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.

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Current issues in recrystallization: a review

Technical Note: On the Relationship between the EPR Test, Sensitization, and IGSCC Susceptibility

Ion-mixed surface microalloys on nickel have been found to inhibit the surface emergence of persistent slip band structures (PSBs) during low cycle fatigue (LCF), thus improving crack initiation resistance. The formation of PSBs in the bulk is not greatly affected. This has been demonstrated by experiments on nickel polycrystals modified by ion mixing of NiAl layers using 1 × 10 16 ions cm −2 of 0.5 MeV Kr + ions. When fatigued, these specimens show fewer surface-breaking PSBs compared with unmodified nickel. The existence of subsurface PSBs in ion microalloyed nickel was revealed by etch-restrain experiments. After the surface-modified region was removed by electropolishing, new surface slip offsets, associated with bulk PSBs, were observed immediately after application of an additional 1 2 fatigue cycle. When PSBs do break the surface during LCF of ion microalloyed nickel, the process is promoted by high local cyclic displacements in the surface film, generated by subsurface PSBs.

Ion implantation and fatigue crack initiation: Interaction of persistent slip bands with modified surface layers☆

A computer code (BASF) has been constructed to perform automatic iterative fitting of Rutherford backscattering spectra using only the experimental spectrum and the parameter set defining the experiment. The code may be used to analyze samples containing anywhere from two to five elements. The code output consists of the total amount of each element present and a composition versus depth profile. The code's performance was verified on both computer generated and experimental backscattering spectra. Samples consisting of nickel substrates onto which layers of pure nickel and pure aluminum have been alternately evaporated in thicknesses of 130 and 100 A, respectively, were used to produce backscattering spectra. These spectra, when analyzed, demonstrated that the code was able to determine the total aluminum content to within 3% and the ratio of aluminum to nickel to within 1% of the thickness monitor readings taken during evaporation. The code has shown the ability to recognize sharp interfaces in well resolved spectra. The code performs equally well on slowly varying concentration profiles which are created during the annealing of layered samples. Limitations on the code and its use include the precise knowledge of the relevant experimental parameters used as input, and complete specification of all elements in the sample. The ultimate limits on the code's accuracy are the resolution of the spectrum and the accuracy of the computed stopping powers. This code provides a significant advantage over other spectrum fitting codes in that the process is fully automated and does not require constant user interaction. Further, it provides the capability of accurately determining concentration profiles in layered samples where the layer thickness is of the order 100 A.

Automatic iterative fitting of Rutherford backscattering spectra from multielement samples

Grain boundary properties are known to affect the intergranular stress corrosion cracking (IGSCC) and irradiation assisted stress corrosion cracking behavior of austenitic alloys in high temperature water. However, it is only recently that sufficient evidence has accumulated to show that the disposition of deformation in and near the grain boundary plays a key role in intergranular cracking. Grain boundaries that can transmit strain to adjacent grains can relieve stresses without undergoing localized deformation. Grain boundaries that cannot transmit strain will either experience high stresses or high strains. High stresses can lead to wedge-type cracking and sliding can lead to rupture of the protective oxide film. These processes are also applicable to irradiated materials in which the deformation can become highly localized in the form of dislocation channels and deformation twins. These deformation bands conduct tremendous amounts of strain to the grain boundaries. The capability of a boundary to transmit strain to a neighboring grain will determine its propensity for cracking, analogous to that in unirradiated metals. Thus, IGSCC in unirradiated materials and IASCC in irradiated materials are governed by the same local processes of stress and strain accommodation at the boundary.

Gary S. Was

Papers

Technical Note: On the Relationship between the EPR Test, Sensitization, and IGSCC Susceptibility

Ion implantation and fatigue crack initiation: Interaction of persistent slip bands with modified surface layers☆

Automatic iterative fitting of Rutherford backscattering spectra from multielement samples

Localized deformation induced IGSCC and IASCC of austenitic alloys in high temperature water

Corrosion and Stress Corrosion Cracking of Iron and Nickel-base Austenitic Alloys in Supercritical Water