Alumina Grain Boundaries Research Articles

Ab initio modeling of clean and Y-doped grain boundaries in alumina and intergranular glassy films (IGF) in β-Si3N4

Based on large and relaxed grain boundary (GB) models in alumina and intergranular glassy film (IGF) models in polycrystalline β-Si3N4, ab initio modeling and theoretical tensile experiments were carried out for both clean and Y-doped models. It is shown that the increased covalent bonding between Y and O or N through the participation of the Y-4d and Y-3p orbitals is the mechanism by which Y ions enhance the mechanical and elastic properties of the Y-doped GB and IGF models. In alumina, this explains the improved creep behavior in the presence of Y doping. Preliminary results on the electronic structure and bonding of a specific GB model (Σ37) in α-Al2O3 is presented. For the IGF models, the distribution patterns of Y ions in the glassy region were investigated by total energy calculations. Y ions prefer to be at the interfacial region between the IGF and bulk crystal. Defect-like states of different origin can be identified near the valence band and the conduction band edges. These theoretical predictions obtained from the calculation of the fundamental electronic structure of the materials can be used to derive local strain fields of dissimilar “particles” that may be linked to continuum level theories via finite element methods.

Excess Free Volume of High Angle Grain Boundaries in Aluminium

A new method is introduced to determine the absolute value of the boundary excess free volume. Along with the boundary energy the excess free volume belongs to the major thermodynamic properties of grain boundaries. The method utilizes the dependence of the contact angle at triple junctions of grain boundaries in Al-tricrystals on hydrostatic pressure. We investigated <111> tilt boundaries in the pressure range up to 14 kbar. In particular, for a 40° <111> tilt boundary with 2° twist component the boundary free volume was found to be equal to 5.03×10-11 m3/m2.

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.

Studying alumina boundary migration using combined microscopy techniques

Thermal grooving and migration of grain boundaries in alumina have been investigated using a variety of microscopy techniques. Using two different methods, polycrystalline alumina was used to investigate wet, (implying the presence of a glassy phase), and dry grain boundaries. In the first, single-crystal Al2O3 was hot-pressed via liquid phase sintering (LPS) to polycrystalline alumina with an anorthite glass film at the interface. Pulsed laser deposition was used to deposit approximately 100-nm thick glass films. Specimens were annealed in air at 1650°C for 20 h to induce boundary migration. Boundary characterization was carried out using visible light (VLM) and scanning electron (SEM) microscopies. Effects on migration due to surface orientation of grains were investigated using electron backscatter diffraction (EBSD). The second method dealt with heat treating dry boundaries in polycrystalline alumina to monitor boundary migration behavior via remnant thermal grooves. Heat treatments were conducted at 1650°C for 30 min. The same region of the sample was mapped using VLM and atomic force microscopy (AFM) and followed over a series of 30 min heat treatments. Boundary migration through a pore trapped inside the grain matrix was of particular interest.

Wetting and crystallization at grain boundaries: Origin of aluminum-induced crystallization of amorphous silicon

It has been shown experimentally that the grain boundaries in aluminium in contact with amorphous silicon are the necessary agents for initiation of the crystallization of silicon upon annealing temperatures as low as 438K. Thermodynamic analysis has shown (i) that Si can “wet” the Al grain boundaries due to the favorable Si∕Al interface energy as compared to the Al grain-boundary energy and (ii) that Si at the Al grain boundaries can maintain its amorphous state up to a thickness of about 1.0nm. Beyond that thickness crystalline Si develops at the Al grain boundaries.

Liquid Ga Penetration along Al Grain Boundaries: Effect of External Stress and Ga Undercooling

The effects of compressive stress and undercooling of Ga to liquid gallium penetration along grain boundaries (GBs) of aluminum were investigated. It was shown that the penetration rate does not change with the temperature if gallium is in liquid state. The effect of compressive stress applied to Al samples was demonstrated. The time to wetting of all aluminum GBs increased several orders of magnitude if the compressive stress was between 0.1 and 6 MPa. It was proved that solid-liquid transformation does not take place during penetration process.

Dynamic Instability in Intergranular Fracture

A molecular-dynamics model for crack propagation under steady-state conditions is used to study dynamic instabilities along a grain boundary in aluminum that occur when the crack speed approaches 1/3 of the material's Rayleigh wave speed. Instead of crack branching, as is characteristic for a crack propagating in a homogeneous environment, the instability of an intergranular crack results in a periodic series of dislocation bursts. These bursts limit the crack speed and produce velocity oscillations with a large increase in energy dissipation that increases the grain boundary toughness.

Energies of [001] small angle grain boundaries in aluminum

Energies of symmetric [001] tilt and twist small angle grain boundaries in aluminum have been examined as a function of misorientations. Boundary energies were evaluated relative to solid/liquid interfacial energies by a dihedral angle method on surface grooves. Energies of (100) s tilt boundaries with the Burgers vector of a[100] were rather smaller than those of (110) s tilt boundaries with a/2[110]. Energies of twist boundaries at θ < 4∘ were slightly larger than those of (110) s tilt boundaries. The energies were well explained as a function of misorientations by the Read-Shockley type equation. The equation also correctly described the difference in elastic energy factors between the (110) s tilt and twist boundaries. However, the term b in the equation may take a constant value independent of the Burgers vectors.

The Temperature Influence on the Faceting of Ʃ3 Grain Boundaries in Aluminium

The Temperature Influence on the Faceting of Ʃ3 Grain Boundaries in Aluminium

Drag effect of triple junctions on grain boundary and grain growth kinetics in aluminium

The motion and geometry of connected 2D grain boundary systems with triple junctions in aluminium was investigated in situ in an SEM with a specially designed heating stage. The results show that triple junctions can have a marked influence on grain boundary motion and grain growth kinetics. The grain area change with annealing time was recorded in situ in the SEM. An analysis of the experimental data reveals that there is no unique relationship between growth rate and the number n of grain sides (von Neumann–Mullins relation). This is attributed to the effect of triple junction drag on grain growth.

The influence of misorientation deviation on the faceting of Σ3 grain boundaries in aluminium

Abstract The faceting of two tilt grain boundaries (GBs) in 99.999 % wt. purity AMK1010231 bicrystals has been studied. The first GB is a coincidence twin Σ3 GB (Σ is the inverse density of c...

Ab initio theoretical tensile test on Y-doped Σ = 3 grain boundary in α-Al 2O 3

It has been known for a long time that Y ions segregate to the grain boundaries (GBs) in polycrystalline alumina with the beneficial effects of enhanced mechanical properties and increased creep resistance. No detailed microscopic theory exists to explain this so called “Y-effect”. We provide a quantum mechanical explanation for this effect through a series of carefully designed large-scale computations. The results of our theoretical tensile experiments show that the maximum stresses in pure crystalline α-Al 2O 3, undoped Σ = 3 GB, and Y-doped Σ = 3 GB models are, respectively 55, 31 and 39 GPa at the uni-axial strains of 17%, 14% and 16% in the direction perpendicular to the GB. The participation of the Y-4d and Y-4p orbitals enhances the covalent character of the Y–O bond, making it stronger than the Al–O bond it replaces. This could be the major reason for the improved mechanical properties of Y-doped α-Al 2O 3.

Remnant grooves on alumina surfaces

Thermal grooving of grain boundaries in alumina has been studied using a combination of visible-light microscopy (VLM) and atomic-force microscopy (AFM). The partial angles of grooves that develop during heat treatments at 1650 °C were estimated by AFM. The observation of remnant grooves is key to determining the magnitude of the movement of a grain boundary. Imaging using VLM and AFM enabled the smoothing of remnant grooves to be observed through a progression of heat treatments, which enables the anisotropic nature of the surface diffusivity of alumina to be monitored.

Effect of Triple Junctions on Grain Boundary and Grain Growth Kinetics in Aluminium - 10 ppm Magnesium Sheet

The motion and geometry of connected grain boundary systems with triple junctions in aluminium -10 ppm magnesium was investigated in-situ with a special designed SEM heating stage. The results show that triple junctions can have a marked influence on grain boundary motion. The grain area change with annealing time was from a hot stage in the SEM. An analysis of the experimental data reveals that there is no unique relationship between growth rate and the number n of grain sides (Von Neumann-Mullins relation). This is attributed to the effect of triple junction drag on grain growth.

Distribution of grain boundaries in aluminum as a function of five macroscopic parameters

The grain boundary character distribution in commercially pure Al has been measured as a function of lattice misorientation and boundary plane orientation. The results demonstrate a tendency to terminate grain boundaries on low index planes with relatively low surface energies and large interplanar spacings. The most frequently observed grain boundary plane orientation is (1 1 1). However, there are also instances where boundaries terminated by higher index planes have significant populations. For example, certain twist configurations on {1 1 w} planes, which correspond to symmetric [1 1 0] tilt boundaries, also have relatively high populations. The population of symmetric [1 1 0] tilt boundaries exhibits an inverse relationship with previously measured energies.

Stability of the chevron domain at triple-line reconstructions

The line of emergence of a grain boundary at the surface of a face centered cubic crystal may decompose into a nanoscale triangular prism with a chevronlike structure. This behavior has been found in gold, and we address here the generality of this phenomenon. The stability of the chevron structure is studied and discussed for the case of a special tilt grain boundary in copper, nickel, and aluminum. Atomistic simulations predict a chevron decomposition for Cu and Ni, but not for Al. The stability of Al against line decomposition was confirmed by atomic resolution electron microscopy. The distinct behavior of these crystals is related to their stacking fault energies.

Distribution of grain boundaries in aluminum as a function of five macroscopic parameters

The grain boundary character distribution in commercially pure Al has been measured as a function of lattice misorientation and boundary plane orientation. The results demonstrate a tendency to terminate grain boundaries on low index planes with relatively low surface energies and large interplanar spacings. The most frequently observed grain boundary plane orientation is (1 1 1). However, there are also instances where boundaries terminated by higher index planes have significant populations. For example, certain twist configurations on {1 1 w } planes, which correspond to symmetric [1 1 0] tilt boundaries, also have relatively high populations. The population of symmetric [1 1 0] tilt boundaries exhibits an inverse relationship with previously measured energies.

Kinked Grain Boundaries in Alumina Doped with Y2O3

Polycrystalline alumina specimens with and without MgO doping show smoothly curved grain boundaries after heat treatment at 1400°C indicating their rough structure. When heat-treated at 1400° and 1500°C for 24 h after packing in an alumina–YAG powder mixture, many grain boundaries (without any liquid phase) develop kinks of large and small scales as observed by scanning electron microscopy and transmission electron microscopy. The addition of Y2O3 at concentrations close to the solubility limit is thus shown to induce the grain boundary transition to singular structures.

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.

X-ray absorption fine-structure study on the fine structure of lutetium segregated at grain boundaries in fine-grained polycrystalline alumina

Local atomic structures around the Lu3+ ion in a 0.1 mol% LuO1.5-doped fine-grained Al2O3, in which the doped Lu3+ ions segregate to the grain boundaries, was characterized by Lu L3-edge X-ray absorption fine structure. Structural parameters in LuO1.5-doped Al2O3 were determined by a curve-fitting method, and the results showed that six O2− ions coordinate with the Lu ion in LuO1.5-doped Al2O3. In addition, it was also revealed that the Lu–O interatomic distance in Lu-doped Al2O3 was close to that in Lu2O3, which was about 19% longer than the Al–O interatomic distance in undoped Al2O3. The present results indicated that the local atomic structures around Lu in Al2O3 are close to that in Lu2O3. It is thus supposed that atomic distances between Al3+ and O2− ions in the vicinity of Lu-segregated grain boundaries are shortened in comparison with that in undoped Al2O3. A first-principles molecular orbital calculation was performed for the [Al2O9]−12 model cluster, and the shortening of the Al–O interatomic distance was found to have an effect of increasing the ionicity in Al3+ ions.