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 original mater ia l s f rom which the alloys were prepared were commerc ia l molybdenum (99.97~c), e lectrolyt ic rhenium (99.99%) remel ted in an a rc furnace for additional purification, and iodide zirconium. Carbon was added in the form of Mo2C and spect ra l ly pure graphite. Ingots of the al loys weighing 250 g (150 mm long and 8-10 mm in diameter) were melted in a furnace with a nonconsumable tungsten electrode and water -cooled copper hearth in an a tmosphere of helium at a p ressure of 300 mm Hg.

Refractory metals and alloys.

Scandium is well known to produce grain refinement in Al-based alloys, and its potency is generally attributed to intermetallic ${\mathrm{Al}}_{3}\mathrm{Sc}$ formation within liquid phase. However, the influence of Sc atoms and ${\mathrm{Al}}_{3}\mathrm{Sc}$ on the local structure of the surrounding melt, and subsequent nucleation remains unclear. Towards that end, we have probed structural changes in three bulk compositions, i.e., $\mathrm{Al}\text{\ensuremath{-}}x\mathrm{Sc}$ $(x=0, 0.4, 1.0 \mathrm{at}.%)$, and near liquid-$\mathrm{Al}/{\mathrm{Al}}_{3}\mathrm{Sc}$ interfacial regions using ab initio molecular dynamics. In-liquid ordering was determined using unsupervised learning techniques, i.e., structural fingerprinting, dimensionality reduction, and cluster analysis. Sc atoms ordered the surrounding liquid Al atoms by forming Sc-centered polyhedrons, while liquid-$\mathrm{Al}/{\mathrm{Al}}_{3}\mathrm{Sc}$ interface manifested planar ordering that resembled ${{100}}_{fcc\ensuremath{-}\mathrm{Al}}$. Both structures were geometrically persistent but constitutionally transient, i.e., they exchanged Al atoms with the surrounding liquid. This behavior was rationalized on the basis of their mixed metallic and covalent bond character. At a lower temperature, ${{100}}_{fcc\ensuremath{-}\mathrm{Al}}$ interfacial planes heteroepitaxially nucleated equilibrium $fcc\text{\ensuremath{-}}\mathrm{Al}$, while Sc-centered polyhedrons sequentially formed metastable $hcp$- and $bcc\text{\ensuremath{-}}\mathrm{Al}$. Using our simulations and extant experimental reports, we postulate two transformation pathways during Al-Sc solidification: (i) $\mathrm{Sc}\ensuremath{-}centered\phantom{\rule{4pt}{0ex}}polyhedrons\ensuremath{\rightarrow}{\mathrm{Al}}_{3}\mathrm{Sc}\ensuremath{\rightarrow}liquid\ensuremath{-}Al/A{l}_{3}Sc\phantom{\rule{4pt}{0ex}}interfacial\phantom{\rule{4pt}{0ex}}ordering\ensuremath{\rightarrow}fcc\ensuremath{-}\mathrm{Al}$; and (ii) $\mathrm{Sc}\ensuremath{-}centered\phantom{\rule{4pt}{0ex}}polyhedrons\ensuremath{\rightarrow}hcp\ensuremath{-}\mathrm{Al}\ensuremath{\rightarrow}bcc\ensuremath{-}\mathrm{Al}\ensuremath{\rightarrow}fcc\ensuremath{-}\mathrm{Al}$. The two in-liquid ordered structures provide an atomistic basis for the potency of Sc element, and, also, serve as possible structural metrics for designing novel Al-based alloys. 

Investigation of in-liquid ordering mediated transformations in Al-Sc via 
ab initio
 molecular dynamics and unsupervised learning

Abstract Thermo-resistant nanoalloys are a new class of materials that combine high-temperature refractory compounds (such as carbides, nitrides, borides, and oxides) with nanoscale particles of metals, ceramics, or carbon. These composites exhibit remarkable thermal stability and anti-ablation/oxidation properties, making them highly attractive for various high-temperature applications in aerospace, energy, and high-temperature manufacturing. Despite their potential, the fabrication of these materials is challenging due to their complex synthesis and processing. Many researchers have summarized the challenges and suggested solutions to produce high-density, superior physicochemical properties of nano refractory materials for specific applications. Thus, in view of these perspectives, the present review provides an overview of the production criteria, processing, and synthetic routes for producing high-temperature nano refractory material composites with exceptional thermal and anti-ablation/oxidation properties. The review also highlights the challenges encountered by researchers and their solutions for fabricating these materials. Potential applications of high temperature refractory materials are found in various industries, such as refractory ceramics, high-temperature components in wear resistant, neutron shielding, and high power-density microelectronics manufacturing to name a few.

/pdf/current-status-of-synthesis-and-consolidation-strategies-for-2pqm3hr7.pdf

Current status of synthesis and consolidation strategies for thermo-resistant nanoalloys and their general applications

New-generation structural materials with superior properties are a constant demand in applications involving extreme environments. Here, we demonstrate the fabrication of a high-strength, high-dense W-TaC-Ta2O5 nanocomposite for such applications on a large scale by a simple, cost-effective, scalable, bottom-up powder metallurgy approach using plasma sintering. The first clear microstructural evidence of the scavenging effect of carbide particles in the W-MC composites (M = Ta, Zr, Hf, Ti) is demonstrated through atom probe studies. Localized plastic deformation and the unique stress-induced amorphization in tungsten are observed due to dislocation activities, and these phenomena are corroborated by molecular dynamics (MD) simulations. Optimized composition and processing conditions yield high Vickers hardness ~540 HV10 and super-high transverse rupture strength (TRS) ~ 1650 MPa, in upscaled components of 100 mm diameter. The enhanced mechanical properties are attributed to the cumulative effect of the grain boundary strengthening and dispersion strengthening from the refined tungsten grains and the second phase intragranular nanocrystalline particles, respectively, the coherent particle-matrix interfaces, the low oxygen-segregation at grain boundaries and the ‘dual nanocrystalline-amorphous’ microstructure present in the matrix.

Ultrahigh transverse rupture strength in tungsten-based nanocomposites with minimal lattice misfit and dual microstructure

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Ultrahigh transverse rupture strength in tungsten-based nanocomposites with minimal lattice misfit and dual microstructure