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

抗原变异可使得多种致病微生物易于逃避宿主免疫应答。表达在感染红细胞表面的恶性疟原虫红细胞表面蛋白1（PfPMP1）与感染红细胞、内皮细胞、树突状细胞以及胎盘的单个或多个受体作用，在黏附及免疫逃避中起关键的作用。每个单倍体基因组var基因家族编码约60种成员，通过启动转录不同的var基因变异体为抗原变异提供了分子基础。

疟原虫var基因转换速率变化导致抗原变异[英]／Paul H, Robert P, Christodoulou Z, et al//Proc Natl Acad Sci U S A

Microstructures and properties of high-entropy alloys

Mechanical properties of nanocrystalline materials

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

It is well known that mechanical properties of materials are dependent on the volume of material that is deformed. This size effect has been investigated for metal nanostructures and thin films. However, to date there are no known studies of how confined geometries affect properties of BCC metals. The current investigation began with the aim of shedding new light on this open question. Because it allows for isothermal tensile tests of thin films in a temperature regime from cryogenic to elevated temperatures, the approach taken is that of bulge testing. Molybdenum was chosen as a starting material because of its insensitivity to oxidation at temperatures below 300 °C. After describing the coating procedure and the parameters associated with the coating process, the coating microstructure is presented. Qualitative results for investigation of the oxidation of Mo thin films are also presented. We then describe the method to obtain free-standing thin films. Finally, the apparatus for performing bulge tests and several stress–strain curves is presented. Due to limitations with the measurement equipment, for the current round of experiments, it was not possible to test the Molybdenum films at temperatures high enough to induce a brittle-to-ductile transition.

Investigation of the bulge test response of molybdenum thin films at room temperature and at 100 °C

Microstructural and crystallographic characterizations of electromigration induced voiding and damage in Al and Al-2% Cu interconnects are presented. Scanning electron and focussed ion beam micrographs show that extended voiding in wide lines and transgranular slit voids in near bamboo lines are the preferred failure morphologies. Electron back scattered diffraction analysis of transgranular slit failure sites show a preferred slit void orientation. Estimates of stresses required for stress assisted void growth in unpassivated interconnects are shown to be reasonably close to measured stress levels in films and interconnects. The transgranular void process is shown to be preferred over boundary voiding based on usual estimates for the variation of surface energy and random boundary energies in Al. Finally, line edge void growth into transgranular slit failures at favorably stressed and crystallographically oriented grain sites is presented as an empirical model for the observed electromigration induced failures in near bamboo interconnects lines.

/pdf/morphology-and-crystallography-of-electromigration-induced-4dw76jsuu2.pdf

Morphology and crystallography of electromigration induced transgranular slit failures in aluminum alloy interconnects

When confronted by severe geometric constraints, dislocations may respond in unforeseen ways. One example of such unexpected behavior is parallel glide in unpassivated, ultrathin (200 nm and thinner) metal films. This involves the glide of dislocations parallel to and very near the film/substrate interface, following their emission from grain boundaries. In situ transmission electron microscopy reveals that this mechanism dominates the thermomechanical behavior of ultrathin, unpassivated copper films. However, according to Schmid’s law, the biaxial film stress that evolves during thermal cycling does not generate a resolved shear stress parallel to the film/substrate interface and therefore should not drive such motion. Instead, it is proposed that the observed dislocations are generated as a result of atomic diffusion into the grain boundaries. This provides experimental support for the constrained diffusional creep model of Gao et al.[1], in which they described the diffusional exchange of atoms between the unpassivated film surface and grain boundaries at high temperatures, a process that can locally relax the film stress near those boundaries. In the grains where it is observed, parallel glide can account for the plastic strain generated within a film during thermal cycling. One feature of this mechanism at the nanoscale is that, as grain size decreases, eventually a single dislocation suffices to mediate plasticity in an entire grain during thermal cycling. Parallel glide is a new example of the interactions between dislocations and the surface/interface, which are likely to increase in importance during the persistent miniaturization of thin film geometries.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S1946427400141284

Parallel Glide: A Fundamentally Different Type of Dislocation Motion in Ultrathin Metal Films

A model is proposed for stress voiding in passivated thin film conductors. The rate limiting step is argued to be the formation of vacancies at dislocation jogs which then diffuse to void sites.

/pdf/a-simple-model-for-stress-voiding-in-passivated-thin-film-3xzkz0z7is.pdf

A simple model for stress voiding in passivated thin film conductors

The search for mathematical methods of predicting the increase in density of a metal powder during compaction has attracted numerous scientists, as it is both a technologically interesting and a scientifically challenging problem. This review outlines the way from simple curve fitting approaches to first attempts at constructing physical models which might form a rational basis for the improvement of powder compactibility or compaction procedures. The models described range from considerations of the deformation of a single spherical pore to reasoning in terms of the deformation behaviour of spherical particles in regular and - more recently - random packings. This paper will not deal with the way in which friction affects the distribution of pressure and density in a compact, nor with the effects of lubricants.

/pdf/fundamental-aspects-of-the-compaction-of-metal-powders-7zh01ga6pb.pdf

Eduard Arzt

Papers

Investigation of the bulge test response of molybdenum thin films at room temperature and at 100 °C

Morphology and crystallography of electromigration induced transgranular slit failures in aluminum alloy interconnects

Parallel Glide: A Fundamentally Different Type of Dislocation Motion in Ultrathin Metal Films

A simple model for stress voiding in passivated thin film conductors

Fundamental aspects of the compaction of metal powders