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

/pdf/dislocations-in-solids-1gaufoliyg.pdf

Dislocations in solids

Severe plastic deformation (SPD) is effective in producing bulk ultrafine-grained and nanostructured materials with large densities of lattice defects. This field, also known as NanoSPD, experienced a significant progress within the past two decades. Beside classic SPD methods such as high-pressure torsion, equal-channel angular pressing, accumulative roll-bonding, twist extrusion, and multi-directional forging, various continuous techniques were introduced to produce upscaled samples. Moreover, numerous alloys, glasses, semiconductors, ceramics, polymers, and their composites were processed. The SPD methods were used to synthesize new materials or to stabilize metastable phases with advanced mechanical and functional properties. High strength combined with high ductility, low/room-temperature superplasticity, creep resistance, hydrogen storage, photocatalytic hydrogen production, photocatalytic CO2 conversion, superconductivity, thermoelectric performance, radiation resistance, corrosion resistance, and biocompatibility are some highlighted properties of SPD-processed materials. This article reviews recent advances in the NanoSPD field and provides a brief history regarding its progress from the ancient times to modernity. Abbreviations: ARB: Accumulative Roll-Bonding; BCC: Body-Centered Cubic; DAC: Diamond Anvil Cell; EBSD: Electron Backscatter Diffraction; ECAP: Equal-Channel Angular Pressing (Extrusion); FCC: Face-Centered Cubic; FEM: Finite Element Method; FSP: Friction Stir Processing; HCP: Hexagonal Close-Packed; HPT: High-Pressure Torsion; HPTT: High-Pressure Tube Twisting; MDF: Multi-Directional (-Axial) Forging; NanoSPD: Nanomaterials by Severe Plastic Deformation; SDAC: Shear (Rotational) Diamond Anvil Cell; SEM: Scanning Electron Microscopy; SMAT: Surface Mechanical Attrition Treatment; SPD: Severe Plastic Deformation; TE: Twist Extrusion; TEM: Transmission Electron Microscopy; UFG: Ultrafine Grained GRAPHICAL ABSTRACT IMPACT STATEMENT This article comprehensively reviews recent advances on development of ultrafine-grained and nanostructured materials by severe plastic deformation and provides a brief history regarding the progress of this field.

Nanomaterials by severe plastic deformation: review of historical developments and recent advances

The study of deformation twinning has long history. However new, sometimes surprising, findings have shown that the phenomenon of deformation twinning still is not completely understood. During recent years, some debates are taking place in the scientific literature concerning deformation twinning mechanisms in metals with hcp structure. These debates deal with the importance of special twin boundary dislocations named disconnections, growth and nucleation of twins, non-Schmid behavior of twinning, difference of deformation produced by twins from simple shear. They invoked new propositions for atomistic mechanisms of deformation twinning. The purpose of this review is to compare the classical theories of interfacial defects with the new findings and prove that many of these findings can be understood in terms of these well-established theories. The main attention is paid to summarizing the explanations of different phenomena in terms of disconnection mechanisms in order to show that there is no contradiction between these mechanisms and the new findings.

Review of non-classical features of deformation twinning in hcp metals and their description by disconnection mechanisms

This work has received funding from the Euratom research and training programme 2014-2018 under grant agreement No. 755039 (M4F project).

https://publikationen.bibliothek.kit.edu/1000136279/123076715

Multiscale modelling for fusion and fission materials: the M4F project

Tilt ${112}$ grain boundaries (GBs) in bcc metals perform shear-coupled grain-boundary migration by the creation and glide of disconnections. Disconnection dipoles may be created at the pristine GB at high stresses or may be generated at the core of a GB dislocation that acts as a source of disconnections. We characterize this source in terms of its Burgers vector, denoted ${\stackrel{P\vec}{b}}_{1/\ensuremath{-}1}$, and describe the mechanism that allows the source to move conservatively with the GB. The ${\stackrel{P\vec}{b}}_{1/\ensuremath{-}1}$ grain-boundary dislocation is created, for instance, during the absorption of a crystal dislocation by the ${112}$ grain boundary. In addition, ${\stackrel{P\vec}{b}}_{1/\ensuremath{-}1}$ accommodates ${112}$ vicinal grain boundaries that are formed by segments of ${112}$ planes separated by ${\stackrel{P\vec}{b}}_{1/\ensuremath{-}1}$ grain-boundary dislocations. The presence of these ${\stackrel{P\vec}{b}}_{1/\ensuremath{-}1}$ dislocations facilitates the conservative displacement of both the pristine and the vicinal GBs. We show that the creation of disconnections is the key for the absorption of edge and screw dislocations by the GB and the drag of mixed dislocations by the GB during its migration. These conservative processes are efficient ways to accommodate plastic deformation by the growth and shrink of ${112}$ twins, and shear-coupled motion of the ${112}$ GB and its vicinal GBs.

/pdf/atomic-processes-of-shear-coupled-migration-in-112-twins-and-4ghstz9rnp.pdf

Atomic processes of shear-coupled migration in { 112 } twins and vicinal grain boundaries in bcc-Fe

The sustainability and capacity of macroscopic deformation by polycrystalline metals and metallic alloys is controlled by the propagation of dislocation-mediated slip through grains In this paper, the interaction of a pileup of $1/2\ensuremath{\langle}111\ensuremath{\rangle}$ dislocations with the ${332}$ tilt grain boundary (GB) is studied as a function of temperature in three bcc metals: iron (Fe), chromium (Cr), and tungsten (W) The interaction results in the transformation of the crystal dislocation into GB dislocations The ${332}$ tilt GB absorbs the crystal dislocations of the pileup, neither the transmission nor reflection of dislocations was observed The reaction product at the GB is determined by the crystallography of the GB and the features of the crystal dislocations involved, specifically, the orientation of the Burgers vector and the glide plane of the dislocation In general, the decomposition results in the formation of a sessile GB dislocation with a riser that facets the GB and several elementary disconnections that glide away In some cases, the riser increases its length with the number of dislocations absorbed and a new asymmetrical grain boundary of ${112}/{110}$ type is created For a given external shear stress, the number of dislocations absorbed depends on the orientation of the Burgers vector, glide plane of the pileup, and material

/pdf/interaction-of-a-dislocation-pileup-with-332-tilt-grain-4vl5odqtid.pdf

N. Kvashin

Papers

Atomic processes of shear-coupled migration in { 112 } twins and vicinal grain boundaries in bcc-Fe

Interaction of a dislocation pileup with {332} tilt grain boundary in bcc metals studied by MD simulations

On the migration of {3 3 2} 〈1 1 0〉 tilt grain boundary in bcc metals and further nucleation of {1 1 2} twin

{111} tilt grain boundaries as barriers for slip transfer in bcc Fe

Effect of radiation defects on the early stages of nanoindentation tests in bcc Fe and Fe-Cr alloys