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

Chemical bonding effects on the brittle-to-ductile transition in metallic glasses

Effect of the fictive temperature on the modulus, hardness, yield strength, dynamic mechanical and creep response of Zr44Ti11Cu10Ni10Be25 metallic glasses

To overcome the brittleness of metallic glasses (MGs), their structure, chemistry, or loading conditions are usually controlled. Here, the local stress state in MGs was modulated without affecting their structure. The elastically designed MG heterostructures provide enhanced ductility together with strain hardening during loading. The stress heterogeneity leads to shear band multiplication that consequently enhances the macroscopic ductility of MGs. In addition, the residual compressive stress significantly increases the strength of the glass and is responsible for the observed strain hardening.

Origin of strain hardening in monolithic metallic glasses

Rejuvenation in metallic glasses (MGs) can be achieved by means of thermal cycling treatments at cryogenic temperatures. Here, atomic scale studies were carried out to explore thermal cycling in CuZr MGs at different species contents. Enhanced ductility was observed due to several factors. All samples presented higher potential energy and less-dense structures upon treatments, indicating rejuvenated states. Furthermore, populations of high-centrosymmetric Voronoi polyhedra decreased, while liquid-like structures increased, which are usually related to low rigidity. Atomic connectivity was explored as a measure of the medium range order, revealing that thermal cycling reduced the fraction of 3-atom connections, while increased 2-atom and 4-atom connections. Since 3-atom connections are associated to more-ordered structures, its weakening explains the enhanced plasticity achieved by MGs upon thermal cycling treatments. This study provides new insights into rejuvenation of MGs, delivering atomic scale descriptions at both the short range and medium range order. 

Cryogenic thermal cycling rejuvenation in metallic glasses: Structural and mechanical assessment

The atomic structure of metallic glasses (MGs) plays an important role in their physical and mechanical properties. Numerous molecular dynamics (MD) simulations have been performed to reveal the structure of MGs at the atomic scale. However, the cooling rates utilized in most of the MD simulations (usually on the order of 109–1012 K/s) are too high to allow the structure to relax into the actual structures. In this study, we performed long-term pressurized sub-Tg annealing for up to 1 μs using MD simulation to systematically study the structure evolution of Cu50Zr50 MG. We find that from relaxation to rejuvenation, structural excitation of MGs and transition during sub-Tg annealing depend on the level of hydrostatic pressure. At low hydrostatic pressures, up to 2 GPa in this alloy, the relaxation rate increases with the increasing pressure. The lowest equivalent cooling rate reaches 3.3 × 106 K/s in the sample annealed at 2 GPa hydrostatic pressure, which is in the order of the cooling rate in melt spinni...

Structural modification through pressurized sub-T-g annealing of metallic glasses

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Structural modification through pressurized sub-T-g annealing of metallic glasses