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 alloying (MA) is a solid-state powder processng technique involving repeated welding, fracturing, and rewelding of powder particles in a high-energy ball mill. Originally developed to produce oxide-dispersion strengthened (ODS) nickel- and iron-base superalloys for applications in the aerospace industry, MA has now been shown to be capable of synthesizing a variety of equilibrium and non-equilibrium alloy phases starting from blended elemental or prealloyed powders. The non-equilibrium phases synthesized include supersaturated solid solutions, metastable crystalline and quasicrystalline phases, nanostructures, and amorphous alloys. Recent advances in these areas and also on disordering of ordered intermetallics and mechanochemical synthesis of materials have been critically reviewed after discussing the process and process variables involved in MA. The often vexing problem of powder contamination has been analyzed and methods have been suggested to avoid/minimize it. The present understanding of the modeling of the MA process has also been discussed. The present and potential applications of MA are described. Wherever possible, comparisons have been made on the product phases obtained by MA with those of rapid solidification processing, another non-equilibrium processing technique.

Mechanical Alloying And Milling

Nanocrystalline metals--with grain sizes of less than 100 nm--have strengths exceeding those of coarse-grained and even alloyed metals, and are thus expected to have many applications. For example, pure nanocrystalline Cu (refs 1-7) has a yield strength in excess of 400 MPa, which is six times higher than that of coarse-grained Cu. But nanocrystalline materials often exhibit low tensile ductility at room temperature, which limits their practical utility. The elongation to failure is typically less than a few per cent; the regime of uniform deformation is even smaller. Here we describe a thermomechanical treatment of Cu that results in a bimodal grain size distribution, with micrometre-sized grains embedded inside a matrix of nanocrystalline and ultrafine (<300 nm) grains. The matrix grains impart high strength, as expected from an extrapolation of the Hall-Petch relationship. Meanwhile, the inhomogeneous microstructure induces strain hardening mechanisms that stabilize the tensile deformation, leading to a high tensile ductility--65% elongation to failure, and 30% uniform elongation. We expect that these results will have implications in the development of tough nanostructured metals for forming operations and high-performance structural applications including microelectromechanical and biomedical systems.

/pdf/high-tensile-ductility-in-a-nanostructured-metal-siwm61h9tg.pdf

High tensile ductility in a nanostructured metal.

The isolation of graphene in 2004 from graphite was a defining moment for the “birth” of a field: two-dimensional (2D) materials In recent years, there has been a rapidly increasing number of papers focusing on non-graphene layered materials, including transition-metal dichalcogenides (TMDs), because of the new properties and applications that emerge upon 2D confinement Here, we review significant recent advances and important new developments in 2D materials “beyond graphene” We provide insight into the theoretical modeling and understanding of the van der Waals (vdW) forces that hold together the 2D layers in bulk solids, as well as their excitonic properties and growth morphologies Additionally, we highlight recent breakthroughs in TMD synthesis and characterization and discuss the newest families of 2D materials, including monoelement 2D materials (ie, silicene, phosphorene, etc) and transition metal carbide- and carbon nitride-based MXenes We then discuss the doping and functionalization of 2

Recent Advances in Two-Dimensional Materials beyond Graphene

Interpretation of X-ray Powder Diffraction Patterns . By H. Lipson and H. Steeple. Pp. viii + 335 + 3 plates. (Mac-millan: London; St Martins Press: New York, May 1970.) £4.

X-Ray Diffraction

The microstructures of La0.7Ca0.3MnO3 (LCMO) and La1.0Nd0.2Sr1.8Mn2O7 (LNSMO) bulk materials, LCMO thin films prepared by different methods, and LCMO/Gd0.7Ca0.3MnO3 (GCMO) multilayers have been studied by high-resolution electron microscopy. Twins and microdomains in the LCMO bulk materials as well as the intergrowth structure, ( 10 1) crystal shear Structure, and a complex defect structure in LNSMO bulk materials were observed. The displacement of oxygen ion in LCMO films grown on LaAlO3 (LAO) by pulsed laser deposition (PLD) method induces the structure transformation from orthorhombic to monoclinic. The evolution of strain in LCMO films prepared by reactive sputtering (RS) method on both LAO and SrTiO3 substrates was studied as a function of film thickness. The distribution and size of oriented domains observed in LCMO films prepared by RS were different from those prepared by PLD. The inner strains exist in LCMO and GCMO layers, which might result in lower T, and larger resistivity maximum in LCMO/GCMO multilayers.

Characterization of the microstructure in CMR materials by HREM

Crystallization of the Bi1.8Pb0.2Sr2Ca2Cu3Oz (BPSCCO) glass has been investigated by transmission electron microscopy in the temperature range from 500-degrees-C to 860-degrees-C. Experimental results show that the microstructure of the annealed BPSCCO glass is closely determined by the annealing temperatures. The superconducting phases are block-shaped when the BPSCCO glass is annealed at a temperature lower than 600-degrees-C, while the superconducting phases adopt a lamella like shape, with length/width ratio of about 4-5, above 600-degrees-C. The 2212 phase exists from 600-degrees-C to 860-degrees-C, but the 2223 is stable only in the narrow range between 800-degrees-C and 850-degrees-C. High-resolution lattice images revealed that the superconducting phases formed via the glass process are composed of several narrow lamellae with a common c-axis, and there are no secondary phases at the interfaces between the lamellae. It has also been found that the crystal defects can be produced through intergrowth of the 2212 and 2201 phases.

Study of the crystallization process of bi1.8pb0.2sr2ca2cu3oz glass using transmission electron-microscopy

Characterization of atomic configuration on an atomic scale is required for the development of advanced materials, as well as chemical compositions and electronic structures. In this paper, by using FEGTEM, studies of phase transformations and synthetic characterization of precipitated phase were carried out on a nanometer/an atomic scale in TiAl intermetallics and Cu-AlNi shape memory alloy, respectively. In TiAl intermetallics, a deformation-induced phase transformation at room temperature was observed on the several-ten-nanometer region in the Ti50Al48Cr2 alloy. It is a diffusionless phase transformation and a Ti3Al alpha(2) phase is formed as a metastable phase. On the other hand, a Ti2Al zeta phase was identified in another Al-rich TiAl with a composition of Ti48.3Al51.7 It was formed during the precipitation of Ti3Al alpha(2) plate in the TiAl gamma matrix. It was found that this precipitation was achieved through a two-step transition and Ti2Al zeta is an intermediate phase in the phase transformation. Characterization of chi particles was carried out in the shape memory Cu-Al-Ni alloy containing small amount of Ti and Mn. Many small precipitates of 18-100 nm in size were formed in the chi(L) phase. The composition of chi L phase was (NiAl)(2)CuTi, while the small precipitates were almost Cu-3(Al,Ni,Ti,Mn).

HREM observation and compositional study of microstructure and phase transformation in TiAl-based and Cu-Al-Ni alloys

STM observation of oxidation morphology of HOPG and boron-doped HOPG

Correction of aberration for a high-resolution electron hologram by means of the amplitude contrast criterion of image wave.

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