Atomistic simulations of kinks in 1/2a<111> screw dislocations in bcc tantalum

Two types of equilibrium core structures (denoted symmetric and asymmetric) for 1/2a screw dislocations in bcc metals have been found in atomistic simulations. In asymmetric (or polarized) cores, the central three atoms simultaneously translate along the Burgers vector direction. This collective displacement of core atoms is called polarization. In contrast, symmetric (nonpolarized) cores have zero core polarization. To examine the possible role of dislocation core in kink-pair formation process, we studied the multiplicity, structural features, and formation energies of 1/3a kinks in 1/2a screw dislocations with different core structures. To do this we used a family of embedded atom model potentials for tantalum (Ta) all of which reproduce bulk properties (density, cohesive energy, and elastic constants) from quantum mechanics calculations but differ in the resulting polarization of 1/2a screw dislocations. For dislocations with asymmetric core, there are two energy equivalent core configurations [with positive (P) and negative (N) polarization], leading to 2 types of (polarization) flips, 8 kinds of isolated kinks, and 16 combinations of kink pairs. We find there are only two elementary kinks, while the others are composites of elementary kinks and flips. In contrast, for screw dislocations with symmetric core, there are only two types of isolated kinks and one kind of kink pair. We find that the equilibrium dislocation core structure of 1/2a screw dislocations is an important factor in determining the kink-pair formation energy

TENSILE DEFORMATION BEHAVIOR AND MECHANICAL PROPERTY STUDY OF SIX FCC METALS

poster abstractNanomaterials have enhanced mechanical properties in comparison to their respective bulk materials. To understand the effect of the size and shape on the mechanical properties of nanomaterials, we used molecular dynamics (MD) methods to simulate the deformation process of copper, gold, nickel, palladium, platinum, and silver nanowires of three cross-sectional shapes (quare, circular, and octagonal) and four diameters (varied from one to eight nanometers). In this work, the nanowires were subjected to a uniaxial tensile load in the [100] direction at a strain rate of 108 s-1 at a simulation temperature of 300 K. The embedded-atom method was employed to describe the many-body atomic interaction energy in metallic systems. The nanowires were stretched to failure and the corresponding stress-strain curves were produced. From these curves, mechanical properties including the elastic modulus, yield stress and strain, and ultimate strain were calculated. In addition to the MD approach, an energy method was applied to calculate the elastic modulus of each nanowire through exponential fitting of an energy function. Both methods used to calculate Young’s modulus qualitatively gave similar results indicating that as diameter decreases, Young’s modulus decreases. The atomic structures generated from MD simulations were examined in details to investigate the deformation and yield behavior of each nanowire. It was found that most nanowires yield and fail through partial dislocation nucleation and propagation leading to {111} slip. However, the octagonal platinum nanowire, whose diameter is 5 nm, was found to yield through reconstruction of the {011} surfaces into the more energetically favorable {111} surfaces

). Size Dependency of the Elastic Modulus of ZnO Nanowires: Surface Stress Effect

Relation between the elastic modulus and the diameter (D) of ZnOnanowires was elucidated using a model with the calculated ZnOsurface stresses as input. We predict for ZnOnanowires due to surface stress effect: (1) when D\u3e20nm, the elastic modulus would be lower than the bulk modulus and decrease with the decreasing diameter, (2) when 20nm\u3eD\u3e2nm, the nanowires with a longer length and a wurtzite crystal structure could be mechanically unstable, and (3) when D\u3c2nm, the elastic modulus would be higher than that of the bulk value and increase with a decrease in nanowire diameter

Predicting Young’s Modulus of Nanowires from First-Principles Calculations on their Surface and Bulk Materials

Using the concept of surface stress, we developed a model that is able to predict Young’s modulus of nanowires as a function of nanowire diameters from the calculated properties of their surface and bulk materials. We took both equilibrium strain effect and surface stress effect into consideration to account for the geometric size influence on the elastic properties of nanowires. In this work, we combined first-principles density functional theory calculations of material properties with linear elasticity theory of clamped-end three-point bending. Furthermore, we applied this computational approach to Ag, Au, and ZnOnanowires. For both Ag and Aunanowires, our theoretical predictions agree well with the experimental data in the literature. For ZnOnanowires, our predictions are qualitatively consistent with some of experimental data for ZnO nanostructures. Consequently, we found that surface stress plays a very important role in determining Young’s modulus of nanowires. Our finding suggests that the elastic properties of nanowires could be possibly engineered by altering the surface stress of their lateral surfaces