Molecular dynamics simulations of cold welding of nanoporous amorphous alloys: effects of welding conditions and microstructures.

Nanoscale cold welding is a promising method in the bottom-up fabrication of nanodevices. Herein, cold welding mechanisms of Cu50Zr50 nanoporous amorphous alloys (NPAAs) are investigated by molecular dynamics simulations, along with the mechanical properties of the welded products. Effects of welding conditions and microstructural parameters are considered. Our results demonstrate that the welded joint has superior mechanical properties. The ultimate strength of the welded NPAAs can be as high as 94-99% that of the original NPAAs but 62-75% for the yield strength and elastic modulus. Voronoi analysis declares that the changes in atomic clusters of NPAAs caused by cold welding are mild. The welding conditions do not have remarkable influences on the mechanical responses of the welded structure. The NPAAs with smaller ligament sizes are more suitable for cold welding, benefiting from the size effect of amorphous alloys. We also successfully use cold welding to fabricate gradient NPAAs and repair fractured NPAAs. It is found that the ultimate tensile strength of the NPAAs changes very little with each successful cold welding. After ten fracture-welding cycles, the ultimate strength of the as-welded specimen is slightly lower than that of the raw materials.

Nanoindentation characteristics of nanocrystalline B2 CuZr shape memory alloy via large-scale atomistic simulation.

Nanoindentation tests are performed by molecular dynamics simulation to explore the mechanical properties of nanocrystalline B2 CuZr shape memory alloys with average grain sizes ranging from 6 to 18 nm. Some paramount aspects are monitored, including indentation force-depth curve, hardness, yield strength, and elastic recovery. The results demonstrate an inverse Hall-Petch effect, i.e., the hardness decreases with the decrease in grain size. For the single crystalline B2 CuZr, dislocation nucleation and propagation are the major plastic mechanisms. However, grain cleavage, grain boundary compression, and grain rotation prevail over the plastic behaviors of nanocrystalline B2 CuZr alloys. The elastic recovery becomes stronger with the increase in grain size. Besides, the effects of temperature, indenter size, and indenter speed on the nanoindentation responses are evaluated quantitively.

Characterization of the deformation behaviors under uniaxial stress for bicontinuous nanoporous amorphous alloys.

In this paper, the deformation behaviors of Cu50Zr50 bicontinuous nanoporous amorphous alloys (BNAMs) under uniaxial tension/compression are explored by molecular dynamics simulations. Scaling laws between mechanical properties and relative density are investigated. The results demonstrate that the bending deformation of the ligament is the main elastic deformation mechanism under tension. Necking and subsequent fracture of ligaments are the primary failure mechanism under tension. Under tensile loading, shear bands emerge near the plastic hinges for the BNAMs with large porosities. The typical compressive behaviors of porous structure are observed in the BNAMs with large porosities. However, for small porosity, no distinguished plateau and densification are captured under compression. The tension-compression asymmetry of modulus increases with increasing porosity, whereas the BNAMs can be seen as tension-compression symmetry of yield strength. The modulus and yield strength are negatively correlated with temperature, but a positive relationship between the tensile ductility and temperature is shown. This work will help to provide a useful understanding of the mechanical behaviors of the BNAMs.

Mechanical properties and clamping behaviors of snow crab claw.

The high-performing biomimetic behaviors of crustaceans are the optimal results of long-time wise adaption to their living environment. One outstanding prototype is crab claw, which has the combining advantages of lightweight and high strength. To promote relevant engineering applications, it is imperative to explore its mechanical behaviors and structural characteristics. In this work, mechanical test and finite element analysis (FEA) are performed to reveal the fundamental mechanical properties and clamping behaviors of snow crab (Chionoecetes opilio) claw, respectively. A lightweight modeling method, parametric lofting modeling, for the 3D modeling of the claw is employed, which is compared with the traditional reverse engineering modeling method based on tomography image. Our results demonstrated that the hardness and modulus of the regions near the top of the claw are larger than those of the regions near of bottom of the claw. Moisture is a critical factor in controlling the tensile behavior of the claw and the wet specimens exhibit higher modulus and strength under tensile loading. Besides, The parametric lofting method is highly flexible and efficient in generating 3D geometrical model. The investigation of clamping behaviors provides not only insights into mechanical behaviors and intrinsic mechanisms but also a practical guide for their potential applications, such as designing high-performing artificial clamping muscles for clinical operations, aerospace applications, and robotics.