Eutectic Ceramics Research Articles

Microstructure evolution and toughening mechanism of Al2O3/YSZ directionally solidified eutectic ceramic

Al2O3/YSZ directionally solidified eutectic ceramics (DSECs) with a diameter of 10 mm were prepared via the induction heating zone melting (IHZM), and effects of the pulling rate of the crucible on the microstructure and mechanical properties were investigated. The eutectic components included α-Al2O3, t-ZrO2 and m-ZrO2 phases. The DSECs’ microstructure evolved from a mixture of discontinuous lamellar and round colony structure to the round colony structure, then to the faceted colony structure, and finally to a cell structure with increasing the pulling rate. The formation kinetics and growth process of the round colony structure were described and discussed in detail. The reason that the faceted colony structure formed could attributed to the large increase of solid-liquid interface curvature. When the pulling rate was 300 mm/h, the actual growth rate vg = 81.9 ± 17.5 mm/h was calculated from λ2v=37.7 corresponding to the intracolony; or vg = 99.5 ± 58 mm/h was calculated from with λ2v=631 corresponding to the intercolony. The DSECs’ hardness and fracture toughness increased with the pulling rate, and got the maximum values of 18.6 GPa and 9.39 MPa m1/2 at 300 mm/h, respectively, which could be attributed to the refinement of phases and the decrease of m-ZrO2 phase fraction.

Preparation of large-size Al2O3/GdAlO3/ZrO2 ternary eutectic ceramic rod by laser directed energy deposition and its microstructure homogenization mechanism

Laser 3D printing based on melt growth has great potential in rapid preparation of Al2O3-based eutectic ceramics. In this work, large-scale Al2O3/GdAlO3/ZrO2 ternary eutectic ceramic rod with diameter of 4–5 mm and height higher than 250 mm was additively manufactured by laser directed energy deposition. Especially, heat treatment was applied to eliminate the microstructure heterogeneity in the as-deposited eutectic ceramic, and the microstructure homogenization mechanism was studied in depth. The results indicate that colonies and banded structures completely disappear after the heat treatment, producing a homogeneous network eutectic structure. The microstructure homogenization is revealed to experience three stages of discontinuous coarsening, continuous coarsening and microstructure coalescence. Additionally, it is found that the eutectic spacing linearly increases with the heat treatment time, meaning that the coarsening behavior of the laser 3D-printed Al2O3/GdAlO3/ZrO2 eutectic ceramic satisfies well with the Graham-Kraft model.

One-step additive manufacturing and microstructure evolution of melt-grown Al2O3/GdAlO3/ZrO2 eutectic ceramics by laser directed energy deposition

Synchronized-powder-feeding-based laser directed energy deposition (LDED) has great application potential for the rapid fabrication of large-scale composite ceramics with complex shapes. In this study, near-full-density Al2O3/GdAlO3/ZrO2 ternary eutectic ceramics with different shapes and smooth surfaces were directly prepared by using an improved LDED device. Spherical ceramic powders with eutectic composition and good flowability were obtained by centrifugal spray drying. The microstructure characteristics and microstructure evolution of the rapidly solidified 3D-printed eutectic ceramic were systematically elucidated. In particular, the formation mechanism of the observed periodic banded structures was revealed through a unique laser partial remelting technique. The result indicated that the appearance of the banded structure is attributed to the drastic abnormal coarsening of the nanoscale microstructures adjacent to the molten pool. On the basis these results, a physical model was proposed to illustrate the microstructure evolution of the 3D-printed Al2O3/GdAlO3/ZrO2 eutectic ceramic.

Microstructure of directionally solidified Al2O3/EAG eutectic ceramics prepared with high-temperature gradient

Directionally solidified Al2O3/Er3Al5O12 (EAG) eutectic ceramics are characterized by high melting point, improved high-temperature flexural strength, and high fracture toughness. Therefore, they have recently attracted considerable attention as candidates for high-temperature (>1500 °C) structural materials. In this work, directionally solidified Al2O3/EAG eutectic ceramics were prepared by the optical floating zone technique. 2D and 3D microstructures were observed by scanning electron microscopy and X-ray computerized tomography. The effect of thermal gradient and the solidification rate on Al2O3/EAG eutectic microstructure was investigated. The microstructure was completely irregular, exhibiting 3D interconnected microstructure. The size of eutectic microstructure decreased with increasing growth rate. Stable coupled-growth can be obtained when the solidification rate is less than 60 mm/h. Fracture toughness of Al2O3/EAG eutectic ceramics prepared with a withdrawal rate of 40 mm/h, measured by indentation tests, was KIC = 5.7 ± 0.3 MPa m1/2 and the hardness was HV = 14.5 ± 0.4 GPa. Interfaces between Al2O3 and the EAG phases play important role in improving fracture toughness. The characterization of microstructure and mechanical properties of Al2O3/EAG eutectic ceramics may provide guidance for microstructure design and reveal the deformation mechanisms of Al2O3/EAG eutectic ceramics.

Directionally Solidified Al2O3/Er3Al5O12 and Al2O3/Yb3Al5O12 Eutectic Ceramics Prepared by Optical Floating Zone Melting

Directionally Solidified Al₂O₃/Er₃Al₅O₁₂ and Al₂O₃/Yb₃Al₅O₁₂ Eutectic Ceramics Prepared by Optical Floating Zone Melting

Phase growth patterns for Al2O3/GdAlO3 eutectics over wide ranges of compositions and solidification rates

Phase selection and growth characteristics of directionally solidified Al2O3/GdAlO3 (GAP) faceted eutectic ceramics are investigated over wide ranges of compositions and solidification rates to explore the eutectic coupled zone. Through the observation of the quenched solid-liquid interface, the competitive growth of primary faceted Al2O3 phase, primary non-faceted GAP phase and Al2O3/GAP eutectic with different morphologies is detected. Microstructure transitions from wholly eutectic to primary Al2O3 (GAP) dendrite plus eutectic and then to wholly eutectic are found in Al2O3-20 mol% Gd2O3 hypoeutectic (Al2O3-26 mol% Gd2O3 hypereutectic) ceramics with the increase of solidification rate. The dendrite growth of faceted Al2O3 and non-faceted GAP phases are well predicted by KGT model, which have introduced appropriate dimensionless supersaturation Ω to characterize the anisotropic growth of dendrites. Based on the maximum interface temperature criterion, the competitive growth of primary phase and eutectic is analyzed theoretically and the predicted coupled zone of Al2O3/GAP eutectic ceramics is in good agreement with the experimental results. Besides, the influence of microstructure with these different morphologies on the flexural strength of Al2O3/GAP eutectic ceramics is studied.

Preferential growth of directionally solidified Al2O3/SmAlO3 eutectic ceramics

Directionally solidified Al2O3/SmAlO3 (SAP) eutectic ceramics were successfully prepared with an optical floating zone furnace. The crystallographic orientation and interfacial structures were investigated by electron backscattered diffraction and transmission electron microscopy. It was found that Al2O3 has one preferred growth direction but the SAP has two. The preferred growth direction of the directionally solidified Al2O3/SAP eutectic ceramics was 〈101¯0〉Al2O3 || 〈100〉SAP || 〈011〉 SAP. The near-coincidence site lattice models were proposed to interpret the intergrowth of the two SAPs. It was found that the two interfaces have nearly the same volume strain, validating the coexistence of both orientation relationships.

Mg2SiO4-MgAl2O4 directionally solidified eutectics: Hardness dependence modelled through an array of screw dislocations

Mg2SiO4-MgAl2O4 eutectic ceramics have been fabricated by means of the laser floating zone (LFZ) technique. The microstructure has revealed as an unusual one at lower growth rate, composed of broken lamellae of MgAl2O4 distributed randomly along one matrix, composed of Mg2SiO4. At higher growth rates, a cell structure with intra-cell lamella structure is dominant. Contrary to most eutectic systems, hardness is not dependent upon the inter-spacing, but it does depend on one characteristic length of lamellae: their perimeter. One simple model based upon the dislocation is proposed, which successfully accounts for such extraordinary hardness law. Accordingly, Mg2SiO4-MgAl2O4 eutectic ceramics fabricated at 50 mm/h growth rate with the smallest MgAl2O4 lamella perimeter favorably showed more elevated hardness (13.4 GPa from Vickers indentation and 15.3 GPa from nanoindentation) and strength (∼430 MPa) than those found in the monolithic Mg2SiO4 matrix.

Evolutions of rod diameter, molten zone and temperature gradient of oxide eutectic ceramics during laser floating zone melting

Laser floating zone melting (LFZM) is an important directional solidification (DS) technique in oxide ceramic fabrication owing to its very high temperature gradient and rapid cooling rate. In this work, variations and evolutions of rod diameter, molten zone, and temperature gradient are explored in fabrication of Al2O3-based eutectic ceramics. At low solidification rate of 4 μm/s, periodical oscillations with almost the same period (~275 s) are produced in melting zone shape/size, as-grown rod diameter, temperature distribution, and temperature gradient. The temperature gradient fluctuates between 3.6 × 103 K/cm and 6.9 × 103 K/cm with an average value of about 5.3 × 103 K/cm. As increasing the solidification rate, the temperature gradient decreases and the periodical oscillation weakens. Based on the evolutions of the temperature field, interface location and molten zone shape, the formation mechanism of periodical oscillation during LFZM processing is evaluated.

Ultrafast formation of Al2O3–Y3Al5O12 eutectic ceramic by flash sintering

AbstractIn this study, a dense Al2O3–Y3Al5O12 (YAG) ceramic was synthesized by flash sintering a powder mixture of Al2O3 and Y2O3 in less than 150 seconds at a furnace temperature of 1350°C. The resultant ceramic has a well‐defined eutectic structure consisting of alternating Al2O3 and YAG layers. The hardness and fracture toughness of the ceramic were measured to be 18.5 GPa and 4.3 MPa.m1/2, respectively. These values are comparable to those of similar eutectic ceramics made by directional solidification techniques. The results suggest a new method for making high‐performance eutectic ceramics, which could be applied in other systems.

Directed laser deposition of Al2O3–ZrO2 melt-grown composite ceramics with multiple composition ratios

Melt-grown Al2O3–ZrO2 hypoeutectic, eutectic and hypereutectic ceramics were fabricated using directed laser deposition (DLD) technology. Flexural strength of DLD-fabricated Al2O3–ZrO2 composites was obtained for the first time. Toughening mechanisms of ZrO2 in different composition ratios were revealed. The results show that hypoeutectic ceramics form composite microstructure with α-Al2O3 primary dendrites discretely distributing in eutectic continuous matrix, while the discrete phase becomes t-ZrO2 in hypereutectic ceramics. Eutectic ceramic has lamellar fine eutectic colony structure. Hypoeutectic ceramics show the best hardness, while eutectic ceramics have the highest flexural strength of 237 MPa. Fracture toughness reaches the maximum of 5.91 MPa m1/2 under hypereutectic composition, which is 16.8% higher than that of eutectic ratio. Primary t-ZrO2 dendrites in hypereutectic ceramics have phase transformation toughening effect. However, as a eutectic structure phase, t-ZrO2 in hypoeutectic and eutectic ceramics has no phase transformation toughening effect because its size is smaller than the critical size of phase transformation.

Microstructure analysis of Al2O3/Er3Al5O12/ZrO2 DSECs prepared using high-frequency zone melting method

The microstructure of directionally solidified eutectic ceramics (DSECs) is mainly determined by the eutectic growth mode, properties of each phase, and solidification processing parameters. The Al2O3/Er3Al5O12(EAG)/ZrO2 DSECs were prepared using high-frequency induction melting, and their phase components, and microstructures were investigated. The results indicated that the samples were composed only of Al2O3, EAG, and ZrO2 phases. As an solidification rate increased from 5 to 500 mm/h, the microstructure was refined, and the distribution of the eutectic phases changed. This could be explained by the relationship between the undercooling, the growth rate and the melting entropy. The regular microstructure appeared in the samples prepared at low growth rate and it exhibited the form of triangles or squares. A polygonal colony structure was found in the DSECs obtained at 50 mm/h, which was related to the non-uniformed distribution of temperature fields near the solid-liquid interface. Furthermore, both of the internal microcrack and amorphous structure were identified in the rapidly solidified samples prepared at 500 mm/h.

Comparison of Al2O3/Er3Al5O12/ZrO2 ceramics with eutectic composition prepared using hot-pressing sintering and melt growing

Al2O3/Er3Al5O12(EAG)/ZrO2 ceramics with eutectic composition were prepared with hot-pressing sintering and melt growing. The samples were divided into four sets, non-directionally solidified eutectic ceramics (N-DSEC), directionally solidified eutectic ceramics (DSEC), rapidly quenched eutectic ceramics (RQEC) and hot-pressing sintered ceramics (HPSC), respectively. The relationship between their microstructures and mechanical properties was investigated. The results show that RQEC has the highest hardness of 17.3 GPa, HPSC has the highest fracture toughness of 6.8 MPa·m1/2, while DSEC has the highest flexural strength of 721.8 MPa. The increase of density, the improvement of microstructural uniformity and the decrease of defects are beneficial to the mechanical properties of the samples. The refinement of microstructure can increase the hardness, but not significantly within a certain range of solidification rate. The fracture toughness increases with the interface amount at the low growth rate. However, but too fast solidification rate may produce excessive residual stress and amorphous structure in the samples, and decrease the fracture toughness. In addition, the reasonable weakening of interface bonding strength could improve the fracture toughness of Al2O3/EAG/ZrO2 ceramics with high density. However, the weakened interface at the sample surface is inclined to become the starting point of failure, which is harmful to the flexural strength.

Interfacial stability and ionic conductivity enhanced by dopant segregation in eutectic ceramics: the role of Gd segregation in doped CeO2/CoO and CeO2/NiO interfaces

Ionic charge balance at the interface of Gd-doped CeO2/CoO eutectic ceramics produces Gd segregation and increases the interfacial ionic conductivity.

Effect of Number and Location of Eutectic Chips on Luminescent Properties of Ceramic Eutectic Packaged LED

Effect of Number and Location of Eutectic Chips on Luminescent Properties of Ceramic Eutectic Packaged LED

Medium infrared transparency of MgO-MgAl2O4 directionally solidified eutectics

MgAl2O4-MgO eutectic ceramics have been fabricated by the laser floating zone method. Increasing growth rates from 10 to 50 mm/h, the microstructure transformed from irregular MgO rod-to-lamellae phase and it approached to almost homogeneous rod morphology. At the highest used velocity of 750 mm/h, the cell structure was completely dominant and the samples were free from transversal cracks. Although the highest flexure strength was found at 750 mm/h growth rate, the maximum optical transmittance in the medium-infrared range was obtained for 50 mm/h growth rate and for 1 mm thick samples reached values higher than 75% in the wavelengths between 4 and 5.3 μm. The enhanced transmittance for the sample with 50 mm/h growth rate can be explained in terms of the close refraction indexes of the component phases and the characteristic lengths of the resulting microstructure showing fully dense ceramics with the finest and almost homogeneous microstructure.

Phase evolution and microstructure analysis of Al2O3–ZrO2(Y2O3) eutectic powders with ultra-fine nanostructure

It is difficult to prepare high-quality eutectic particles with regular morphologies and fine microstructure by current technologies. In this work, the Al2O3–ZrO2(Y2O3) eutectic powders with ultra-fine nanostructure is fabricated by a novel combustion synthesis combined with spray cooling (CS-SC) method. The phase structure and microstructure of eutectic particles with different Y2O3 content are discussed in details. The doping of Y2O3 inhibits the transformation of cubic-tetragonal and tetragonal-monoclinic during the solidification process, which gives rise to a rich variety of ZrO2 polymorphs. Moreover, the microstructure of eutectic particles papered by CS-SC is independent of Y2O3 content, which are predominantly manipulated by cooling rate. The evolution of eutectic structure with particle size is also detailly analyzed. The cooling rate declines with the of particle size, which leads to increase of growth rate and consequently resulting in coarse structure. But all for this, the mean phase spacings of all particles remain nanoscale. The CS-SC has much higher growth rate and degree of supercooling than other solidification techniques. This work provides a basis for preparing Al2O3–ZrO2(Y2O3) eutectic ceramics concurrent with large dimension and ultra-fine structure via sintering method.

The influence of manganese oxide on the sintering and properties of the eutectic ceramics ZrO2-Al2O3-SiO2 system

The results of investigation of sintering, microstructure and phase composition of the system ZrO2-Al2O3-SiO2 with MnO admixture are shown. It was approved that the usage of MnO admixture promoted sintering temperature decrease up to 1250-1350 °C. The product obtained is characterized by a high content of tetragonal phase as well as high strength while bending.

The Influence of Manganese Oxide on the Sintering and Properties of the Eutectic Ceramics of the ZrO2–Al2O3–SiO2 System

The Influence of Manganese Oxide on the Sintering and Properties of the Eutectic Ceramics of the ZrO2–Al2O3–SiO2 System

Effect of scanning speed on the solidification process of Al2O3/GdAlO3/ZrO2 eutectic ceramics in a single track by selective laser melting

Abstract Al2O3/GdAlO3/ZrO2 ternary eutectic ceramics with fine microstructure were directly fabricated from mixed pure ceramic powders by selective laser melting. The specimen forming quality, molten pool morphology and microstructure characteristic were investigated as functions of scanning speed. Solidification defects such as cracks and pores were effectively suppressed when the scanning speed was 12 mm/min. The relative density of the as-solidified eutectic specimens decreased from 98.7% to 95.7% with increasing the scanning speed up to 48 mm/min. The melting in this study was governed by conduction mode, leading to a decrease tendency of both melting width and depth with the increase of the scanning speed. Different from ordinary cognition, the eutectic spacing in top zone of the molten pool first decreased and then increased with increasing the scanning speed from 6 mm/min to 48 mm/min. The transition point appeared at 12 mm/min, where the dominant factor affecting the solidification rate changed from the scanning speed to the value of the angle between microstructure growth direction and laser scanning direction.