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

Mechanisms of WC plastic deformation in cemented carbide

The hard WC-Co composites usually have weak toughness. The present work solved this issue by a new strategy of fabricating nanocrystalline composites, which consists of concurrent crystallization, carbonization, and sintering densification of the amorphous compound powder. The resultant nanocrystalline WC-Co composite has distinctly larger proportion of interfacial coherency at the grain- and phase-boundaries compared with that in the conventional counterparts. The formation mechanisms of the coherent WC grain boundaries, Co twinning boundaries, and step-like semi-coherent WC/Co phase boundaries were demonstrated on the atomic scale. Moreover, the contributions of the characteristic interfaces to the resistance against intergranular fracture, which dominates the failure of nanocrystalline composites in the deformation, were illustrated based on the analyses of atomic configuration and crystallographic relationship at interfaces. The reasons were disclosed for the advanced comprehensive properties with simultaneously ultrahigh hardness and fracture toughness of the prepared nanocrystalline composite. The present technology and mechanisms for properties enhancement will be applicable to a large variety of composite materials.

Nanocrystalline WC-Co composite with ultrahigh hardness and toughness

This review covers areas where our understanding of the mechanisms underlying nanoindentation has been increased by atomistic studies of the nanoindentation process While such studies have been performed now for more than 20 years, recent investigations have demonstrated that the peculiar features of nanoplasticity generated during indentation can be analyzed in considerable detail by this technique Topics covered include: nucleation of dislocations in ideal crystals, effect of surface orientation, effect of crystallography (fcc, bcc, hcp), effect of surface and bulk damage on plasticity, nanocrystalline samples, and multiple (sequential) indentation In addition we discuss related features, such as the influence of tip geometry on the indentation and the role of adhesive forces, and how pre-existing plasticity affects nanoindentation

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Atomistic Studies of Nanoindentation—A Review of Recent Advances

The current study applies towards investigate the sintering behavior, mechanical properties as well as stability of predesigned Co gradient of nano-laminated multi-layer graphene (MLG) reinforced WC-Co composites, highlighting the critical roles of MLG as exceptional toughening phase and superior cobalt gradient stability enhancement phase. During liquid phase sintering, the stability of predesigned cobalt gradient can be significantly enhanced via controlling MLG content. The three crucial cobalt gradient stability enhancing mechanisms are MLG preferential orientation, MLG inducing the generation of the liquid cobalt phase and MLG wrapping matrix grains. MLG intrinsic toughening mechanism i.e. MLG bending together with extrinsic toughening mechanisms such as MLG wrapping, interface debonding, MLG wall and network, MLG induced weak interface, MLG pullout, crack deflection, crack bridging and crack stopping is the major toughening mechanisms. Besides, stabilization of the predesigned Co-gradient and grain reinforcement are also the important contribution to the improved hardness to toughness relationship.

Multi-layer graphene reinforced nano-laminated WC-Co composites

Deformation and plastic coordination in WC-Co composite — Molecular dynamics simulation of nanoindentation

The deformation characteristics of WC, as a typical hard ceramic material, were studied on the nanoscale using atomistic simulations for both the single-crystal and polycrystalline forms under uniaxial compression. In particular, the effects of crystallographic orientation, grain boundary coordination and grain size on the origin of deformation were investigated. The deformation behavior of the single-crystal and polycrystalline WC both depend strongly on the orientation towards the loading direction. The grain boundaries play a significant role in the deformation coordination and the potential high fracture toughness of the nanocrystalline WC. In contrast to conventional knowledge of ceramics, maximum strength was obtained at a critical grain size corresponding to the turning point from a Hall-Petch to an inverse Hall-Petch relationship. For this the mechanism of the combined effect of dislocation motion within grains and the coordination of stress concentration at the grain boundaries were proposed. The present work has moved forward our understanding of plastic deformability and the possibility of achieving a high strength of nanocrystalline ceramic materials.

https://iopscience.iop.org/article/10.1088/1361-6528/aa9270/pdf

Qing Feng

Papers

Deformation and plastic coordination in WC-Co composite — Molecular dynamics simulation of nanoindentation

Compression deformation of WC: atomistic description of hard ceramic material.