Tilt Grain Boundaries In Aluminum Research Articles

Hydrogen at symmetric tilt grain boundaries in aluminum: segregation energies and structural features

Aluminum is envisioned to be an important material in future hydrogen-based energy systems. Here we report an ab initio investigation on the interactions between H-atoms and common grain boundaries (GBs) of fcc Al: Σ9, Σ5, Σ11 and Σ3. We found that upon segregation to the GBs, single H-atoms can cause displacement of Al-atoms. Increasing their concentration revealed large cooperative effects between H-atoms that favor the segregation when other H-atoms are bound at neighboring sites. This makes these GBs able to accommodate high concentrations of H-atoms with considerable segregation energies per atom. Structural analyses derived from Laguerre–Voronoi tessellations show that these GBs have many interstitial sites with higher symmetry than the bulk tetrahedral interstitial site. Many of those sites have also large volumes and higher coordination numbers than the bulk sites. These factors are the increased driving force for H-atom segregation at the studied GBs in Al when compared to other metals. These GBs can accommodate a higher concentration of H-atoms which indicates a likely uniform distribution of H-atoms at GBs in the real material. This suggests that attempting to mitigate hydrogen uptake solely by controlling the occurrence of certain GBs may not be the most efficient strategy for Al.

Role of sessile disconnection dipoles in shear-coupled grain boundary migration

Shear-coupled grain boundary (GB) migration has been evidenced as an effective plastic mechanism in absence of the usual intragranular dislocation activity. GB migration occurs through the nucleation and further motion of disconnections. For perfect GBs, the activation barrier for migration is dominated by the disconnection nucleation. In this study, we examine the effects of a dipole of sessile disconnections on the shear-coupled GB migration using molecular dynamics simulations on a $\mathrm{\ensuremath{\Sigma}}41[001](540)$ symmetric tilt GB in aluminum. The first effect is observed on disconnection nucleation: we show that the disconnection dipole can operate as a source of mobile disconnections. The corresponding yield stress is weakly affected, but the GB migration energy barrier can be reduced by $35%$. The second effect is seen on the disconnection mobility: the sessile disconnection dipole impedes the disconnection motion by repulsing or attracting it. We conclude that the influence of such dipole on the GB migration favors the nucleation of mobile disconnections on one hand but slows down their motion on the other hand.

Molecular-dynamic investigation of the interaction of vacancies with symmetrical tilt grain boundaries in aluminum

The molecular-dynamic method has been used to study the interaction of lattice vacancies with symmetrical grain boundaries (GBs) in aluminum. The fraction of trapped vacancies has been found to depend linearly on the distance to the GB plane. The average velocity of the vacancy migration toward the boundary decreases exponentially with an increase in the distance between the GB plane and vacancy. The radius of the region of trapping of a vacancy by the boundary is limited to two to three lattice parameters and grows with an increase in temperature. Four types of boundaries, which are characterized by different capability for the trapping of vacancies, have been determined.

First-Principles Calculation of Grain Boundary Excess Volume and Free Volume in Nanocrystalline and Ultrafine-Grained Aluminum

We applied first-principles calculations to study the relationship between the grain boundary energy (GBE) and grain boundary excess free volume (BFV) at the tilt grain boundaries in aluminum. GBE increased linearly as the grain BFV increased and the value of the proportionality constant between them, i, was 13.8GPa for aluminum. The grain boundary elastic energies calculated on the basis of first principles using a dummy boundary as well as the classical elasticity theory were close to the grain boundary energies. We examined the free volume in nanocrystalline and ultrafine-grained materials and proposed a method for estimating GBE using the density of nanocrystalline and ultrafinegrained materials with the proportionality constant i. The GBE of nanocrystalline aluminum fabricated by ball milling and subsequent consolidation was estimated to be comparable to or lesser than that in coarse-grained aluminum. [doi:10.2320/matertrans.L-M2013816]

First-principles calculation of grain boundary energy and grain boundary excess free volume in aluminum: role of grain boundary elastic energy

We examined the grain boundary energy (GBE) and grain boundary excess free volume (BFV) by applying the first-principles calculation for six [110] symmetric tilt grain boundaries in aluminum to clarify the origin of GBE. The GBE increased linearly as BFV increased. The elastic energy associated with BFV, namely the grain boundary elastic energy, was estimated as a function of BFV and the shear modulus. The grain boundary elastic energies were close in value to the GBEs. The charge density distributions indicated that the bonding in the grain boundary region is significantly different from the bonding in the bulk. The grain boundary elastic energies were 15–32% higher than the GBEs. This overestimation of the grain boundary elastic energy is caused by the characteristics of the electronic bonding at the grain boundary, which is different from bonding in the bulk. We have concluded that GBE results mainly from the grain boundary elastic energy.

Solute Segregation at Σ11(113)[110] Grain Boundary and Effect of the Segregation on Grain Boundary Cohesion in Aluminum from First Principles

We investigate the energy of segregation of solute Ca at symmetric tilt grain boundary in aluminum from the first-principles calculations. As energy of segregation of Ca is negative, Ca atoms tend to segregate at the grain boundary. Furthermore, on basis of the Rice-Wang model, we study the effect of the segregation of Ca on the grain boundary embrittlement of aluminum. Our first-principles calculations of energies of segregation at grain boundary and free surface show that Ca behaves as embrittler.

Inclination dependence of grain boundary energy and its impact on the faceting and kinetics of tilt grain boundaries in aluminum

The shape evolution and migration of <1 0 0> and <1 1 1> tilt grain boundaries with rotation angles θ in the range between 6° and 24° were investigated in situ in a scanning electron microscope at elevated temperatures. The results revealed that boundaries with misorientation θ < 15° did not attain a continuously curved shape in the entire temperature range up to the melting point and, thus, did not move under a capillary driving force. Instead, they remained straight or formed several facets which were inclined to the initial boundary orientation. Molecular statics simulations suggest that the observed behavior of low-angle boundaries is due to the anisotropy of grain boundary energy with respect to boundary inclination. This anisotropy diminishes with increasing misorientation angle, and high-angle boundaries assume a continuously curved shape and move steadily under the curvature driving force.

First-Principles Studies on Grain Boundary Energies of [110] Tilt Grain Boundaries in Aluminum

The grain boundary structure and its energy are necessary for the fundamental understanding of the physical properties of materials. In aluminum, three distinct atomic structures of a Σ9(221)[110] tilt grain boundary have been reported in previous studies using atomistic simulations and a high-resolution transmission electron microscopy (HRTEM). In this work, we studied the atomic structure and energy of the Σ9 tilt grain boundary in aluminum using first-principles calculations. A comparison of the grain boundary energies among the three distinct Σ9 tilt grain boundaries determined through first-principles calculations allowed us to identify the most stable atomic structure of Σ9 tilt grain boundary in aluminum.

Faceting of low-angle 〈1 0 0〉 tilt grain boundaries in aluminum

The faceting of symmetrical 〈1 0 0〉 tilt grain boundaries with rotation angles 8.4°, 12.0°, 14.3° and 16.0° was investigated in situ in a hot-stage scanning electron microscope at elevated temperatures. The observed transition “faceted-curved boundary” was found to depend on misorientation angle and temperature. This behavior is attributed to the anisotropy of grain boundary energy with respect to boundary inclination.

806 第一原理計算によるアルミニウム[110]傾角粒界の粒界エネルギーと粒界すべりの関係(OS08.電子・原子・マルチシミュレーションに基づく材料特性評価(2))

806 第一原理計算によるアルミニウム[110]傾角粒界の粒界エネルギーと粒界すべりの関係(OS08.電子・原子・マルチシミュレーションに基づく材料特性評価(2))

Molecular-dynamics simulation-based cohesive zone representation of intergranular fracture processes in aluminum

A traction–displacement relationship that may be embedded into a cohesive zone model for microscale problems of intergranular fracture is extracted from atomistic molecular-dynamics (MD) simulations. An MD model for crack propagation under steady-state conditions is developed to analyze intergranular fracture along a flat Σ99 [1 1 0] symmetric tilt grain boundary in aluminum. Under hydrostatic tensile load, the simulation reveals asymmetric crack propagation in the two opposite directions along the grain boundary. In one direction, the crack propagates in a brittle manner by cleavage with very little or no dislocation emission, and in the other direction, the propagation is ductile through the mechanism of deformation twinning. This behavior is consistent with the Rice criterion for cleavage vs. dislocation blunting transition at the crack tip. The preference for twinning to dislocation slip is in agreement with the predictions of the Tadmor and Hai criterion. A comparison with finite element calculations shows that while the stress field around the brittle crack tip follows the expected elastic solution for the given boundary conditions of the model, the stress field around the twinning crack tip has a strong plastic contribution. Through the definition of a Cohesive-Zone-Volume-Element—an atomistic analog to a continuum cohesive zone model element—the results from the MD simulation are recast to obtain an average continuum traction–displacement relationship to represent cohesive zone interaction along a characteristic length of the grain boundary interface for the cases of ductile and brittle decohesion.

Copper Segregation to the 5 (310)/[001] Symmetric Tilt Grain Boundary in Aluminum

New insight into the atomic segregation of copper to an aluminum grain boundary has been obtained using multiple, complementary atomic resolution electron microscopy techniques coupled with ab-initio electronic structure calculations. The copper segregation is site specific and changes the structure of the boundary by occupying interstitial sites. Minor elemental constituents in materials can have profound effects on their engineering performance. This change in structure can be associated with these strong effects. The observed structural change will alter the mass transport behavior of the boundary and has implications for the understanding of electromigration mechanisms.

Internal atomic stress near Σ5 tilt grain boundary in aluminium under tension

It is important to clarify the mechanical properties of inhomogeneous structures from the viewpoint of atomic scale to understand the mechanical behaviour of microscopic materials. Thus, the mechanical properties of local regions with inhomogeneous and non-uniform structure have been intensively investigated on the basis of the atomic stress and the elastic constant. In this study, in order to examine the mechanical behaviour of local regions under external load, we conduct atomic tensile simulations on a Σ5-(013) tilt grain boundary in aluminium using the many-body effective medium theory potential function. The structure of the grain boundary consists of two types of atomic layers, one of which has a higher elastic constant near the grain boundary and the other has a lower one. Although the grain boundary consists of identical atomic species, the local region near the grain boundary displays the mechanical properties similar to those of a composite material.

Atomic Stress and Elastic Coefficient Caused by Grain Boundary Structure.

Mechanical property of materials is intensively dependent on their atomic structures. In regions with disarranged atomic structures such as grain boundaries, the property can be much different from that in the single crystal. In this study, atomic simulations of a Σ5 tilt grain boundary in aluminum are conducted to elucidate distribution of residual stress and local elastic coefficient near the grain boundary. Local stress and elastic coefficient within the range of 0.7 nm from the grain boundary differ from those in grain. The grain boundary consists of two kinds of atomic layers with different elastic coefficient, and this causes the characteristic local stresses.

Interaction of vacancies with a grain boundary in aluminum: A first-principles study

We present a theoretical study of the interaction of vacancies with a tilt grain boundary in aluminum based on the density functional theory. The grain boundary volume expansion and vacancy induced contraction are calculated and compared for the nearest-neighbor atoms from the defects, and the former is found to be smaller than the latter. The formation energy of a vacancy placed at various layers in the grain boundary has been calculated and we find that the grain boundary does not always act as sinks for vacancies. In fact, it costs more energy to form a vacancy at the boundary plane than in the bulk, although the rest of the grain boundary region does attract vacancies. The microscopic mechanisms of grain boundary sliding and migration are investigated thoroughly with and without a vacancy. We find that although the vacancy can hinder the grain boundary motion by tripling the energy barrier of sliding and migration, it can not inhibit or even delay the migration process. The vacancy placed at the first layer from the interface is found to be trapped at the layer and not able to follow the migrating interface.

Molecular dynamics study on the structural changes of the tilt grain boundary in aluminium

Abstract The structure and physical properties of two Al tilt grain boundaries (GBs) at different temperatures have been studied using the molecular dynamics method. We find that, with increasing temperature, the GB structure changes by steps. Three characteristic temperatures T 0, T 1 and T 2 have been identified for the changes in the GB structure. A comparison with available experimental data has also been made.

Molecular dynamics study on the structural changes of the tilt grain boundary in aluminium

Molecular dynamics study on the structural changes of the tilt grain boundary in aluminium

Structural Study of a [100] 45° Twist Plus 7.5° Tilt Grain Boundary in Aluminium by HREM

AbstractA [100] 45° twist plus 7.5° tilt grain boundary in aluminium prepared by cold rolling and annealing has been studied by high-resolution electron microscopy. The direct interpretability of the image features in terms of atomic column positions allows structural models of the grain boundary to be developed. The boundary exibits a high concentration of steps due to the 7.5° tilt from a perfect [100] 45° quasiperiodic misorientation. Occurrence of co-incidence and pseudo co-incidence of atomic planes across the interface appears to play an important role in the formation of steps along this boundary. Local relaxation of atoms resulting from the perturbation of the [100] 45° twist bi-crystal determines the boundary structure

Investigation of Copper Segregation to the Σ5(310)/[001] Symmetric Tilt Grain Boundary in Aluminum

AbstractThe Σ5 (31O)/[001] symmetric tilt grain boundary (STGB) in the face centered cubic (FCC) metal aluminum with 1at% copper has been studied. The model grain boundary has been fabricated by ultra-high vacuum diffusion bonding of alloy single crystals. The segregation of the copper has been encouraged by annealing the sample after bonding at 200 °C. TEM samples of this FCCmaterial were prepared with a new low voltage ion mill under very low angles.The atomic structure of the Σ5(310)/[001] STGB for this system was modeled with electronic structure calculations. These theoretical calculations of the interface structure indicate that the Cu atoms segregate to distinct sites at the interface. High resolution electron microscopy (HRTEM) and analytical electron microscopy including electron energy spectroscopic imaging and X-ray energy dispersive spectrometry have been used to explore the segregation to the grain boundary. The HRTEM images and the analytical measurements were performed using different kinds of microscopes, including a Philips CM300 FEG equipped with an imaging energy filter. The amount of the segregated species at the interface was quantified in a preliminary way. To determine the atomic positions of the segregated atoms at the interface, HRTEM coupled with image simulation and a first attempt of a holographic reconstruction from a through-focal series have been used.

Analysis of Structure and Properties of .SIGMA.9 Tilt Grain Boundary in Aluminum by ab initio Molecular Dynamics.

In order to understand precisely the structure and properties of interface in solids, it is important to elucidate the disordered atomic configuration on the basis of quantum mechanics. In this study, the structure and energy of Σ9 tilt grain boundary in aluminum is analyzed by ab initio molecular dynamics simulation based on the density functional theory. The results obtained are summarized as follows. (1) The grain boundary has a pair of atoms which are close to each other, and one of them moves along the grain boundary during relaxation. (2) The maximum valence charge density near the grain boundary is 0.03 a.u.-3, which is nearly equal to that in single crystal. (3) The grain boundary energy is 0.431 J/m2, which is smaller than that of Σ5 tilt grain boundary, 0.458 J/m2. Moreover, the validity of Morse potential and EMT (Effective Medium Theory) potential is examined by comparing the atomic force and energy with those obtained in ab initio simulation. (1) The grain boundary structure and the force acting on each atom evaluated by EMT are similar to those in ab initio simulation. However, the grain boundary energy obtained by EMT potential is much smaller than that obtained by ab initio simulation. (2) Morse potential does not give adequate values of atomic force as well as grain boundary energy.