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

Friction stir welding/processing of metals and alloys: A comprehensive review on microstructural evolution

The double-edge effect of second-phase particles on the recrystallization behaviour and associated mechanical properties of metallic materials

Bulk nanostructured (ns)/ultrafine-grained (UFG) metallic materials possess very high strength, making them attractive for high strength, lightweight and energy efficient applications. The most effective approach to produce bulk ns/UFG metallic materials is severe plastic deformation (SPD). In the last 30 years, significant research efforts have been made to explore SPD processing of materials, SPD-induced microstructural evolutions, and the resulting mechanical properties. There have been a few comprehensive reviews focusing mainly on SPD processing and the mechanical properties of the resulting materials. Yet no such a review on SPD-induced microstructural evolutions is available. This paper aims to provide a comprehensive review on important microstructural evolutions and major microstructural features induced by SPD processing in single-phase metallic materials with face-centered cubic structures, body-centered cubic structures, and hexagonal close-packed structures, as well as in multi-phase alloys. The corresponding deformation mechanisms and structural evolutions during SPD processing are discussed, including dislocation slip, deformation twinning, phase transformation, grain refinement, grain growth, and the evolution of dislocation density. A brief review on the mechanical properties of SPD-processed materials is also provided to correlate the structure with mechanical properties of SPD-processed materials, which is important for guiding structural design for optimum mechanical properties of materials.

Structural evolutions of metallic materials processed by severe plastic deformation

Friction stir welding (FSW), a highly efficient solid-state joining technique, has been termed as “green” technology due to its energy efficiency and environment friendliness. It is an enabling technology for joining metallic materials, in particular lightweight high-strength aluminum and magnesium alloys which were classified as unweldable by traditional fusion welding. It is thus considered to be the most significant development in the area of material joining over the past two decades. Friction stir processing (FSP) was later developed based on the basic principles of FSW. FSP has been proven to be an effective and versatile metal-working technique for modifying and fabricating metallic materials. FSW/FSP of aluminum alloys has prompted considerable scientific and technological interest since it has a potential for revolutionizing the manufacturing process in the aerospace, defense, marine, automotive, and railway industries. To promote widespread applications of FSW/FSP technology and ensure t...

Recent Advances in Friction Stir Welding/Processing of Aluminum Alloys: Microstructural Evolution and Mechanical Properties

Al3(Sc,Zr)-based precipitates in Al–Mg alloy: Effect of severe deformation

In this work, a simple but effective approach for improvement of strength of friction-stir welded 6061-T6 aluminum alloys was elaborated. It involves friction-stir welding (FSW) at relatively high tool rotational speed and welding speed followed by standard post-weld aging. The selected combination of FSW parameters provides high welding temperature as well as rapid cooling rate. This leads to complete dissolution of strengthening precipitates in stir zone but hinders their coarsening in heat-affected zone. In turn, this enables to avoid the formation of softened region during subsequent aging and thus substantially enhances strength and improves ductility.

Optimization of processing-microstructure-properties relationship in friction-stir welded 6061-T6 aluminum alloy

Deformation structures and strengthening mechanisms in an AlMgScZr alloy

The effect of prior work hardening on the microstructure and mechanical properties of a friction-stir welded (FSWed) Al–5.4Mg–0.2Sc–0.1Zr alloy was investigated. The microstructure of the final stir zone was shown to be weakly dependent on the initial condition of the base material; FSW led to the formation of fully recrystallized microstructures with average grain sizes ranging from 1.2 to 1.5 μm and a moderate dislocation density of ~3.5±1.5×1013 m−2 in the stir zone. The nano-scale Al3(Sc,Zr) dispersoids coarsened from 9 to ~15 nm but retained a coherent relationship with the matrix. In contrast, the joint efficiency of the obtained welds was very sensitive to the initial material condition. Nearly full strength joints were obtained in both annealed (O) and partially hardened and stabilized (H323) conditions. However, the joint efficiency was only 65% in the fully hardened condition (H18). The relatively low weld strength for the alloy in the H18 condition was attributed to the elimination of dislocation and substructure strengthening during FSW.

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Friction-stir welding of an Al–Mg–Sc–Zr alloy in as-fabricated and work-hardened conditions

The effect of friction-stir welding (FSW) on the microstructure and mechanical properties of Al–5.4Mg–0.2Sc–0.1Zr sheets with ultra-fine grained (UFG) microstructure was studied. The UFG material was produced by equal-channel angular pressing (ECAP) followed by either hot or cold rolling. FSW was found to be very effective for preservation of the UFG microstructure as well as the constituent coherent nano-scale dispersoids in the welded material. Despite the preservation effect, however, essential material softening was observed in the weld zone. This observation was attributed to recrystallization presumably occurring during FSW. The joint efficiency for the yield strength of the obtained friction-stir welds was found to be 85% in the hot rolled condition and only 57% in the cold rolled state. The relatively low joint efficiency was attributed to the recrystallization softening as well as to the formation of a “zigzag line” defect in the stir zone.

Vladislav Kulitskiy

Papers

Al3(Sc,Zr)-based precipitates in Al–Mg alloy: Effect of severe deformation

Optimization of processing-microstructure-properties relationship in friction-stir welded 6061-T6 aluminum alloy

Deformation structures and strengthening mechanisms in an AlMgScZr alloy

Friction-stir welding of an Al–Mg–Sc–Zr alloy in as-fabricated and work-hardened conditions

Friction-stir welding of ultra-fine grained sheets of Al–Mg–Sc–Zr alloy