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

Note: Times Cited: 875 Reference EPFL-ARTICLE-206025doi:10.1021/cr0501846View record in Web of Science URL: ://WOS:000249839900009 Record created on 2015-03-03, modified on 2017-05-12

Complex Hydrides for Hydrogen Storage

Materials Challenges in Nuclear Energy

Many traditional approaches for strengthening steels typically come at the expense of useful ductility, a dilemma known as strength-ductility trade-off. New metallurgical processing might offer the possibility of overcoming this. Here we report that austenitic 316L stainless steels additively manufactured via a laser powder-bed-fusion technique exhibit a combination of yield strength and tensile ductility that surpasses that of conventional 316L steels. High strength is attributed to solidification-enabled cellular structures, low-angle grain boundaries, and dislocations formed during manufacturing, while high uniform elongation correlates to a steady and progressive work-hardening mechanism regulated by a hierarchically heterogeneous microstructure, with length scales spanning nearly six orders of magnitude. In addition, solute segregation along cellular walls and low-angle grain boundaries can enhance dislocation pinning and promote twinning. This work demonstrates the potential of additive manufacturing to create alloys with unique microstructures and high performance for structural applications.

https://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=1443&context=ameslab_manuscripts

Additively manufactured hierarchical stainless steels with high strength and ductility

Handbook of SiC properties for fuel performance modeling

Cohesion properties of incoherent Fe/W interfaces: A DFT study

To study the effect of tungsten, vanadium and tantalum on the microstructures in CLAM (China Low Activation Martensitic) steel after irradiation respectively, the microstructures of Fe-M (M = V, W, Ta) model alloys were investigated after implanted deuterium ions using an ion accelerator at 773 K. After implanted deuterium ion, TEM (Transmission Electron Microscope) observation and EDX (Energy Dispersive X-ray Spectrom) analysis have been carried out. The result showed that tiny voids were observed in all model alloys after implanted the same dose of deuterium ions. The swelling rate in FeTa alloy was the smallest among the three alloys. Unlike FeW and FeV alloys, there was the segregation in FeTa alloy under a fluence of 5×1017 D+/cm2 at 773 K. A theoretical analysis showed that the void growth in FeTa alloy slowed down due to tantalum segregation near voids. It indicates that tantalum plays an important role in the improved irradiation resistance of CLAM steel.

Effect of Deuterium Ion Implantation on Microstructures of Fe-M (M =V, W, Ta) Alloys

Development and application of a window-type environmental cell in high voltage electron microscope

Duplex stainless steel has been used as a structural material for light water reactors. It is well known that the thermal aging during operation causes spinodal decomposition to Cr-rich (α') phase and Fe-rich (α) phase in the ferrite phase, resulting in brittlement. In order to understand the mechanism of this phenomena and the effect of irradiation, a duplex model alloy (Fe-25Cr-10Ni-2.5Mo-1Mn) prepared by arc melting and thermally aged in the appropriate condition was subjected to accelerated irradiation by a multi-beam ultra-high voltage electron microscope and partially an ion accelerator in this study. Spinodal decomposition of Fe and Cr was confirmed in the model alloy aged at 450 °C. In addition, the electron beam irradiation to aged model alloy resulted in the decrease in the amplitude of spinodal decomposition. From this experiment, it was suggested that spinodal decomposition could be suppressed by accelerated irradiation.

Microstructure change of duplex stainless steels after thermal aging and electron beam irradiation

Li-C-H system, which can store about 5.0 mass% of rechargeable H2, has been reported as a promising hydrogen storage system by Ichikawa et al. [Appl. Phys. Lett. 86, 241914 (2005); Mater. Trans. 46, 1757 (2005)]. This system was investigated from the thermodynamic and structural viewpoints. However, hydrogen absorption/desorption mechanism and the state of hydrogen atoms absorbed in the composite have not been clarified yet. In order to find new or better hydrogen storage system, graphite powder and nano-structural graphite ball-milled under H2 and Ar atmosphere were prepared and milled with Li and Mg under Ar atmosphere in this study. Microstructural analysis for those samples by transmission electron microscope revealed that LiC6 and/or LiC12 were formed in Li-C-H system. On the other hand, MgC2 was found in Mg-C-H system ball-milled under H2 atmosphere, but not in the system ball-milled under Ar atmosphere. These results indicated that nano-structure in composites of nano-structural graphite is different from that of alkali (-earth) metal. For these reasons, metal-C-H system can be recognized to be a new family of hydrogen storage materials.

/pdf/microscopic-characterization-of-metal-carbon-hydrogen-2sqsqgwsbf.pdf

Naoyuki Hashimoto

Papers

Cohesion properties of incoherent Fe/W interfaces: A DFT study

Effect of Deuterium Ion Implantation on Microstructures of Fe-M (M =V, W, Ta) Alloys

Development and application of a window-type environmental cell in high voltage electron microscope

Microstructure change of duplex stainless steels after thermal aging and electron beam irradiation

Microscopic characterization of metal-carbon-hydrogen composites (metal = Li, Mg)