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

Films of Al, Al2O3 and Al/Al2O3 microlaminates were formed by ion beam assisted deposition (IBAD) at R ratios from 0.0025 to 0.5 and film thicknesses between 150 and 2600 nm. Oxide films were amorphous while metal layers were crystalline with small grains and texture for both PVD and IBAD conditions. The average stress in the oxide film is tensile at R=0 and becomes compressive, saturating at approximately 15 eV/atom. The residual stress in the Al films is tensile over all R ratios and the stress in the microlaminate roughly follows a rule of mixtures. Deformation of ductile substrates on which films had been deposited revealed that the critical strain to fracture was strongly dependent on residual stress. Large compressive stresses in monolithic films produced by ion beam assisted deposition delayed the onset of crack initiation while the presence of multiple layers, in general, lowered the crack density at saturation, suggesting a possible ductilizing effect.

Mechanical Properties and Residual Stresses in Oxide/Metal Multilayer Films Synthesized by Ion Beam Assisted Deposition

Many irradiation effects in Fe-Cr-Ni alloys such as radiation-induced segregation, radiation-enhanced diffusion, and void swelling are known to vary with bulk alloy composition. The development of microstructural and microchemical changes during irradiation and during post-irradiation annealing is determined by the rate of diffusion of point defects and alloying elements. To accurately predict the changes in grain boundary chemistry due to radiation-induced segregation and post-irradiation annealing, the composition dependence of diffusion parameters, such as the migration energy, must be known. A model has been developed which calculates migration diffusivity. The advantages of this calculational method are that a single set of input parameters can be used for a wide range of bulk alloy compositions, and the effects of local order can easily be incorporated into the calculations. A description of the model is presented, and model calculations are compared to segregation measurements from seven different iron-chromium-nickel alloys, irradiated with protons to doses from 0.1 to 3.0 dpa at temperatures between 200 C and 600 C. Results show that segregation trends can be modeled using a single set of input parameters with the difference between model calculation and measurement being less than 5 at%, but usually less than 2 at%. Additionally, model predictions are compared to grain boundary composition measurements of neutron irradiated 304 stainless steel following annealing. For the limited annealing data available, model calculations correctly predict the magnitude and time scale for recovery of the grain boundary composition.

/pdf/modeling-the-effect-of-irradiation-and-post-irradiation-1gjycc498n.pdf

Modeling the Effect of Irradiation and Post-Irradiation Annealing on Grain Boundary Composition in Austenitic Fe-Cr-Ni Alloys

Novel Techniques to Mitigate Corrosion and Stress Corrosion Cracking in Supercritical Water

Journal of Nuclear Materials: Preface

Radiation-induced segregation (RIS) has been implicated as a potential contributor to irradiation assisted stress corrosion cracking in light water reactor core components. To better understand changes to grain boundary chemistry during irradiation, RIS was measured in ultra-high purity (UHP) 304 stainless steel using Auger electron spectroscopy (AES). Variations in measured grain boundary concentration, both within a sample and between samples, are reduced by refinements in both the radiation and the AES techniques. These refinements include improvements in temperature control, uniformity of sample-to-sample dose, grain boundary acceptance criteria, amount of intergranular fracture, and amount of beam current used in analysis. AES measurements on samples irradiated at 400°C to 1.0 dpa show how implementing the technique refinements reduces the variability in the measured concentrations. Additionally, measurements from regions of ductile tearing in samples irradiated to 0.1 and 1.0 dpa at 400°C, to 1.0 dpa at 200°C, and from unirradiated samples show that sensitivity factors must be determined to obtain the most accurate measurement of grain boundary composition.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S1946427400262945

Gary S. Was

Papers

Mechanical Properties and Residual Stresses in Oxide/Metal Multilayer Films Synthesized by Ion Beam Assisted Deposition

Modeling the Effect of Irradiation and Post-Irradiation Annealing on Grain Boundary Composition in Austenitic Fe-Cr-Ni Alloys

Novel Techniques to Mitigate Corrosion and Stress Corrosion Cracking in Supercritical Water

Journal of Nuclear Materials: Preface

Refinement of Irradiation and Analysis Techniques for Radiation-Induced Segregation