Computer Simulations Of Grain Boundaries Research Articles

Zener Pinning of Grain Boundaries and Structural Stability of Immiscible Alloys

Immiscible Cu-Ta alloys produced by mechanical alloying are currently the subject of intensive research due to their mechanical strength combined with extraordinary structural stability at high temperatures. Previous experimental and simulation studies suggested that grain boundaries (GBs) in Cu-Ta alloys are stabilized by Ta nano-clusters coherent with the Cu matrix. To better understand the stabilization effect of Ta, we performed atomistic computer simulations of GB–cluster interactions in Cu-Ta alloys with various compositions and GB velocities. The study focuses on a single plane GB driven by an applied shear stress due to the shear-coupling effect. The results of the simulations are in close quantitative agreement with the Zener model of GB pinning. This agreement and the large magnitude of the unpinning stress confirm that the structural stability of these alloys is due to the drastically decreased GB mobility rather than a reduction in GB energy. For comparison, we simulated GB motion in a random solid solution. While the latter also reduces the GB mobility, the effect is not as strong as in the presence of Ta clusters. GB motion in the random solution itself induces precipitation of Ta clusters due to short-circuit diffusion of Ta in GBs, suggesting a possible mechanism of cluster formation inside the grains.

Thermodynamics of coherent interfaces under mechanical stresses. I. Theory

We present a thermodynamic theory of plane coherent solid-solid interfaces in multicomponent systems subject to nonhydrostatic mechanical stresses. The interstitial and substitutional chemical components are treated separately using chemical potentials and diffusion potentials, respectively. All interface excess quantities are derived using Cahns (1979) generalized excess method without resorting to geometric dividing surfaces. We present expressions for the interface free energy as an excess quantity and derive a generalized adsorption equation and an interface Gibbs-Helmholtz equation that does not contain the interface entropy. The interface stress tensor emerges naturally from the generalized adsorption equation as an appropriate excess over bulk stresses and is shown to be generally nonunique. Another interface property emerging from the generalized adsorption equation is the interface excess shear. This property is specific to coherent interfaces and represents the thermodynamic variable conjugate to the shear stress applied parallel to the interface. The theory reveals a number of Maxwell relations describing cross effects between thermal, chemical, and mechanical responses of coherent interfaces. In Part II of this work, this theory will be applied to atomistic computer simulations of grain boundaries.

Computer Simulation of Tilt Grain Boundaries in Alloys with L1<sub>2</sub> and B2 Superlattices

Computer simulation of grain boundaries (GB) was carried out in alloys Ni 3 Al with L1 2 superlattice and NiAl with B2 superlattice. This work studies simmetrical tilt grain boundaries Σ=5 [100](012) and Σ=5 [100](013) (special boundaries) with an axis of turn [001] and with angles of misorientation θ = 53.1° and 36.9° The atomic interaction has been described by Morse empirical central-force potentials. The atomic structure and energy of GB were investigated by means of construction of γ-surface using three relaxation techniques rigid two-dimensional relaxation in which the atoms at shift of one grain relatively another remain in sites of corresponding lattices, rigid three-dimensional relaxation and full atomic relaxation It is shown that Σ=5 tilt GBs in Ni 3 Al and NiAl have several steady states Value of the GB excess volume was calculated. Comparison of our results with geometrical model of coincidence site lattice (CSL) was carried out, GBs in model CSL are unstable, the stabilization is achieved by additional displacement on some vector along the plane of defect.

Computer Simulation of Interfaces in Ceramics

ABSTRACTThe methods for computer simulation of grain boundaries and hetero-interfaces in ceramics have long been available. We review developments in the field in two areas; the study of general grain boundaries and the simulation of hetero-interfaces. In both cases we discuss extrapolation from the symmetric case. We will also consider the validity of the continued use of classical potential models in an age of large-scale quantum calculations.

挿入原子法による結晶粒界計算

挿入原子法による結晶粒界計算

Coincidence site lattice misorientations of crystals in orthorhombic systems, with application to YBa 2Cu 3O 7

In recent years many new materials have been involved in grain boundary studies. Many of them, have rather complex crystal lattices with low symmetry. For experimental studies and computer simulations of grain boundaries some theoretical tools are necessary. The main tool is the Coincidence Site Lattice (CSL) model, which has been successfully used for structural characterization of grain boundaries in materials with relatively simple crystal structures, mostly cubic systems. In this paper, a method for determining coincidence rotations in orthorhombic systems is given. This method is based on the vector-quaternion algorithm which is very close to the quadruple algorithm developed by Grimmer and used.

Computer simulation of grain boundaries in Ni 3Al: The effect of grain boundary composition

The authors have performed atomistic simulations on three (001) symmetric tilt grain boundaries: 5(210), (310), and 13(320). Depending on which sub-lattice in each of the two grains is occupied by Al, the grain boundary may have different stoichiometries. All of the simulations show that the Al-rich grain boundaries have the highest grain boundary energies. Thus Al-rich grain boundaries are more likely to fail than those which have the bulk stoichiometry or are Ni-rich. This conclusion is consistent with the observed stoichiometry dependence of the beneficial boron effect. The similarity between the grain boundary energies (cohesive energies) of Ni/sub 3/Al and Ni and the much higher yield stress of Ni/sub 3/Al provides a justification for the ''inherent'' brittleness of Ni/sub 3/Al grain boundaries.

Computer simulation of grain boundaries in ionic crystals

The structures of grain boundaries in ionic crystals have been modelled using the Harwell MIDAS code, for the simulation of surfaces and interfaces. In addition, it is possible to calculate the interaction energies between point defects and grain boundaries. We discuss the computer codes which are used for the simulations and present examples of the calculated structures of a number of tilt and twist boundaries in NiO. The energies of vacancies and impurities close to these boundaries are also included.