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

We show that the indentation size effect for crystalline materials can be accurately modeled using the concept of geometrically necessary dislocations. The model leads to the following characteristic form for the depth dependence of the hardness: H H 0 1+ h ∗ h where H is the hardness for a given depth of indentation, h, H0 is the hardness in the limit of infinite depth and h ∗ is a characteristic length that depends on the shape of the indenter, the shear modulus and H0. Indentation experiments on annealed (111) copper single crystals and cold worked polycrystalline copper show that this relation is well-obeyed. We also show that this relation describes the indentation size effect observed for single crystals of silver. We use this model to derive the following law for strain gradient plasticity: ( σ σ 0 ) 2 = 1 + l χ , where σ is the effective flow stress in the presence of a gradient, σ0 is the flow stress in the absence of a gradient, χ is the effective strain gradient and l a characteristic material length scale, which is, in turn, related to the flow stress of the material in the absence of a strain gradient, l ≈ b( μ σ 0 ) 2 . For materials characterized by the power law σ 0 = σ ref e 1 n , the above law can be recast in a form with a strain-independent material length scale l. ( builtσ σ ref ) 2 = e 2 n + l χ l = b( μ σ ref ) 2 = l ( σ 0 σ ref ) 2 . This law resembles the phenomenological law developed by Fleck and Hutchinson, with their phenomenological length scale interpreted in terms of measurable material parametersbl].

Indentation size effects in crystalline materials: A law for strain gradient plasticity

The current understanding of the fundamentals of recrystallization is summarized. This includes understanding the as-deformed state. Several aspects of recrystallization are described: nucleation and growth, the development of misorientation during deformation, continuous, dynamic, and geometric dynamic recrystallization, particle effects, and texture. This article is authored by the leading experts in these areas. The subjects are discussed individually and recommendations for further study are listed in the final section.

/pdf/current-issues-in-recrystallization-a-review-12181rj4pn.pdf

Current issues in recrystallization: a review

The effect of ion beam mixing of nickel-aluminum alloys with 500 keV krypton ions has been investigated over a range of temperature, composition, ion dose, and post-irradiation thermal treatments. Samples were formed by alternate evaporation of layers of aluminum and nickel. A portion of these samples was subsequently annealed to form intermetallic compounds. Irradiations were performed at both room temperature and 80/sup 0/K using the 2MV ion accelerator at Argonne National Laboratory. Phase transformations were observed during both in situ irradiations in the High Voltage Electron Microscope (HVEM) at Argonne, and also in subsequent analysis of an array of irradiated samples. Electron diffraction indicates the presence of metastable crystalline structures not present in the conventional nickel-aluminum phase diagram. Transformations occur at doses as low as 5 x 10/sup 14/ cm/sup -2/ and continue to develop as the irradiation progresses up to 2 x 10/sup 16/ cm/sup -2/. Layer mixing is followed through Rutherford Backscattering analysis. Samples are also checked with x-rays and Electron Energy Loss Spectroscopy (EELS). A thermodynamic argument is presented to explain the phase transformations in terms of movements on a free energy diagram. This analysis explains the interesting paradox concerning the radiation hardness of the NiAl phasemore » and the amorphous structure of mixed Ni-50% Al layers.« less

/pdf/phase-transformations-in-nickel-aluminum-alloys-during-ion-50zeimrvgq.pdf

Phase Transformations in Nickel-Aluminum Alloys During Ion Beam Mixing

Statistical determination of stress threshold and survival analysis on IASCC initiation data of stainless steels in pressurized water reactor conditions

Films of Mo, MoSix and Mo/MoSix (1.22<x<1.35) multilayers were formed by physical vapor deposition (PVD) and ion beam assisted deposition (IBAD) onto (100) Si, glass and graphite substrates. Ion to atom arrival rate (R) ratios for IBAD varied from 0.01 to 0.1 and film thicknesses varied from 200 to 1100 nm. The Si/Mo ratio decreased with increasing R ratio. The oxygen content of Mo films was greater than silicide films, but both decreased substantially with increasing R ratio. Ar incorporation increased with increasing R ratio to a maximum of 1 at% in Mo and 5 at% in MoSi1.22. Mo films exhibit a strong (110) fiber texture at low R ratios. At the highest R ratio, a tilting of the (110) fiber texture by 15° occurs, along with the development of a distinct azimuthal texture indicative of planar channeling of the ion beam along (110) planes. The microstructure of the multilayer consists of small Mo grains and an amorphous silicide. Average film stress in Mo films increases from tension to a maximum value of 0.63 GPa and becomes compressive with increasing normalized energy. The stress in the MoSix films decreases with increasing normalized energy and saturates at a compressive stress of -0.24 GPa at 25 eV/atom. Indentation fracture experiments using a Vickers indenter with a 300 g load show a fracture behavior that is consistent with a residual stress effect for the IBAD monolithic MoSix and microlaminate, but which is influenced by additional factors in the PVD microlaminate.

Synthesis and Properties of Mo/MoSix Microlaminates Using Ion Beam Assisted Deposition

During stress corrosion crack propagation, stress values in the crack tip vicinity often exceed material yield stress limit. Plastic deformation processes may accompany and influence cracking. Here, stress corrosion crack propagation and deformation mechanisms were investigated using EBSD analysis. The investigated material was 304L Ti-enriched austenitic stainless steel irradiated to 10.4 dpa at 320°C in the BOR-60 fast reactor. Crack growth tests were conducted in a simulated Normal Water Chemistry (NWC) environment in the temperature range 288–320 °C using compact tension specimens. By analyzing crack trajectory and grain structure in the crack vicinity, it was established that grain orientation with respect to the acting stress direction was not a key factor controlling crack propagation. No crystallographic orientation susceptible to cracking was identified. Also, EBSD analysis revealed strong inhomogeneity in plastic strain distribution along the crack path. Most crack-adjacent grains remained virtually strain-free whereas few grains experienced strong plastic strain. These areas were presumed to be “plastic bridges” or “ductile ligaments.”

Plastic Deformation Processes Accompanying Stress Corrosion Crack Propagation in Irradiated Austenitic Steels

Students in Nuclear Engineering and Radiological Sciences at the University of Michigan are required to learn about the various applications of radiation. Because of the broad applicability of accelerators to surface analysis, one of these courses includes a laboratory session on surface analysis techniques such as Rutherford Backscattering Analysis (RBS) and Nuclear Reaction Analysis (NRA). In this laboratory session, the students determine the concentration of nitrogen atoms in various targets using RBS and NRA by way of the 14N(d,α)12C reaction. The laboratory is conducted in a hands‐on format in which the students conduct the experiment and take the data. This paper describes the approach to teaching the theory and experimental methods behind the techniques, the conduct of the experiment and the analysis of the data.

/pdf/surface-analysis-for-students-in-nuclear-engineering-and-2amskxxd2s.pdf

Gary S. Was

Papers

Phase Transformations in Nickel-Aluminum Alloys During Ion Beam Mixing

Statistical determination of stress threshold and survival analysis on IASCC initiation data of stainless steels in pressurized water reactor conditions

Synthesis and Properties of Mo/MoSix Microlaminates Using Ion Beam Assisted Deposition

Plastic Deformation Processes Accompanying Stress Corrosion Crack Propagation in Irradiated Austenitic Steels

Surface analysis for students in Nuclear Engineering and Radiological Sciences