There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

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

J. Appl. Cryst.の発刊に際して

Microstructures and properties of high-entropy alloys

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

Mechanical properties of bulk metallic glasses

Shock compression of powder mixtures can lead to chemical reactions, resulting in the formation of equilibrium as well as nonequilibrium compounds, and rapid increases in temperature. The reactions occur as manifestations of enhanced solid‐state chemical reactivity of powders, caused by configurational changes and defect states introduced during shock compression. Two types of reactions are possible and can be distinguished on the basis of their respective process mechanisms and kinetics. Shock‐induced chemical reactions occur during the shock‐compression state, before unloading to ambient pressure, and in time scales of mechanical equilibrium. In contrast, shock‐assisted reactions occur after unloading to ambient pressure, in an essentially shock‐modified material, in time scales of temperature equilibration. The mechanisms of shock‐assisted reactions include solid‐state defect‐enhanced diffusional processes. Shock‐induced reactions, on the other hand, require mechanisms different from conventional solid‐state nucleation and growth processes. The complex nature of deformation of powders has precluded a detailed understanding of the reaction mechanisms of such high‐rate reaction processes. Results of controlled experiments, however, suggest that shock‐induced chemical reactions involve nondiffusional processes giving rise to mechanochemical effects and solid‐state structural rearrangements. Mechanistic concepts that distinguish between shock‐induced and shock‐assisted chemical reactions are described. The effects of configurational changes introduced during shock compression, and the influence of material properties and shock‐loading characteristics on such effects, are analyzed to identify the mechanisms of complex processes leading to chemical reaction initiation and compound formation.

Shock-induced and shock-assisted solid-state chemical reactions in powder mixtures

Shock-induced chemical reactions and synthesis of materials

http://meyersgroup.ucsd.edu/papers/journals/Meyers%20297.pdf

High-strain-rate response of ultra-fine-grained copper

Half-Heusler alloys with the general formula TiNiSn1−xSbx are currently being investigated for their potential as thermoelectric (TE) materials. A systematic investigation of the effect of Sb doping on the Sn site and Zr doping on the Ti site on the electrical and thermal transport of the TiNiSn system has been performed. Unexpectedly, lattice thermal conductivity κL appears to increase somewhat randomly with small amounts (x<5%) of Sb doping. Subsequently, an investigation of grain structure in these Sb-doped materials has been found to correlate with the anomalous behavior of κL. Furthermore, effects of submicron grain sizes on κL in ball milled and shock compressed samples are also presented.

Naresh N. Thadhani

Papers

Mechanical properties of bulk metallic glasses

Shock-induced and shock-assisted solid-state chemical reactions in powder mixtures

Shock-induced chemical reactions and synthesis of materials

High-strain-rate response of ultra-fine-grained copper

Grain structure effects on the lattice thermal conductivity of Ti-based half-Heusler alloys