Alumina Grain Boundaries Research Articles

Atomic-scale investigation on the structures and mechanical properties of metastable grain boundaries in aluminum

Grain boundary (GB) structural transition in pure metals open up new possibilities for material microstructure design. By managing the distribution of ground-state and metastable GB structures, the beneficial effects of GBs on materials can be maximised. However, the application of GB transitions to develop high-performance aluminum is limited by the inadequate understanding of the metastable GB structure in aluminium and its associated plastic deformation mechanisms. In this work, we firstly used a high-throughput GB generation method to construct multiple GB structure for a given misorientation angle, and screen out the ground-state and metastable GB structures according to the energy principle. Six typical symmetrically tilted GBs were investigated, including Σ5(210), Σ17(410) and Σ17(530) GBs around <001> tilt axis, and Σ9(221), Σ11(332), Σ17(223) GBs around <110> axis. Then, molecular dynamics simulations were carried out to perform uniaxial tensile tests on the six GBs and their corresponding metastable GBs at different temperatures. The results show that the tensile strength of <001> metastable GBs differs slightly from the ground-state GB at various temperatures, but for <110> GBs, the strength of most metastable GBs is significantly stronger or weaker compared to that of ground-state GB at 10 K. An increase in temperature reduced this difference in tensile strength of <110> GBs. According to atomic analysis of the GB structural characteristics, it was found that stress and temperature-induced GB structural transition, GB strain localization, and the prevention of dislocation nucleation or propagation from the dissociated structure are the main causes of the differences in tensile responses between the ground-state and metastable GBs.

In situ carbon nanonetwork structure based on MOF-derived carbon to manufacture ceramic composites: A case study of zirconia-toughened alumina

Highly robust ceramics materials are promising materials for applications in the automobile, aerospace, and ceramic cutting tool industries. However, conventional ceramics are typically brittle, which limits their suitability for advanced applications. In this study, with zeolite imidazolium salt skeleton-8 (ZIF-8) as a carbon source, discharge plasma sintering is used to create carbon nanonetworks derived from metal-organic frameworks (MOFs) in situ on the grain boundaries of zirconia-toughened alumina (ZTA) ceramic. ZTA exhibits maximum bending strength (719 ± 20.5 MPa) and fracture toughness (7.91 ± 0.7 MPa m1/2) at MOF concentrations of 1.0 wt% and 2.0 wt%, respectively, which are 1.37 and 1.88 times higher than that of the unmodified sample. This demonstrates that the MOF-derived carbon nanonetworks significantly improve the mechanical properties of the ceramic. The finite element analysis results indicate that the carbon nanonetworks can effectively disperse the stress inside the material and transfer the concentrated stress from the internal defects to the two-phase grain boundary on the surface of the sample, thereby contributing to strengthening and toughening. Overall, this work validates that the in situ production of carbon nanonetworks from MOFs is an effective strategy to enhance the mechanical properties of ZTA ceramics. Moreover, it offers new approaches for reinforcing and toughening other ceramic materials.

Unveiling the diffusion pathways under high-temperature oxidation of Cr2AlC MAX phase via nanoscale analysis

The MAX phase chromium aluminium carbide (Cr2AlC) is a potential candidate for high-temperature structural and energy applications due to the unique combination of properties, including excellent oxidation and corrosion resistance in harsh environmental conditions. In this work, the oxidation behaviour of bulk Cr2AlC was studied at temperatures of 1000 °C and 1200 °C in humid synthetic air, mimicking a realistic application environment. An α-Al2O3 scale forms at both temperatures, consisting of both inward and outward-growing layers, with a continuous Cr7C3 layer beneath the oxide. A special focus was put on employing atom probe tomography (APT) to analyse the oxide and carbide layers at the nanoscale. Decomposition of Cr2AlC to Cr7C3 is the main source (>70%) of aluminium needed for the oxide scale formation, while Al depletion in the underlying MAX phase remains marginal (<1 at%), exhibiting a pattern different from the common high-temperature oxidation of alloys. Al diffusion to the oxide scale proceeds predominantly through grain boundaries (GBs) in Cr2AlC and through the bulk in Cr7C3. Ionic diffusion through GBs of the large-grained inner alumina layer seems to be the rate-limiting step for the oxide scale growth, resulting in the apparent parabolic oxidation kinetics. No segregation of Cr, C or any impurities, which could affect ionic transport, was found at these alumina GBs by APT, rendering the results of this study as characteristic of a pure Cr2AlC oxidation.

Effect of Copper Segregation at Low-Angle Grain Boundaries on the Mechanisms of Plastic Relaxation in Nanocrystalline Aluminum: An Atomistic Study.

The paper studies the mechanisms of plastic relaxation and mechanical response depending on the concentration of Cu atoms at grain boundaries (GBs) in nanocrystalline aluminum with molecular dynamics simulations. A nonmonotonic dependence of the critical resolved shear stress on the Cu content at GBs is shown. This nonmonotonic dependence is related to the change in plastic relaxation mechanisms at GBs. At a low Cu content, GBs slip as dislocation walls, whereas an increase in Cu content involves a dislocation emission from GBs and grain rotation with GB sliding.

Исследование динамической прочности мелкозернистого оксида алюминия, полученного методом электроимпульсного плазменного спекания

The results of dynamic compressive testing of alumina samples with different grain sizes sintered from nano and fine α-Al2O3 powders are presented. The ceramics were obtained by spark plasma sintering (SPS). The effect of heating rate (Vh), sintering temperature (Ts), holding time (ts), cooling rate (Vc) on hardness, fracture toughness, dynamic strength stress (σY) of Al2O3 has been studied. An amorphous layer of nanometer thickness was present on the surface of nanopowders in the initial state. After sintering, the grain boundaries of the ceramics had a crystalline structure; no inclusions of the amorphous phase were found. It has been suggested that during the SPS process, an amorphous structure containing an excess free volume is transformed into a crystalline phase with the formation of dislocation-type defects at the grain boundaries, which create long-range internal stress fields. It is shown that nanopores less than 50 – 100 nm in size are present at the grain boundaries of sintered ceramics. It is shown that the nonmonotonic nature of the dependence of σY on the temperature and time of the SPS is due to the simultaneous change in the density, the nonequilibrium state of the grain boundaries, and the grain size of the ceramic. It is shown that a decrease in the degree of nonequilibrium of the grain boundaries of alumina due to an increase in the SPS temperature or an increase in the holding time makes it possible to increase the dynamic strength of alumina. It has been established that an increase in the cooling rate leads to the formation of compressive residual stresses and a slight increase in σY of ceramics. The maximum dynamic strength (σY = 1755 MPa) for alumina ceramics with average grain size 1.6 – 2 µm obtained by SPS (Vh = 50 °С/min, Ts = 1520 °С, ts = 50 min).

ABC-FIRE: Accelerated Bias-Corrected Fast Inertial Relaxation Engine

The Fast Inertial Relaxation Engine (FIRE) is a pseudo-dynamical minimisation algorithm commonly used for the structural relaxation of atomistic systems. We introduce an additional factor to FIRE that modifies the bias and the scaling of the velocities of the atoms during the “mixing” step. The robustness of the modified algorithm has been tested in a number of representative configurations having different levels of complexity. Reductions of 9% (Σ5(310) grain boundary in aluminium with 3184 atoms) to 65% (fracture simulation in Fe25Ni25Cr50 with ∼1.6 million atoms) were obtained in the number of iterations needed to reach convergence. We dub this variation of the original algorithm ABC-FIRE, where ABC stands for Accelerated Bias-Corrected.

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.

Investigation of the Densification Behavior of Alumina during Spark Plasma Sintering.

The article presents the results of the investigation of the mechanism of the densification behavior of alumina-based ceramics during spark plasma sintering. The role of the heating rates and additives were investigated. The first (initial) stage of sintering was investigated by the Young–Cutler model. The second (intermediate) stage of sintering was investigated as a process of plastic deformation of a porous body under external pressure. It was shown that, at the initial stage, the formation of necks between the particles is controlled by grain boundary diffusion (the activation energy is Qb ≈ 20 kTm). At this stage, accommodation of the shape of the alumina particles is also occurring (an increase in the packing density). The accommodation process facilitates the shrinkage of the powder, which is reflected in a decrease in the effective activation energy of shrinkage at low heating rates (10 °C/min) to Qb ≈ 17 kTm. At heating rates exceeding 10 °C/min, the intensity of the processes of accommodation of alumina particles turns out to be much slower than the existing diffusion processes of growth of necks between the alumina particles. It was shown that the grain boundary sliding mechanism that occurs in the second stage of sintering can play a decisive role under conditions of spark plasma sintering with a high heating rate. The found value of the activation energy at the second stage of sintering is also close to the activation energy of grain–boundary diffusion of alumina (Qb ≈ 20 kTm). The influences of the second phase particles of MgO, TiO2, and ZrO2 on densification behavior of alumina-based ceramics were investigated. Since at the first stage of sintering the densification relates with the formation of necks between the particles of alumina, the additives (0.5% vol) have no noticeable effect on this process. It was also shown that the second phase particles which are located at the grain boundaries of alumina are not involved in the slip process during the second sintering stage. Analysis shows that additives act only in the final (third) stage of spark plasma sintering of alumina.

Grain boundary relaxation in doped nano-grained aluminum

Simulation studies are done to understand the role of dopants that segregate preferentially to grain boundaries on the stability of nanocrystalline aluminum. A dopant design framework based on thermodynamic principles, is used to identify the specific dopant type with the highest potential to segregate to grain boundaries in nanocrystalline aluminum. Various elements are evaluated as potential dopants and magnesium is identified to have the highest tendency to segregate to grain boundaries and release the excess free energyleading to the relaxation of the grain boundaries. A systematic combination of atomic structure analysis is then done to correlate grain boundary relaxation and the mechanical response of the magnesium-doped nanocrystalline aluminum at ambient temperature. The atomistic simulations reveal that the preferential partitioning of magnesium dopants to the grain boundaries reduces the excess volume within this region which stabilizes the nanostructure. At low contents, the magnesium dopants are observed initially partition to the grain boundaries, but once saturation of the grain boundaries is reached, excess dopants are accommodated in the crystalline interiors. It is found that the addition of the magnesium dopants even in the dilute limit, enhances the strength of the nanocrystalline aluminum. The formation of large, disordered GBs in doped nanocrystalline aluminum under tensile load allowed it to accommodate the deformation and prohibit crack growth.

Description of grain boundary structure and topology in nanocrystalline aluminum using Voronoi analysis and order parameter

Grain boundaries (GB) have a significant impact on mechanical, physical properties and microstructure of polycrystalline materials. Their studies are necessary for designing materials of improved properties. We present molecular dynamics based studies of grain boundary in nanocrystalline aluminum. A Voronoi tessellation based method (Voronoi analysis, VA) and order parameter approach were used for describing structure and topological features in nanocrystalline aluminum. We are portraying new functionality and usefulness of VA combined with order parameter (OP) to picturing structure nature of grain boundary (GB) and its order given in form of configuration entropy. Voronoi analysis unveiled sub-nanoscale topological features contributing to GB which correspond to lattice distortions and liquid-like GB behavior. Grouping of Voronoi cell indices to a crystal lattice distortion group was made together with corresponding to them OP values. Two complementary methods used in this work provide new information and open new possibilities for studying structure and properties of nanomaterials.

Local damage in grain boundary stabilized nanocrystalline aluminum

The excess free volume in grain boundaries of pure nanocrystalline aluminum leads to the grain coarsening, which degrades their desirable mechanical properties. The preferential segregation of dopants to the grain boundaries has been proposed to enhance the stability of the nanostructure. To shed light on the stabilization mechanisms, we present results from virtual mechanical tests on nanocrystalline aluminum samples with similar grain sizes but different magnesium dopant contents. The segregation of magnesium decreases the excess free volume within the grain boundaries and prohibits the nucleation and growth of intergranular cracks.

Optimized etching of porcelain and polycrystalline alumina with a glass phase

Chemical and thermal etching techniques are commonly used in ceramography to enhance microstructural features. While thermal etching works well for high purity ceramic systems, this technique may not be appropriate where glass is present. Porcelain and industrial alumina both contain a glass phase, typically 40−60 vol% and 4−30 vol%, respectively, and these glass chemistries are proposed to be similar. Chemical etching of porcelain is common, but the images published in the literature are frequently over-etched. When glass is present in the grain boundaries of alumina, thermal etching can cause the glass to disappear or to recrystallize, obscuring the microstructure. Because of this, it is proposed that both chemical and thermal etching are necessary to prepare an industrial alumina microstructure for grain size measurements. In addition, it was observed that chemical etching is sensitive to the residual stress in the glass phase, becoming more aggressive when there is residual tension in the glass.

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.

Simulation of the Pore Formation at Grain Boundaries in Aluminum

The possibility of the pore formation at the tilt grain boundaries with the [100] misorientation axis is studied by computer simulation methods. Three special boundaries and three grain boundaries of a general type are studied at temperatures of 400, 500, and 600 K. It is shown that the presence of 3% lattice vacancies is insufficient for the pore formation both at the grain boundary and in grains, whereas in the presence of 6% lattice vacancies, pores are formed only in grains. Pores on grain boundaries are formed at temperatures of 500 and 600 K with a lattice vacancy concentration of 4 and 5%, respectively. Pores are not formed on a special grain boundary Σ5(013).

Improving Microstructural and Mechanical Properties of AA2024 Base Metal by Adding Reinforced Strip Width of AA7075 via Vertical Compensation Friction Stir Welding Technique

Improving microstructural and mechanical properties of AA2024 base metal reinforced with AA7075 via vertical compensation friction stir welding (VCFSW) have become a very vital research area to reinforce the weak part of nugget zone in welded joints. So this paper presents a novel and simple method to improve microstructural and mechanical properties especially ductility and fracture locations of welded aluminum joints by VCFSW technique. Anyhow, most of the welded parts are subjected to the forming processes so improvement in the property of ductility and fracture behavior of welded joints is required. One solution to increase ductility and improve fracture behavior of welded joints is the use of interlayer compensation strip width as a reinforcement which has superior mechanical properties between two blanks of base metal during friction stir welding process. The effect of changing interlayer compensation width was investigated on the microstructural characterization (by an optical microscope, scanning electron microscope and transmission electron microscope) and mechanical properties (using microhardness, tensile and bending tests). Microstructure observations revealed that the tailor-welded joint without compensation interlayer contains micro-voids in the stir zone, while in tailor-welded joints with compensation interlayer, the voids completely disappear on the microscale. X-ray diffraction detected that the use of compensation interlayer of AA7075 between two AA2024 blanks led to the appearance of two precipitates of intermetallic phases, Al2Cu and Mg2Zn, on grain boundaries of aluminum which improved ductility and fracture behavior of welded joints. Ductility enhanced till 54.4% at compensation interlayer width 2 mm when compared to the joint without compensation interlayer. Lower hardness values were detected at nugget zone in joint without compensation interlayer. By contrast, highest hardness values in the softened regions were obtained in compensation interlayer joints. Finally, the highest bending angle occurred without any cracks in case of compensation interlayer of 2 mm at ultimate bending load.

A criterion for slip transfer at grain boundaries in Al

The slip transfer phenomenon was studied at the grain boundaries of pure Aluminum by means of slip trace analysis. Either slip transfer or blocked slip was analyzed in more than 250 grain boundaries and the likelihood of slip transfer between two slip systems across the boundary was assessed. The experimental results indicate that slip transfer was very likely to occur if the residual Burgers vector, ∆b, was below 0.35b and the Luster–Morris parameter was higher than 0.9, and that the ratio of the Luster–Morris parameter and the residual Burgers vector has a threshold above which slip transfer is probable.

Reactive Element Effects in High-Temperature Alloys Disentangled

Reactive elements—REs—are decisive for the longevity of high-temperature alloys. This work joins several previous efforts to disentangle various RE effects in order to explain apparently contradicting experimental observations in alumina forming alloys. At 800–1000 °C, “messy” aluminum oxy-hydroxy-hydride transients initially formed due to oxidation by H2O which in turn undergo secondary oxidation by O2. The formation of the transient oxide becomes supported by dispersed RE oxide particles acting as water equivalents. At higher temperatures, electron conductivity in impurity states owing to oxygen vacancies in grain boundaries (GBs) becomes increasingly relevant. These channels are subsequently closed by REs pinning the said vacancies. The universality of the emerging understanding is supported by a comparative first-principles study by means of density functional theory addressing RE(III): Sc2O3, Y2O3, and La2O3, and RE(IV): TiO2, ZrO2, and HfO2, that upon reaction with water, co-decorate a generic GB model by hydroxide and RE ions. At 100% RE coverage, the GB model becomes relevant at both temperature regimes. Based on reaction enthalpy ΔHr considerations, “messy” aluminum oxy-hydroxy-hydride transients are accessed in both classes. Larger variations in ΔHr are found for RE(III)-decorated alumina GBs as compared to RE(IV). For RE(III), correlation with GB width is found, increasing with increased ionic radius. Similarly, upon varying RE(IV), minor changes in stability correlate with minor structural variations. GB decorations by Ce(III) and Ce(IV) further consolidate the emerging understanding. The findings are used to discuss experimental observations that include impact of co-doping by RE(III) and RE(IV).

Pinning Effect of Oxide Particles on Grain Boundaries of a Low Aluminum Non‐oriented Electrical Steel

In this article the pinning effect of oxide particles on grain boundaries of a low aluminum non‐oriented electrical steel is studied. According to their locations and morphologies in annealed steel samples, oxide particles are classified into three types: globular ones at grain boundaries, globular ones in the interior of grains, and chain‐like ones pinning at grain boundaries. The chemical composition of oxide particles shows a significant influence on the probability pinning at grain boundaries. The elongation ratio of oxide particles becomes larger with the decrease in their viscosity that is determined by the chemical composition and temperature and is calculated using FactSage. Based on the thermodynamic analysis, small particles forming during solidification have more content of SiO2 due to the reaction between the dissolved oxygen and silicon in the steel. Particles of large size and large elongation ratio show larger pinning force on the grain growth. Removing large SiO2–Al2O3–MnO–CaO particles as far as possible, and controlling the composition of oxide particles to achieve larger viscosity than the steel matrix, will retard their elongation during the hot‐rolling process so that the magnetic properties of the steel are improved.

Enhancement ductility of friction stir welded aluminum blanks AA2024 via adding interlayer strip width of AA7075

This paper presents a novel and simple method to improve and enhance the ductility of welded aluminum blanks by friction stir welding technique. Anyhow; most of welded parts are subjected to the forming processes; so the ductility of friction stir tailor-welded aluminum blanks is an important key property. One solution to increase ductility is the use of interlayer compensation strip width as a reinforcement which has superior mechanical properties between two blanks of base metal during friction stir welding process. The effect of changing interlayer compensation width was investigated on the microstructural characterization (by an optical microscope, scanning electron microscope and transmission electron microscope) and mechanical properties (using microhardness, tensile and bending tests). Microstructure observations revealed that the tailor-welded joint without compensation interlayer contains micro-voids in the stir zone; while in tailor-welded joints with compensation interlayer the voids completely disappear on the microscale. x-ray diffraction detected that the use of compensation interlayer of AA7075 between two AA2024 blanks led to the appearance of two precipitates of intermetallic phases, Al2Cu and Mg2Zn, on the grain boundaries of aluminum which improved ductility of welded joints. Ductility enhanced till 54.4 % at compensation interlayer width 2 mm when compared to the joint without compensation interlayer. Lower hardness values were detected at the nugget zone in joint without compensation interlayer. On the other side; highest hardness values in the softened regions were obtained in compensation interlayer joints. Finally; the highest bending angle occurred without any cracks in case of compensation interlayer of 2 mm at ultimate bending load.

Atomistic simulations of energies for arbitrary grain boundaries. Part I: Model and validation

Grain boundary (GB) energies are important data for understanding and predictive modeling of thermal and mechanical behaviors in polycrystalline materials. In this study, a cutoff sphere molecular dynamics model is proposed to calculate the energies of arbitrary GBs. This model eliminates the effect of interaction between free surface and GB in a bicrystal cell by excluding the surface region in the energy computation. The model is tested for four groups of coincident site lattice (CSL) GBs with low-to-medium coincidence index and non-CSL GBs in aluminum. The results show that, at a fraction of computation time as of the original sphere model, the cutoff sphere model without rigid body translation (RBT) provides approximate energy predictions that agree with experimental results and those of the classical block model adopting periodic boundary conditions. The good performance of the cutoff sphere model for non-CSL GBs lies in its faithful reproduction of characteristic structure units and resulting quasi-periodicity in the GB structures. The applicability to arbitrary GBs renders the model an attractive tool to construct energy datasets useful for material behavior simulations. Further improvement in the accuracy of energy predictions is shown to be possible by incorporating RBT at the expense of significantly-increased computational cost.