We applied Taguchi experimental design to a nano-structure multilayer film. An L9 orthogonal array and a molecular dynamics simulation were used to design a metal film made up of eight atomic layers so that the metal film was made to have strong adhesion to a resin with a polyphenyl chain (a sequential benzene-ring chain). By carrying out sensitivity analysis with the orthogonal array, among four metal-film factors (the short-side and long-side lattice mismatches with the resin Δa and Δb, surface energy G, and cohesive energy H of an atomic-layer laminated metal film), the mismatches were found to be the most dominant factors in the adhesion strength. The author also found that reducing the mismatches and increasing the surface energy and cohesive energy are effective in increasing the adhesion strength and the signal-to-noise ratio. Molecular simulations showed that a copper/ruthenium/cobalt-laminated film is an appropriate structure that has strong adhesion to the resin.

View full abstract

To better understand the mechanism of structural instability at the atomistic level, we have proposed a rigorous method of instability mode analysis by solving the eivgenvalue problem of the Hessian dynamical matrix and applied it to dislocation nucleation from surface steps and interfaces. In dislocation nucleation from an interface edge, relation between instability at a low but nonzero temperature and instability modes for vanishing temperature is investigated. An instability mode corresponding to an eigenvector that has a small eigenvalue is activated by thermal fluctuation, resulting in deformation indicated by the eigenvector. This is in contrast to the case of dislocation nucleation from a surface notch, where instability occurrs along a mixed mode. In the case of dislocation nucleation from surface steps, we discuss the area of influence from a step by examining the distribution of eigenvectors utilizing the quasicontinuum (QC) method.

View full abstract

By means of molecular dynamics simulations cooperative grain boundary sliding is investigated during plastic deformation of two-dimensional nanopolycrystal at given temperatures and hydrostatic pressure. The mechanisms of obstacle overcoming are studied. It is demonstrated that the flattering of grain boundaries is the result of cooperative processes of grain boundary migration and dislocation sliding.

View full abstract

Influence of temperature on ideal shear strength (ISS), τ_c, of two fcc metals (Al and Cu) was studied by means of molecular dynamics simulations. To get reliable results we investigated influence of parameters of the applied Parrinello-Rahman stress control method and implemented damping of simulation cell fluctuations to avoid occurrence of structural instability assisted by too high fluctuations. The damping successfully reduces strain and stress fluctuations during simulations if the damping factor is specified properly. We also investigated simulation cell size effect to evaluate minimal number of atoms providing reliable results in order to reduce computational efforts and estimate the possibility of applying ab initio calculations. Recently developed embedded atom method (EAM) interatomic potentials for both metals were also examined to find most appropriate for our study. EAM potential developed by Zope et al. and Mishin et al. were revealed to be most suitable for Al and Cu, respectively. It is essential to choose appropriate simulation parameters and interatomic potentials for the valid evaluation of ISS at elevated temperatures. We find almost linear decrease in ideal strength with increasing temperature for [112](111) shear deformation, while critical strain decreases in a nonlinear manner. At room temperature, reduction of shear strength for Al(Cu) is less than 35%(25%) compared to that at 0 K.

View full abstract

Ab initio electronic structure theory has achieved considerable reliability concerning predictions of physical and chemical properties and phenomena. It provides understanding of matter at the atomic and electronic scale with an unprecedented level of details and accuracy. In the present contribution, the electronic structure theory and state-of-the-art ab initio calculation methods in solids are briefly reviewed and the application of the calculated total energy differences between various phases (lattice stabilities) is illustrated on construction of phase diagrams by the CALPHAD (CALculation of PHAse Diagrams) method in systems containing phases with complex structures, as e.g. sigma phase. Particular examples include description of the sigma phase in the Fe-Cr system and prediction of the phase composition of super-austenitic steels. It is shown that the utilization of ab initio results introduces a solid basis of the energetics of systems with complex phases, allows to avoid unreliable estimates and extrapolations of Gibbs energies and brings more physics into the CALPHAD method.

View full abstract

We utilize the relationship between the shape matrix and strain tensor of calculation cell in Parrinello-Rahman's algorithm, which is used in molecular dynamics (MD) methods, and formulate a strain-controlled MD algorithm based on the finite deformation theory of continuum mechanics. We simulate the atomic behavior in a nano-sized polycrystalline aluminum specimen under two different loading conditions. The first loading condition is a simple shear, and the other is a simple shear after compression, which mimics the loading conditions of actual ECAP processing. Atomic strain measure (ASM) is introduced to investigate the distribution of strain at the atomic scale within the specimen. In both cases, it is observed that grain boundary-structured atoms bordering the dislocation emitted from existing grain boundaries (GB) expand onto the slip plane and develop into a new GB plane, accommodating the rotation of adjacent grains. We propose a possible mechanism for grain refinement under severe plastic deformation. Common neighbor analysis indicates that precompression restricts dislocation emission from the GB. ASM results indicate that this deterioration of dislocation emission is caused by configurational change in the GB region that acts as a dislocation source. That is, preferential accumulation of compressive strain is observed near the GB under compression. In addition, precompression promotes GB sliding.

View full abstract

A three-dimensional level set method (LSM) was proposed for dislocation dynamics of which the dislocation curves were smoothly evolved, especially when topological changes of merging and breaking occur. The intersections of the zero levels of two level set functions are used to represent the dislocation lines in three dimensions. Fast marching method (FMM) or fast sweeping method (FSM) creating signed distance function (SDF) from the given interface can incorporate the existing different shapes into one level set function. Therefore, the dislocation lines can be represented by one set of level set functions. Within some specific restrictions, we succeeded to demonstrate some dislocation movements like cross-slip, double cross-slip and dislocation interactions on cross-slip plane, and discussed the internal stress field around such a complex topological dislocation configuration.

View full abstract

A nonlinear vibration mode, referred to as the intrinsic localized mode (ILM), in graphene is investigated based on precise numerical solutions obtained by the iteration method coupled with molecular dynamics simulations. We obtain ILMs for which the frequency is greater than the maximum frequency of the phonon bands of graphene. The amplitude and structure of ILMs indicate that these ILMs are simply vibration modes due to the nonlinearity of the system. Moreover stability analysis of ILMs in graphene is performed numerically by solving the eigenvalue problem of the monodromy matrix based on Floquet theory for periodic solutions. In all cases, the ILM was found to have unstable perturbation modes. The growth rate of the unstable perturbation modes exhibits complex change behavior as the period of ILMs changes. The structure and growth rate of the unstable perturbation modes can be classified into two types. The most unstable perturbation mode is also localized and provides cooperative motions of neighbor atoms of the ILM.

View full abstract

Defects in crystals such as surfaces, interfaces, and grain boundaries significantly affect mechanical responses of materials under deformation in microscopic scale. While locally-distributed energy and stress obtained by ab-initio method are considered to be useful tools for capturing the local effects of defects inside crystals, topological analysis of electron density proposed by Bader is another way to approach such local effects. Estimating the changes of electron density or its curvatures on some specific points such as bond midpoints, we can obtain the characteristics of the local electronic structure as well as by the partial density of states or the bond order. In this study, we have performed the topological analysis and the PDOS analysis on shear deformation of diamond crystals, carbon, Si, and Ge. Using the results, we conclude that the stress responses depend on the balance between the bonding strength and the angular dependence of sp3 configuration.

View full abstract

While ferroelectric perovskites are promising materials for various applications, stress-induced change in microscopic structure is believed to be a major culprit of functional deterioration. In this study, we investigate stress-induced motion of the 90° domain wall (DW) in PbTiO₃ by shell model atomistic model calculation. Firstly, we examine the validity of the shell model potential for deformation problems. We find a good agreement with an ab initio study in critical shear stress for DW motion at 0 K. Secondly, we consider the effect of simulation cell size to find significant effect on shear modulus and the critical shear stress of DW motion, which may indicate that DW has a long-range interaction due to induced electric field.

View full abstract

Twinning can become an important deformation mechanism in fcc metals once the crystal size is reduced to the sub-micron scale, e.g., in nanocrystals or nanowhiskers. The study of the twinning process, the interactions between propagating twins and between dislocations and twins is therefore important for a better understanding of the mechanical properties of metallic nanostructures. Here we show the results of atomistic simulations of defect-free nanowhiskers under tensile load using different EAM potentials for Au. The mechanisms of twin propagation and twin-stacking fault interaction are described and a modification to the criterion by Chen et al. [M. Chen et al., Science 300, 1275, 2003] for predicting twinning and its size dependence is presented.

View full abstract