Alumina Grain Boundaries Research Articles

General trends between solute segregation tendency and grain boundary character in aluminum - An ab inito study

Quantum mechanical calculations have been performed to establish general trends in propensity for the segregation of solutes across the periodic table at or in the neighborhood of an extended set of commonly observed special grain boundaries in face centered cubic aluminium. To this end, Al has been considered as the matrix and elements from 3d and 4d transition metals as well as those from group II, III and IV have been selected as solute atoms. For transition metal solutes, we find a concave-up parabolic-like dependency of segregation energy as a function of atomic number that is argued to be caused by the competition between chemical bonding and atomic size effects. The analysis is corroborated quantitatively by the computation of crystal orbital Hamiltonian population for solute-Al and Al-Al pairs as well as the Voronoi polyhedral surrounding solutes at a sample GB. The parabolic-like (concave-down trend) dependency of the cohesiveness of grain boundaries is explained by an equivalent trend in the bonding strength of Al-Al pairs at the segregated GBs. We extend this investigation to examine the stability of the solid solution polycrystalline state by comparing the calculated segregation energy against the combined energetic cost of grain boundary and intermetallic precipitate formation. The results may serve as a design tool for tailoring polycrystalline alloys with desired properties.

Dislocation Structures in Low-Angle Grain Boundaries of α-Al2O3

Alumina (α-Al2O3) is one of the representative high-temperature structural materials. Dislocations in alumina play an important role in its plastic deformation, and they have attracted much attention for many years. However, little is known about their core atomic structures, with a few exceptions, because of lack of experimental observations at the atomic level. Low-angle grain boundaries are known to consist of an array of dislocations, and they are useful to compose dislocation structures. So far, we have systematically fabricated several types of alumina bicrystals with a low-angle grain boundary and characterized the dislocation structures by transmission electron microscopy (TEM). Here, we review the dislocation structures in { 11 2 ¯ 0 } / [ 0001 ] , { 11 2 ¯ 0 } / 〈 1 1 ¯ 00 〉 , { 1 1 ¯ 00 } / 〈 11 2 ¯ 0 〉 , ( 0001 ) / 〈 1 1 ¯ 00 〉 , { 1 ¯ 104 } / 〈 11 2 ¯ 0 〉 , and ( 0001 ) / [ 0001 ] low-angle grain boundaries of alumina. Our observations revealed the core atomic structures of b = 1 / 3 〈 11 2 ¯ 0 〉 edge and screw dislocations, 〈 1 1 ¯ 00 〉 edge dislocation, and 1 / 3 〈 1 ¯ 101 〉 edge and mixed dislocations. Moreover, the stacking faults on { 11 2 ¯ 0 } , { 1 1 ¯ 00 } , and ( 0001 ) planes formed due to the dissociation reaction of the dislocations are discussed, focusing on their atomic structure and formation energy.

Dissociation reaction of the 1/3$$ \left\langle {\bar{1}101} \right\rangle $$ edge dislocation in α-Al2O3

It has been reported that dislocations with 1/3 $$ \left\langle {\bar{1}101} \right\rangle $$ edge component of the Burgers vector are formed in {1 $$ \bar{1} $$ 04}/ $$ \left\langle {11\bar{2}0} \right\rangle $$ low-angle grain boundaries of alumina (α-Al2O3). These dislocations dissociate into two partial dislocations with a stacking fault on the (0001) plane (Tochigi et al. in J Mater Sci 46:4428–4433, 2011). However, the dissociation reaction of these dislocations has not been determined so far. In this study, the structures of the dissociated dislocations and the (0001) stacking fault were investigated by transmission electron microscopy and theoretical calculations. It was revealed that the dissociated dislocations were generated from the 1/3 $$ \left\langle {\bar{1}101} \right\rangle $$ perfect edge dislocation by the reaction of 1/3 $$ \left\langle {\bar{1}101} \right\rangle $$ → 1/18 $$ \left\langle {\bar{4}223} \right\rangle $$ + 1/18 $$ \left\langle {\bar{2}4\bar{2}3} \right\rangle $$ . Furthermore, electron energy loss spectroscopy analysis was performed to examine the atomic/electronic structure of the (0001) stacking fault. In the observed spectra, a chemical shift and intensity decrease were found at the oxygen K-edge. Theoretical spectrum analysis using first-principles calculations revealed that the characteristic features of the spectra are originated from the local atomic configurations of the (0001) stacking fault.

Carbon network/aluminum composite made by powder metallurgy and its corrosion behavior in seawater

In this work, a carbon network/aluminum matrix composite material was made by the powder metallurgical approach. Pure aluminum powders with a diameter of less than 30 μm were compressed with sugar particles with the similar size under a pressure of 40 MPa. Sintering at 500 °C in pure argon gas for 2 h was performed to carbonize the sugar. After the sintered product was cooled down, microstructure analysis was carried out. It was found that high temperature sintering allowed the material to generate carbon networks distributed along the grain boundaries in the porous aluminum. An aluminum to sugar ratio of 1 to 1 reached the maximum carbon concentration in the composite for keeping the structural integrity. Characterization of the corrosion behaviors of the carbon network/porous aluminum composite material and a pure aluminum plate in seawater was conducted by measuring their Tafel plots. The Galvanic potential of the carbon network/porous aluminum composite material is about -0.62 V. The pure aluminum shows a more negative Galvanic potential of -0.755 V. As compared with pure aluminum, the composite material showed improvement on the corrosion property. The corrosion current of the composite in seawater is about 100 times lower than that of the pure aluminum. The existence of carbon network in the composite and the oxidized state of the sintered aluminum allows the composite material to be more corrosion resistant in seawater. For the materials processed with higher sugar content (with Al: sugar ratio of 1:1.25), carbon became the dominant phase in the composite. The material has a low strength and ease to get collapsed. The corrosion behavior of the composite made from low Al to sugar ratio raw material is similar to that of the one with Al: sugar ratio of 1:1. For the high carbon content composite material with Al: sugar ratio of 1:1.25, the Galvanic potential of the material is as high as -0.32 V, which is very close to the Galvanic potential of graphite in seawater. It shows the corrosion behavior of pure carbon in seawater.

Oxygen permeation mechanism in polycrystalline mullite at high temperatures

AbstractThe oxygen permeability of polycrystalline mullite wafers, serving as a model environmental barrier coating layer on SiC fiber‐reinforced SiC matrix composites, was evaluated at temperatures above 1673 h with an oxygen tracer gas (18O2). Oxygen permeation occurred by grain‐boundary (GB) diffusion of oxygen from the high oxygen partial pressure (high‐Po2) surface to the low‐Po2 surface, with simultaneous GB diffusion of aluminum in the opposite direction. This GB interdiffusion of both oxygen and aluminum proceeded without acceleration or retardation, maintaining the Gibbs‐Duhem relationship. Oxygen permeation related to the GB diffusion of silicon was negligibly small compared to that generated by aluminum GB diffusion, resulting in decomposition of the mullite near the low‐Po2 surface. The GB diffusion coefficients for oxygen in the vicinity of the high‐Po2 surface were determined directly from the SIMS‐18O line profiles along individual GBs, as assessed from cross sections of the exposed wafer. The coefficients thus obtained were comparable to those determined in the absence of an oxygen potential gradient and those calculated from an oxygen permeation trial under the assumption of nearly ionic conductivity.

Grain Boundary Chemistry and Transport Through Alumina Scales on NiAl Alloys

It is widely accepted that the growth of protective α-Al2O3 scales on Ni-based alloys is governed by the inward diffusion of oxygen through the oxide grain boundaries (GB). However, there is also some outward diffusion of metal ions to the surface, but it is difficult to quantify. In this work we apply atomic force microscopy, scanning electron microscopy and transmission electron microscopy to investigate the outward flux of Al, which manifests as the growth of small ridges along the alumina GBs after the removal of the outermost oxide layer by mechanical polishing or focused ion beam techniques followed by additional oxidation. As a model alumina-former, NiAl with Hf and Zr additions was investigated. In comparison to Zr, Hf was found to reduce the outward Al diffusion. This outward diffusion was six orders of magnitude smaller than the O inward diffusion.

Atomic simulations of twist grain boundary structures and deformation behaviors in aluminum

The structures and behaviors of grain boundaries (GBs) have profound effects on the mechanical properties of polycrystalline materials. In this paper, twist GBs in aluminum were investigated with molecular dynamic simulations to reveal their atomic structures, energy and interactions with dislocations. One hundred twenty-six twist GBs were studied, and the energy of all these twist GBs were calculated. The result indicates that <001> and <111> twist GBs have lower energy than <101> twist GBs because of their higher interplanar spacing. In addition, 12 types of <001> twist GBs in aluminum were chosen to explore the deformation behaviors. Low angle twist GBs with high density of network structures can resist greater tension because mutually hindering behaviors between partial dislocations increase the twist GB strength.

Atomistic migration mechanisms of atomically flat, stepped, and kinked grain boundaries

We studied the migration behavior of mixed tilt and twist grain boundaries in the vicinity of a symmetric tilt $\ensuremath{\langle}111\ensuremath{\rangle} \mathrm{\ensuremath{\Sigma}}7$ grain boundary in aluminum. We show that these grain boundaries fall into two main categories of stepped and kinked grain boundaries around the atomically flat symmetric tilt boundary. Using these structures together with size converged molecular dynamics simulations and investigating snapshots of the boundaries during migration, we obtain an intuitive and quantitative description of the kinetic and atomistic mechanisms of the migration of general mixed grain boundaries. This description is closely related to well-known concepts in surface growth such as step and kink-flow mechanisms and allows us to derive analytical kinetic models that explain the dependence of the migration barrier on the driving force. Using this insight we are able to extract energy barrier data for the experimentally relevant case of vanishing driving forces that are not accessible from direct molecular dynamics simulations and to classify arbitrary boundaries based on their mesoscopic structures.

A simple faceting model for the interfacial and cleavage energies of [formula omitted] grain boundaries in the complete boundary plane orientation space

Interfacial energies play a crucial role in the evolution of polycrystalline microstructures both in structural and functional materials. From a crystallographic perspective, the energy landscape depends both on the misorientation and the boundary-plane orientation of grain boundaries (GBs). Traditionally, however, GB structure–property relationships have been investigated primarily for interfaces with twist or tilt character and with an emphasis on the role of misorientation. In this article, we introduce an automated routine for simulating the minimum energy structures for general GBs. The important role of the boundary-plane orientation is elucidated by simulating 297 distinct Aluminum Σ3 GBs that adequately sample the fundamental zone of the plane orientation space. It is observed that while the energy varies significantly as a function of the boundary-plane orientation, the variation is also smooth and without cusps in the fundamental zone. In this article, a simple energy function, motivated by the two-dimensional faceting model for interface structures, is proposed for the free-surfaces and the Σ3 GBs in Aluminum. The energy function consists of only three fitting parameters and predicts the energy landscape surprisingly well. It is anticipated that this model may be extended to represent the energies of GBs corresponding to higher Σ-misorientations. Finally, as an application of the faceting model for interfaces, the theoretical cleavage energies for Σ3 GBs in Aluminum have been computed. It has been observed that, contrary to conventional wisdom, the Σ3(111) twin boundary does not exhibit the highest cleavage energy and these results are expected to have consequences for grain boundary engineering.

Mechanical responses, texture and microstructural evolution of high purity aluminum deformed by equal channel angular pressing

Ultrafine-grained (UFG) high purity aluminum exhibits a variety of attractive mechanical properties and special deformation behavior. Equal channel angular pressing (ECAP) process can be used to easily and effectively refine metals. The microstructure and microtexture evolutions and grain boundary characteristics of the high purity aluminum (99.998%) processed by ECAP at room temperature are investigated by means of TEM and EBSD. The results indicate that the shear deformation resistance increases with repeated EACP passes, and equiaxed grains with an average size of 0.9 μm in diameter are formed after five passes. Although the orientations distribution of grains tends to evolve toward random orientations, and microtextures (80°, 35°, 0°), (40°, 75°, 45°) and (0°, 85°, 45°) peak in the sample after five passes. The grain boundaries in UFG aluminum are high-angle geometrically necessary boundaries. It is suggested that the continuous dynamic recrystallization is responsible for the formation of ultrafine grains in high purity aluminum. Microstructure evolution in the high purity aluminum during ECAP is proposed.

Atomic dynamics of grain boundaries in bulk nanocrystalline aluminium: A molecular dynamics simulation study

Dynamics of grain boundary (GB) atoms in the bulk nanocrystalline aluminium is investigated by means of a large-scale molecular dynamics simulation. It is found that the GB atoms in the nanocrystalline aluminium display the glassy dynamics. Their dynamic features are, on one hand, similar to those in the glass systems and bicrystal GBs, in terms of the time-correlation functions of mean-square displacement (MSD) and non-Gaussian parameter (NGP). But on the other hand, these GB atoms also demonstrate some different dynamic behaviors due to the more complicated microstructure. In particular, the immobile GB atoms are localized into their equilibrium positions due to the strong cage effect, and they are mainly at the surfaces of grains, especially the large grains, while the mobile GB atoms are active and can easily hop away from the cage around them. These mobile GB atoms gather together at the surfaces of small grains and the triple junction regions of GBs, and they form some abnormally big clusters. The size distribution of these clusters significantly deviates from the power law.

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.

Toward Knowledge‐Based Grain‐Boundary Engineering of Transparent Alumina Combining Advanced TEM and Atomistic Modeling

Transition element dopants (e.g., Y, La) are commonly used as sintering aids in polycrystalline alumina ceramics, which segregate to the grain boundaries and control the grain‐boundary mobility. However, due to the extremely thin (<2 nm) layer of segregated dopants, the experimental characterization of the segregated alumina grain boundaries is a complex task. Computational studies have focused only on tilt grain boundaries, which are only a small fraction in a sintered alumina sample. In this study, a quantitative characterization of the segregation of Y and La at general high angle grain boundaries in transparent alumina is carried out using a unique combination of advanced TEM and near coincidence grain‐boundary atomistic simulations. The result show that high angle grain boundaries may lead to enhanced grain growth in comparison to symmetric tilt twin grain boundaries due to the reduced configuration entropy for dopant segregation and higher order grain‐boundary complexions. On the other hand, multidoping with different dopants was shown to be more beneficial than single doping due to its contribution in increasing the configurational entropy for segregation. The advanced TEM analysis showed Y and La distributions and concentrations on a series of general grain boundaries in very good agreement with the atomistic simulations. This validation of atomistic modeling technique used in this study means, as it a generic method, can be used as a predictive tool to design ceramic microstructure and properties.

An atomistic modeling survey of the shear strength of twist grain boundaries in aluminum

A computational survey of the shear strength of 343 unique grain boundaries was performed. For each boundary, the strength was surveyed as a function of shear direction. The results suggest that: (1) the shear strength cannot be comprehensively predicted by common grain boundary descriptors, (2) the shear strength depends significantly and simply on shear direction due to the faceted geometry of boundary planes, and (3) grain boundary shear strengths in an ordinary material can be represented by a simple statistical distribution.

Modelling with variable atomic structure: Dislocation nucleation from symmetric tilt grain boundaries in aluminium

Plastic deformation is thought to involve highly ‘stochastic’ phenomena, caused by the generation, motion and interactions of crystal defects called dislocations and grain boundaries. Grain boundaries (or GBs) can act as the nucleation source for dislocations in polycrystalline materials, which may become the rate-limiting phenomenon in FCC metals with grain sizes less than 15nm. Atomistic simulations were performed to study the fundamental factors of dislocation nucleation in non-equilibrium bi-crystals of aluminium. It is shown that several metastable grain boundary structures may accommodate an identical lattice misorientation, and form several non-equilibrium states not coincidental with the global-minimum energy at room temperature. Thermodynamic mechanisms for metastability are discussed, with evidence provided by a high resolution image from the literature. A recently developed post-processing technique is applied in a novel manner to analyse the non-linear, post-yield dislocation nucleation response with respect to the cumulative dislocation length. A correlation is established between the atomic free volume and the accumulation of dislocations at the final state of the simulation. A statistically valid relationship has been demonstrated, which links the initial-state (300K, 1atm) GB energy and free volume, with the critical resolved shear stress for dislocation nucleation under compressive loading. The mechanisms underpinning dislocation nucleation in compression are studied and linked to localised asymmetry of the GB normal stress. Overall, this work shows that by isolating key state variables, a comparative analysis of several non-equilibrium bi-crystals may provide an effective means of studying the fundamental factors of GB effects.

Molecular dynamics studies of the roles of microstructure and thermal effects in spallation of aluminum

Spallation behaviors of nanocrystalline aluminum under shock loading are studied by nonequilibrium molecular dynamics simulations. Simulations on single-crystal aluminum are conducted for comparison. Both classical spallation and micro-spallation are studied. Our simulations show that the shock front structure of the nanocrystalline aluminum sample apparently displays two stages plasticity –– the grain boundary mediated stage and the dislocation mediated stage. The spallation mechanism is dominated by cavitation, i.e., nucleation, growth, and coalescence of voids. It is found that void nucleation mechanism is different in single-crystal aluminum and nanocrystalline aluminum. Void nucleation is induced by dislocation activities in single-crystal aluminum, while is induced by mechanical separation and sliding of grain boundaries in nanocrystalline aluminum. Thermal dissipation during cavitation is studied, and the mutual promotion between cavitation and melting is discovered. Thermal dissipation during cavitation leads to temperature arising in the vicinity of voids and promotes melting around voids. On the other hand, melting of materials leads to dropping of spall strength and thus facilitates the cavitation process. Some quantitative discussions on spall strength and thermal dissipation rate are proposed. The spall strength of single-crystal aluminum first increases and then decreases as shock intensity increases, which is attributed to the competition mechanism between strain rate hardening and temperature softening. Grain boundary effect on spall strength is more remarkable in cases of lower shock intensity. The thermal dissipation rate in void nucleation stage is much larger than in void growth–coalescence stage.

アルミナにおける小角粒界を用いた転位設計と構造解析

アルミナにおける小角粒界を用いた転位設計と構造解析

A multiscale model of grain boundary structure and energy: From atomistics to a continuum description

Grain boundaries play an important role in the mechanical and physical properties of polycrystalline metals. While continuum macroscale simulations are appropriate for modeling grain boundaries in coarse-grained materials, only atomistic simulations provide access to the details of the grain boundary (GB) structure and energy. Hence, a multiscale description is required to capture these GB details. The objective of this paper is to consolidate various approaches for characterizing grain boundaries in an effort to develop a multiscale model of the initial GB structure and energy. The technical approach is detailed using various 〈100〉,〈110〉 and 〈111〉 symmetric tilt grain boundaries in copper and aluminum. Characteristic features are: (i) GB energies obtained from atomistic simulations and boundary period vectors from crystallography, (ii) structural unit and dislocation descriptions of the GB structure and (iii) the Frank–Bilby equation to determine the dislocation content. The proposed approach defines an intrinsic net defect density scalar that is used to accurately compute the GB energy for these GB systems. The significance of the present work is that the developed atomistic-to-continuum approach is suitable for realistically inserting the initial GB structure and energy into continuum level frameworks.

Impact of grain boundary character on grain rotation

The capillarity-driven shrinkage of isolated cylindrical grains with structurally different grain boundaries in aluminum was studied by molecular dynamics simulations. Three pairs of grains with 〈100〉 tilt and mixed tilt–twist boundaries with the misorientation angles θ0=5.45°, 16.26° and 22.61° were examined. The simulation results showed that the shrinkage of grains with pure tilt boundaries was accompanied by their rotation towards higher misorientation angles. On the contrary, grains with the mixed boundaries did not rotate significantly during their shrinkage. An analysis revealed that for the observed rotational behavior the grain boundary structure is crucial. In contrast to pure tilt boundaries composed of edge dislocations, for mixed tilt–twist boundaries, which are composed of intersecting dislocations with the mixed edge–screw character, the effective mechanisms of dislocation annihilation are available, which allow the respective grains to shrink without rotation.

Local atomic structures in grain boundaries of bulk nanocrystalline aluminium: A molecular dynamics simulation study

The microstructures of grain boundaries (GBs) in bulk nanocrystalline aluminium have been investigated by a large-scale molecular dynamics method. Bulk nanocrystalline aluminium is obtained directly by liquid quenching at an appropriate cooling rate, which has narrow grain-size distribution and high-angle GBs. It is found that up to 89.75% GB atoms (named as GB1 atoms) are located at the nearest-neighbour coordination shell around nanograins; others (named as GB2 atoms) are mainly at triple junctions. Local atomic structures in the GBs are quantified in terms of a recently developed method, in which the neighbours of an atom are identified with a parameter-free topological criterion rather than a fixed cut-off distance rc. The results demonstrate that though there are a large number of different cluster types in both the GB1 and the GB2 regions, only a few ones with FCC-like order appear with high frequency in the GB1 region and play a crucial role in the microstructural feature of the GB1. The GB1 region displays short-to-long range order. The GB2 region presents ICO- and BCC-like short-range orders whose degrees are in between the liquid and amorphous, but the medium-range order at the cluster-scale is very weak.