Competing roles of interfaces and matrix grain size in the deformation and failure of polycrystalline Cu-graphene nanolayered composites under shear loading

Methodology for Molecular Dynamics Simulation of Plastic Deformation of a Nickel/Graphene Composite

In this study, some features of molecular dynamics simulation for evaluating the mechanical properties of a Ni/graphene composite and analyzing the effect of incremental and dynamic tensile loading on its deformation are discussed. A new structural type of the composites is considered: graphene network (matrix) with metal nanoparticles inside. Two important factors affecting the process of uniaxial tension are studied: tension strain rate (5 ×10−3 ps−1 and 5 ×10−4 ps−1) and simulation temperature (0 and 300 K). The results show that the strain rate affects the ultimate tensile strength under tension: the lower the strain rate, the lower the critical values of strain. Tension at room temperature results in lower ultimate tensile strength in comparison with simulation at a temperature close to 0 K, at which ultimate tensile strength is closer to theoretical strength. Both simulation techniques (dynamic and incremental) can be effectively used for such a study and result in almost similar behavior. Fabrication technique plays a key role in the formation of the composite with low anisotropy. In the present work, uniaxial tension along three directions shows a big difference in the composite strength. It is shown that the ultimate tensile strength of the Ni/graphene composite is close to that of pure crumpled graphene, while the ductility of crumpled graphene with metal nanoparticles inside is two times higher. The obtained results shed the light on the simulation methodology which should be used for the study of the deformation behavior of carbon/metal nanostructures.

Effects of temperature and repeat layer spacing on mechanical properties of graphene/polycrystalline copper nanolaminated composites under shear loading

In the present study, the characteristics of graphene/polycrystalline copper nanolaminated (GPCuNL) composites under shear loading are investigated by molecular dynamics simulations. The effects of different temperatures, graphene chirality, repeat layer spacing, and grain size on the mechanical properties, such as failure mechanism, dislocation, and shear modulus, are observed. The results indicate that as the temperature increases, the content of Shockley dislocations will increase and the maximum shear stress of the zigzag and armchair directions also decreases. The mechanical strength of the zigzag direction is more dependent on the temperature than that of the armchair direction. Moreover, self-healing occurs in the armchair direction, which causes the shear stress to increase after failure. Furthermore, the maximum shear stress and the shear strength of the composites decrease with an increase of the repeat layer spacing. Also, the shear modulus increases by increasing the grain size of copper.

Effects of temperature and grain size on deformation of polycrystalline copper-graphene nanolayered composites

The effects of temperature and grain size on mechanical properties of polycrystalline copper-graphene nanolayered (PCuGNL) composites are investigated by analytical mechanical models and molecular dynamics simulations. The yield of PCuGNL composites under tension depends on temperature, copper grain size, and repeat layer spacing. Graphene-copper interfaces play the dominant role in the ultimate tensile strength of PCuGNL composites. The optimal range for strengthening of repeat layer spacing is 2-10 nm, and the failure stress of PCuGNL composites is weakly dependent on temperature. An analytical model is proposed to accurately characterize the mechanical behaviors of PCuGNL composites.

Temperature-Dependent Mechanical Properties of Graphene/Cu Nanocomposites with In-Plane Negative Poisson's Ratios

Negative Poisson's ratio (NPR), also known as "auxetic", is a highly desired property in a wide range of future industry applications. By employing molecular dynamics (MD) simulation, metal matrix nanocomposites reinforced by graphene sheets are studied in this paper. In the simulation, single crystal copper with crystal orientation [1 1 0] is selected as the matrix and an embedded-atom method (EAM) potential is used to describe the interaction of copper atoms. An aligned graphene sheet is selected as reinforcement, and a hybrid potential, namely, the Erhart-Albe potential, is used for the interaction between a pair of carbon atoms. The interaction between the carbon atom and copper atom is approximated by the Lennard-Jones (L-J) potential. The simulation results showed that both graphene and copper matrix possess in-plane NPRs. The temperature-dependent mechanical properties of graphene/copper nanocomposites with in-plane NPRs are obtained for the first time.

Unusually high flexibility of graphene-Cu nanolayered composites under bending

The mechanical properties of graphene-Cu nanolayered (GCuNL) composites under bend loading are investigated via an energy-based analytical model and molecular dynamics (MD) simulations. For an anisotropic material, if it has a weak strength in a certain direction, improving the mechanical properties along this direction is normally difficult for its composites. Here, we find that the flexibility of GCuNL composites can be improved considerably by graphene interfaces, despite graphene's small bending stiffness. The graphene interfaces can delocalize slip bands in the inner Cu layers of GCuNL composites, and impede local nucleation of dislocations, thus greatly increasing the yield and failure bend angles. As the thickness decreases, the flexibility of GCuNL nanofilms increases. However, the GCuNL nanofilms are thermodynamically unstable due to interface instability when the repeat layer spacing is less than 2 nm. The energy-based analytical model for large deformation can accurately characterize the bending response of GCuNL nanofilms.

Shock wave propagation, plasticity, and void collapse in open-cell nanoporous Ta

We systematically investigate the wave propagation, plasticity and void collapse, as well as the effects of porosity, specific surface area and impact velocity, in a set of open-cell nanoporous Ta, during shock compression, via performing large-scale non-equilibrium molecular dynamics simulations. The shock wave propagation presents an impedance, sensitive to porosity, but not to specific surface area. Such surprising phenomena are due to the similar sensitivities in density and stress variations to porosity or specific surface area. Upon impact, shock front shapes change from ramped to steep ones, with increasing porosity, specific surface area or impact velocity, owing to the transition from the heterogeneous to homogeneous plasticity along transverse directions. This transition of plasticity arises by (i) the strong impedance on large deformation bands as porosity increases; and (ii) the transition from deformation twinning to dislocation slips, and to amorphization, as the specific surface area or impact velocity increases. Shock-induced plasticity, including their nucleation, growth and interactions, also facilitates the collapse of voids.