging time in the temperature range 300–400 °C, indicating that the Er and Sc diffusion kinetics are accelerated. Silicon also enhance the Zr diffusion kinetics, accelerating precipitate growth during aging at 300 °C and precipitate coarsening at 400 °C. Addition of Si modifies the concentration profiles within the nanoprecipitates, enhancing the chemical homogeneity of Sc and Er in their cores, rather than forming Er-enriched-cores/Sc-enriched-shells that we have observed in prior research. Finally, the microhardness of the alloys, containing 0.12 and 0.18 at% Si, only diminishes slightly from the peak values after isothermal aging at 375 °C for about 2000 h, suggesting that the studied alloys can be practically utilized at this operating temperature.

Role of silicon in the precipitation kinetics of dilute Al-Sc-Er-Zr alloys

Contrary to the often reported findings from molecular dynamics computer simulation that metals soften as their grain sizes fall below 10--15 nm, we do not observe such softening in nanocrystalline specimens when they are first thermally relaxed. We offer a simple model that illustrates that the increased hardening is a consequence of grain-boundary relaxation, which suppresses grain-boundary sliding and forces the material to deform by dislocation glide. These observations provide an explanation for why some experiments observe an inverse Hall-Petch relationship at grain sizes below 10--20 nm while others do not.

Limits of hardness at the nanoscale: Molecular dynamics simulations

Atom probe tomographic study of a friction-stir-processed Al–Mg–Sc alloy

Yield strength in nanocrystalline Cu during high strain rate deformation

The fundamental processes of shear-induced chemical mixing in heterogeneous Cu-based alloy systems have been studied by molecular dynamics computer simulations. These simulations reveal that two very disparate mechanisms operate depending on whether or not the two phases are coherent. For the coherent systems, mixing occurs as dislocations transfer across phase boundaries. The mixing in these systems is “superdiffusive,” and for spherical precipitates, the rate of mixing increases quadratically with precipitate radius. In systems that have incoherent phases, the mixing occurs by a local shuffling of atoms at the interface, and for them, the mixing is diffusive, with the mixing rates of spherical precipitates scaling linearly with particle radius. The morphologies of the interfaces for the two situations are also different. Coherent precipitates form rough interfaces that are relatively sharp, whereas the interfaces of incoherent precipitates are smooth but diffuse. These simulations also show that for incoherent precipitates, shear-induced mixing can be very different at different crystallographic interfacial planes as well as for different strain directions.

Nhon Q. Vo

Papers

Role of silicon in the precipitation kinetics of dilute Al-Sc-Er-Zr alloys

Limits of hardness at the nanoscale: Molecular dynamics simulations

Atom probe tomographic study of a friction-stir-processed Al–Mg–Sc alloy

Yield strength in nanocrystalline Cu during high strain rate deformation

Atomic Mixing in Metals Under Shear Deformation