Influence of symmetric tilt grain boundaries and/or Cr-rich α′ precipitates on irradiation damage in Fe-Cr-Al alloys: A molecular dynamics investigation

Effect of symmetrical and asymmetrical tilt grain boundaries with <001> as the tilt axis on the shock response of bi-crystalline Ni

Symmetrical Σ7 tilt grain boundaries of alumina (Al2O3) were studied using bicrystals. Three types of Σ7 boundaries were successfully fabricated, that is, rhombohedral twin (Σ7{1[Onemacr]02}) and two types of [0001] symmetrical tilt grain boundaries with grain‐boundary planes {4[Fivemacr]10} and {2[Threemacr]10} (Σ7{4[Fivemacr]10} and Σ7{2[Threemacr]10}). Their atomic structures and grain‐boundary energies were investigated using high‐resolution transmission electron microscopy (HRTEM) and a thermal grooving technique, respectively. HRTEM observations showed that the Σ7{1[Onemacr]02} boundary had a completely symmetrical atomic arrangement with respect to the grain‐boundary plane. In contrast, Σ7{2[Threemacr]10} and Σ7{4[Fivemacr]10} boundaries exhibited asymmetrical atomic structures, which were confirmed by analyzing the atomic configurations using static lattice calculations. Thermal grooving experiments showed that the grain‐boundary energies strongly depended on the properties of the grain‐boundary planes.

The results of calculations of the atomic and electron structure of Pd and TiFe with symmetrical Σ5 tilt grain boundaries obtained using the methods of electron density functional theory are reported. Hydrogen sorption at tilt grain boundaries and corresponding surfaces is considered. It is shown that the hydrogen absorption energy increases in magnitude by ∼0.2 eV at the Pd Σ5(210) grain boundary and by ∼0.5 eV in B2-TiFe with the Σ5(310) grain boundary. The binding energy of hydrogen in palladium, as well as in TiFe, in the most preferred positions at the surface is higher than near grain boundaries. It is found that, as in the case of a defect-free material, the following tendency is observed at a symmetrical tilt grain boundary: the strong bond of the impurity at the grain boundary in the metal or alloy matrix reduces the sorption energy of hydrogen.

Insights into the interfacial thermal transport properties of in-plane graphene/h-BN heterostructure with grain boundary

Tilt grain boundaries energy and structure in NiTi alloys

To obtain a fundamental understanding of the effect of structure and geometry of grain boundary on the diffusion kinetics in nanocrystalline materials, the influence of grain boundary misorientation on the effective diffusion coefficient (apparent diffusivity) in nanocrystalline aluminum was investigated using molecular dynamics simulations. Nine series of [001] symmetric tilt grain boundaries, including high and low symmetric boundary planes, were studied. The apparent diffusivity in the samples was calculated in the temperature range from 423 K to 823 K by monitoring the mean square displacement of atoms as a function of simulation time. A temperature dependence of the effective diffusion coefficient according to the Arrhenius law was obtained for all samples. It is found that the apparent diffusivity is anisotropic and it is a strong function of grain boundary misorientation at low and high temperatures. At all temperatures, Σ29 [001]/(520) symmetric tilt grain boundary with misorientation angle of 43.68° exhibits the highest effective diffusion coefficient among the investigated grain boundaries. The simulation results show that the activation energy and pre-exponential factor are affected significantly by the grain boundary misorientation angle. Moreover, the results indicated that the misorientation dependence of activation energy for diffusion exhibits two local maxima, which correspond to two symmetric tilt grain boundaries. Additional calculation of misorientation dependence of the pre-exponential factor shows two local minima at the same symmetric tilt grain boundaries. The misorientation dependence of the effective diffusion coefficient was explained on the basis of grain boundary energy and the crystallographic structure of grain boundary.

The grain boundary energies (GBEs) of symmetric tilt grain boundaries (STGBs) and asymmetric tilt grain boundaries (ATGBs) for W at 0 and 2400 K and β-Ti at 1300 K were calculated by means of molecular statics and dynamics simulations to investigate the effects of temperature on GBE and the relationships between GBEs and grain boundary (GB) planes. Generally, the variation trends of GBE with the tilt angle are similar for the three cases, when the tilt axis is specified. It is of course that these similarities result from their similar GB microstructures in most cases. However, the variation trends of GBE with tilt angle are somewhat different between β-Ti at 1300 K and W at 2400 K for STGBs with <100> and <110> tilt axes. This difference mainly stems from the following two reasons: firstly, the GB microstructures of W at 2400 K and β-Ti at 1300 K are different for some STGBs; secondly, the atoms at the STGB of β-Ti at 1300 K tend to evolve into the local ω- or α-like structures distributed at the STGB for some STGBs with <110> tilt axis, which makes the corresponding STGBs more stable, thereby decreasing the GBEs. Furthermore, a geometric parameter θ, the angle between the misorientation axis and the GB plane, was defined to explore the relationships between GBEs and GB planes. It was found that the relationships between GBEs and GB planes can be described by some simple functions of sin(θ) for the GBs with definite lattice misorientation, which can well explain and predict the preferred GB planes for the GBs having the same lattice misorientation. Our calculations not only extend the investigation of GBs to higher temperature, but also deepen the understanding on the temperature contributions to the microstructure evolution at GBs and on the relationships between GBEs and possible geometric parameters.

Intergranular corrosion of a curved ∑=5 grain boundary in an FeSi alloy bicrystal

The virial stress is the most commonly used definition of stress in discrete particle systems. This quantity includes two parts. The first part depends on the mass and velocity (or, in some versions, the fluctuation part of the velocity) of atomic particles, reflecting an assertion that mass transfer causes mechanical stress to be applied on stationary spatial surfaces external to an atomic‐particle system. The second part depends on interatomic forces and atomic positions, providing a continuum measure for the internal mechanical interactions between particles. Historic derivations of the virial stress include generalization from the virial theorem of Clausius (1870) for gas pressure and solution of the spatial equation of balance of momentum. The virial stress is stress‐like a measure for momentum change in space. This paper shows that, contrary to the generally accepted view, the virial stress is not a measure for mechanical force between material points and cannot be regarded as a measure for mechanical stress in any sense. The lack of physical significance is both at the individual atom level in a time‐resolved sense and at the system level in a statistical sense. It is demonstrated that the interatomic force term alone is a valid stress measure and can be identified with the Cauchy stress. The proof in this paper consists of two parts. First, for the simple conditions of rigid translation, uniform tension and tension with thermal oscillations, the virial stress yields clearly erroneous interpretations of stress. Second, the conceptual flaw in the generalization from the virial theorem for gas pressure to stress and the confusion over spatial and material equations of balance of momentum in theoretical derivations of the virial stress that led to its erroneous acceptance as the Cauchy stress are pointed out. Interpretation of the virial stress as a measure for mechanical force violates balance of momentum and is inconsistent with the basic definition of stress. The versions of the virial‐stress formula that involve total particle velocity and the thermal fluctuation part of the velocity are demonstrated to be measures of spatial momentum flow relative to, respectively, a fixed reference frame and a moving frame with a velocity equal to the part of particle velocity not included in the virial formula. To further illustrate the irrelevance of mass transfer to the evaluation of stress, an equivalent continuum (EC) for dynamically deforming atomistic particle systems is defined. The equivalence of the continuum to discrete atomic systems includes (i) preservation of linear and angular momenta, (ii) conservation of internal, external and inertial work rates, and (iii) conservation of mass. This equivalence allows fields of work‐ and momentum‐preserving Cauchy stress, surface traction, body force and deformation to be determined. The resulting stress field depends only on interatomic forces, providing an independent proof that as a measure for internal material interaction stress is independent of kinetic energy or mass transfer.

Atomistic simulations based on energy minimisation method were employed to compute the structural and defect properties of the symmetric and asymmetric Ni tilt grain boundaries (GBs). The GB structures have been investigated in terms of global GB metrics (GB energy, excess volume) and at the atomic-scale analysis (atomic site energy, binding energy and displacement field of vacancies). The GB properties are treated by the notion of the plane inclination angle between the two symmetric tilt grain boundaries: coherent twin boundary and symmetric incoherent twin boundary configurations. We observed correlations: (i) between the GB energy and the net expansion at the boundary and (ii) between the vacancy properties and GB energetics. We identified that the GB sink efficiency character, which reflects the defect absorption capacity, can be influenced by the GB energy. The minimum defect formation energy in each GB tends to decrease with increasing grain boundary energy. In addition, energetic and structural analyses are linked together to characterise vacancies segregation as a function of the defect location within the GB interface. Our results show a non-symmetric trend especially near the GB plane between the vacancy binding energy and the displacement field generated around the vacancy core. However, the distribution between the binding energy and the displacement field of a vacancy permits to identify whether the atomic relaxations around the GB-defect site are isotropic or anisotropic.

The channeling effect of symmetrical tilt grain boundaries on helium bubbles in tungsten

We demonstrate that, based on their unique geometry, the migration of all symmetrical and certain asymmetrical tilt grain boundaries is necessarily accompanied by sliding parallel to the interface. By contrast, for all other types of grain boundaries no crystallographic necessity exists for migration to be coupled with sliding. Except in the case of the coherent (111) twin boundary in the fcc lattice, the coherently-twinned translational configuration is identified as the saddle-point configuration for the migration of the symmetrical tilt boundaries.

We present the results of a parallel study of the atomic structure of the Σ = 5 (210) [001] symmetric tilt grain boundary in bcc Mo, by computer simulation and High Resolution Electron Microscopy (HREM). Excess energy values of different boundary configurations are obtained via a quasi-dynamic minimisation scheme while cohesion is described by a new n-body, central-force, phenomenological potential which satisfactorily reproduces static and dynamic properties of the bulk material. HREM observations and numerical modelling both show the symmetric configuration of the Σ = 5 (210) [001] symmetric tilt grain boundary (GB) to be of lowest energy.

We apply self-consistent-field theory to T junctions and symmetric tilt grain boundaries in block copolymer systems with and without the addition of homopolymer. We find that, in the absence of homopolymer, T junctions have a larger free energy per unit area than that of the symmetric tilt junctions with which they compete except for a range of angles between about 100° and 130°. With the addition of homopolymer, this range increases. These results are quite consistent with experiment. As the angle between grains increases towards 180°, the T junction undergoes a morphological change somewhat similar to that which occurs in symmetric tilt grain boundaries. At the onset of this change, the free energy per unit area decreases markedly.

Quantifying and connecting atomic and crystallographic grain boundary structure using local environment representation and dimensionality reduction techniques