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

The Varying Temperature Irradiation Experiment in the High Flux Isotope Reactor (HFIR) was carried out in the framework of the Japan-USA fusion cooperation program (JUPITER). The objective of the experiment is to investigate the performance of fusion structural materials subject to temperature variation during operation. The experiment will also contribute to re-examining the available fission reactor irradiation data including temperature transient. The irradiation for the experiment was performed from July 1998 to May 1999. Some results of the Post-Irradiation Examination are now available. This paper summarizes the highlights of the experimental results and insights obtained through the experiment about the temperature variation effects on materials performance under irradiation.

Summary of the Varying Temperature Irradiation Experiment in HFIR

When the severe accident or some crucial troubles happen in nuclear power plants, the stop of coolant system and the increase in temperature to far exceeding from the normal operating condition could be predicted. The sudden increase in temperature would change microstructure of structural materials. In order to investigate the influence of annealing on microstructure development, in-situ electron irradiation experiment was carried out in 316L model alloy. Electron irradiation at 288 °C resulted in the formation of black dots, Flank loops and cavities in 316L model alloy. Black dots and Frank loops formed in the early stage of electron-irradiation and transferred to Flank loops and perfect loops, respectively. Annealing experiment at 500 °C for 120 min indicated that the number density and the mean size of Frank loops decreased after annealing. In addition, the mean size of cavities decreased, while the number density seemed not to be changed after annealing. Comparing the size distribution change before and after annealing experiment, it is suggested that Frank loops could be shrunk as a result of absorbing vacancies released from cavities during annealing.

Microstructure development during annealing in electron-irradiated 316L model alloy

Mg–Nb2O5 evaporated composites, which were prepared by evaporating Mg thermally on single crystals of Nb2O5, were investigated in this work. We attempted to hydrogenate the sample under 5 bar H2 atmosphere at 250 °C for 2 h. The electron microscope was used to observe the microstructure of the as-prepared and hydrogenated samples. It was found that the phase transition occurred along the particular orientation where Mg(002) is parallel to MgH2(101) or MgO(200). Density functional theory calculations were carried out on MgH2 and MgO slabs, showing that MgH2(101) and MgO(200) have the lowest surface-formation energy. On the basis of the observation and calculation, the phase-transition process of Mg during hydrogenation and oxidation was discussed. Finally, a structural model including Mg–Mg distance adjustment and layer shift was proposed to demonstrate the phase-transition process.

Phase Transition of Mg during Hydrogenation of Mg–Nb2O5 Evaporated Composites

In-situ observations of ferritic/martensitic steels by electron irradiation with a 1.25MeV high voltage microscope at 573K were carried out to study damage evolution in the steels. The development of interstitial type loops and cavities in both of the two steels, F82H-IEA and F82HODS, showed smaller and more numerous defects in the ODS steel. The cavities were formed preferentially at the interface between oxide particles and matrix. The results suggest that ODS particles may function to suppress the nucleation and growth of loops and cavities arising from irradiation. The effect of pre-implanted helium was also studied. The pre-implanted helium led to a homogenous distribution of black dots and cavities in the steels, and these may act as sinks for point defects arising from irradiation, causing a suppression of the subsequent growth of loops and cavities. The hardening corresponding to the microstructural evolution was estimated by assuming parameters extracted from the ion irradiation. [doi:10.2320/matertrans.MD201318]

Effect of Oxide Particles and Pre-Implanted Helium on Defect Evolution during Electron Irradiation

The onset of plasticity during nanoindentation is a new method to investigate the irradiation damage of structural materials in fission and fusion reactors. In this paper, nanoindentation experiment was carried out to helium implanted F82H-IEA and nano-sized oxide dispersion strengthened F82H-ODS steels for studying the elastic-plastic transition at a constant loading rate. The onset of plasticity shifted after helium implantation. By a statistical thermal activation model, activation volume was extracted to discuss the strength of barrier for dislocation motion. The results reveal an increase in the pinning force and number density of effective obstacles for dislocation motion in He-implanted F82H-IEA, and a decrease in the local pinning force without changing the density of effective obstacles in He-implanted F82H-ODS.

/pdf/onset-of-plasticity-of-helium-implanted-ferritic-martensitic-3znf7p5cx3.pdf

Naoyuki Hashimoto

Papers

Summary of the Varying Temperature Irradiation Experiment in HFIR

Microstructure development during annealing in electron-irradiated 316L model alloy

Phase Transition of Mg during Hydrogenation of Mg–Nb2O5 Evaporated Composites

Effect of Oxide Particles and Pre-Implanted Helium on Defect Evolution during Electron Irradiation

Onset of plasticity of helium-implanted ferritic/martensitic steels during nanoindentation