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

Structural heterogeneities and mechanical behavior of amorphous alloys

Inspired by the gradient structures of biological materials, researchers have explored compositional and structural gradients for about 40 years as an approach to enhance the properties of engineering materials, including metals and metallic alloys. The synthesis of various gradient nanostructured materials, such as gradient nanograined, nanolaminated nd nanotwinned metals and alloys, has provided new opportunities to understand gradient-related mechanical behaviour. These emerging gradient materials often exhibit unprecedented mechanical properties, such as strength–ductility synergy, extraordinary strain hardening, enhanced fracture and fatigue resistance, and remarkable resistance to wear and corrosion, which are not found in materials with homogeneous or random microstructures. This Review critically assesses the state of the art in the field of gradient nanostructured metallic materials, covering topics ranging from the fabrication and characterization of mechanical properties to underlying deformation mechanisms. We discuss various deformation behaviours induced by structural gradients, including stress and strain gradients, the accumulation and interaction of new dislocation structures, and unique interfacial behaviour, as well as providing insight into future directions for the development of gradient structured materials. Gradient nanostructured metals and alloys are an emerging class of materials that exhibit a combination of excellent mechanical properties that are not possessed by their homogeneous counterparts. This Review assesses the fabrication, characterization and deformation behaviour of these materials, as well as the challenges and future directions of the field.

Mechanical properties and deformation mechanisms of gradient nanostructured metals and alloys

https://scholars.cityu.edu.hk/files/58110090/1_s2.0_S1369702120301073_main.pdf

Nanostructural metallic materials: Structures and mechanical properties

http://li.mit.edu/A/Papers/12/Wang12DingAM.pdf

Sample size matters for Al88Fe7Gd5 metallic glass: Smaller is stronger

High strength and high ductility are often mutually exclusive properties for structural metallic materials. This is particularly important for aluminum (Al)-based alloys which are widely commercially employed. Here, we introduce a hierarchical nanostructured Al alloy with a structure of Al nanograins surrounded by nano-sized metallic glass (MG) shells. It achieves an ultrahigh yield strength of 1.2 GPa in tension (1.7 GPa in compression) along with 15% plasticity in tension (over 70% in compression). The nano-sized MG phase facilitates such ultrahigh strength by impeding dislocation gliding from one nanograin to another, while continuous generation-movement-annihilation of dislocations in the Al nanograins and the flow behavior of the nano-sized MG phase result in increased plasticity. This plastic deformation mechanism is also an efficient way to decrease grain size to sub-10 nm size for low melting temperature metals like Al, making this structural design one solution to the strength-plasticity trade-off. Strengthening a metallic alloy without sacrificing ductility remains challenging. Here, the authors develop a hierarchical nanostructured aluminium alloy composed of nanograins surrounding by metallic glass shells that has both ultrahigh strength and good ductility.

/pdf/hierarchical-nanostructured-aluminum-alloy-with-ultrahigh-1llrxw4dhj.pdf

Hierarchical nanostructured aluminum alloy with ultrahigh strength and large plasticity

In this article, the shear-banding behavior in bulk metallic-glasses (BMGs) is studied using a focused ion beam (FIB)-based nanoindentation method, which involves cylindrical nanoindentation of a FIB-milled BMG microlamella and is capable of revealing the subsurface shear-band patterns down to the submicron scale. The results of the current study on a Zr-based BMG clearly show that short shear bands, with the lengths of a few hundred nanometers, could be severely kinked before growing into a longer one, which implies that structural heterogeneity plays an important role in the microplasticity of BMGs. Furthermore, through the three-dimensional finite-element simulation combined with the theoretical calculation based on the Mohr–Coulomb law, it is found that the yield strengths exhibit a large scatter as a consequence of the structural heterogeneity when microplasticity occurs in the Zr-based BMG, which is consistent with our recent findings obtained from the microcompression experiments.

/pdf/revelation-of-the-effect-of-structural-heterogeneity-on-36oonez8ku.pdf

Revelation of the effect of structural heterogeneity on microplasticity in bulk metallic-glasses

Surface Mechanical Attrition Treatment (SMAT) process, as one of the typical severe plastic deformation technologies, can be utilized to generate layup structure with finer size of grains and large residual stresses at the near treated surface area on the alloys. Such technology not only improves the mechanical behaviors but also keeps neatness of alloys without inducing extra compositions. In this paper, four major controllable parameters of SMAT process are investigated to demonstrate their influence on ball-impacting intensity of the treatment, which the improvement of mechanical behaviors are greatly dependent on. The high speed camera is employed to capture the motion of the balls and measure the impact velocities by counting the number of frames. The compared results indicate that the amount of balls not only varies the average velocities during SMAT process, but also offsets the effect of distance between treated surface and source of balls. In addition, the influence of ball sizes, the mentioned distance and power of excitation exhibits apparently on intensity of SMAT process when limited amount of balls are used.

Qing Wang

Papers

Hierarchical nanostructured aluminum alloy with ultrahigh strength and large plasticity

Revelation of the effect of structural heterogeneity on microplasticity in bulk metallic-glasses

Investigation of Controllable Parameters of Mechanical Treatment on Aluminum-Based Composite in Marine Application