Al2O3–Y3Al5O12(YAG)–ZrO2 ternary composite rapidly solidified from the eutectic melt

In situ fabrication of new ceramic eutectic composites by rapid solidification of eutectic drops is a cheap and quick method compared to fabrication of directional solidification or multi-step fabrication methods of fiber reinforced/layered composites for high temperature use. This study reports the fabrication of ceramic composites during rapid solification of eutectics melts in the ternary oxide alumina–yttria–zirconia system. Layered ternary eutectics are obtained in the alumina–YAG–zirconia subsystem. The microstructure of Al₂O₃–Y₃Al₅O₁₂–ZrO₂ composites rapidly solidified from melts is presented.     

Effect of solidification path on the microstructure of Al2O3–Y2O3–ZrO2 ternary oxide eutectic ceramic system

The solidification path of the Al₂O₃–Y₂O₃–ZrO₂ ternary oxide eutectic composite ceramic is determined by a high temperature DTA and laser floating zone (LFZ) directional solidification method to investigate the effect of solidification path on the microstructure of the ternary oxide. The DTA and microstructure analyses show that the YAG or Al₂O₃ tends to form as primary phase under the unconstrained solidification conditions, and then the system enters ternary eutectic solidification during cooling from 1950 °C at rate of 20 °C/min. The as-solidified composite ceramic shows a divorced irregular eutectic structure consisting of Al₂O₃, YAG and ZrO₂ phases with a random distribution. The primary phases are however completely restrained at the directional solidification conditions with high temperature gradient, and the ternary composite by LFZ presents well coupled eutectic growth with ultra-fine microstructure and directional array. Furthermore, the eutectic transformation and growth mechanism of the composite ceramic under different solidification conditions are discussed.     

Microstructure and cytotoxicity of Al2O3-ZrO2 eutectic bioceramics with high mechanical properties prepared by laser floating zone melting

Developing new generation of strong, tough and stable bioceramics used in dental filed has been highly desired for attaining the clinical requirement of secure and reliable therapy. In this paper, a novel Al₂O₃-ZrO₂ eutectic bioceramics with nearly fully density and extremely aesthetic luster was in-situ prepared by innovative laser floating zone melting (LFZM) method. The influence of solidification rates on microstructure evolution, mechanical properties and cytotoxicity was investigated. The eutectic bioceramics displayed a special three dimensional interpenetrating microstructure evolving with increasing the solidification rate. The eutectic colony structure occurred when solidification rate overpassed 8?µm/s, and lamellar spacing was below 1?µm when solidification rate exceeded 30?µm/s. The eutectic bioceramics solidified at 100?µm/s exhibited optimal mechanical properties with an average hardness of 16.53?GPa, fracture toughness of 6.5?MPa?m^1/2 and flexural strength of 1.37?GPa. The cytotoxicity of Al₂O₃-ZrO₂ eutectic bioceramics was evaluated by MTT methods according to ISO 10993-5 standard. Non-cytotoxic behavior was detected for the eutectic bioceramics, indicating this eutectic bioceramic could be used as promising dental restoration material.     

Growth rate effect on colony formation in directional solidification of Al2O3/YAG/ZrO2

Mechanical properties of Al₂O₃/Y₃Al₅O₁₂/ZrO₂ ternary eutectic ceramics are strongly affected by structural defects as pores or colonies. Experimental investigation of the microstructure of this ternary composite indicates that the colonies are generally observed when the solidification occurs at high rates. In this work, the influence of the growth rate on the solid-liquid interface shape and formation of colonies in directional solidification of Al₂O₃/Y₃Al₅O₁₂/ZrO₂ by Bridgman, Edge-defined Film-fed Growth (EFG), and Czochralski (Cz) methods is numerically and experimentally investigated. Numerical modeling of the Bridgman growth process shows large curvatures of the solid-liquid interface when the pulling rate is increased up to 80 mm/h. The ingots solidified at rates between 5 and 80 mm/h exhibit colony type microstructure. The analysis of EFG growth of ceramic ribbons reveals less curved solid-liquid interfaces in this system. Numerical modeling shows significant increase in the interface curvature with increasing pulling rate. The microstructure of ribbons grown at pulling rates between 6 and 12 mm/h exhibits colonies only for the ingots solidified at higher rate. Simulations carried out for Czochralski growth process show that the solidification front is almost plane in this system. These results are in agreement with experimental observations showing good structural quality of Cz grown crystals with a flat solid-liquid interface. Finally it is concluded that formation of colonies in directional solidification of this ternary eutectic composite is linked to large curvatures of the growth interface.     

Superplastic deformation of directionally solidified nanofibrillar Al2O3–Y3Al5O12–ZrO2 eutectics

Nanofibrillar Al₂O₃–Y₃Al₅O₁₂–ZrO₂ eutectic rods were manufactured by directional solidification from the melt at high growth rates in an inert atmosphere using the laser-heated floating zone method. Under conditions of cooperative growth, the ternary eutectic presented a homogeneous microstructure, formed by bundles of single-crystal c-oriented Al₂O₃ and Y₃Al₅O₁₂ (YAG) whiskers of ≈100 nm in width with smaller Y₂O₃-doped ZrO₂ (YSZ) whiskers between them. Owing to the anisotropic fibrillar microstructure, Al₂O₃–YAG–YSZ ternary eutectics present high strength and toughness at ambient temperature while they exhibit superplastic behavior at 1600 K and above. Careful examination of the deformed samples by transmission electron microscopy did not show any evidence of dislocation activity and superplastic deformation was attributed to mass-transport by diffusion within the nanometric domains. This combination of high strength and toughness at ambient temperature together with the ability to support large deformations without failure above 1600 K is unique and shows a large potential to develop new structural materials for very high temperature structural applications.     

Self assemble silicide architectures by directional solidification

Directional solidification of eutectics has self assembly characteristics that can fabricate two dimensional periodic arrays of two or more phases. The ordering of the phases can be utilized as a “lithographic technique” to produce porous structures for fabrication of miniature devices. Directional solidification by the Bridgman technique was applied to the eutectic systems Si–TiSi₂ and Si–YSi₂. Patterned growth of TiSi₂ rods occur in the silicon matrix during solidification. Micro-channel structure of pillar arrays of TiSi₂ was produced by silicon dissolution by KOH. Average TiSi₂ rod diameter was 2.5 μm with 99% of the population falling in the 2–3 μm range. Colony formation could not be suppressed in the Si–TiSi₂ system. Patterned growth is not observed in Si–YSi₂ system. Eutectic and hyper-eutectic Si–YSi₂ compositions formed anomalous eutectic microstructure. Microstructure shows dependence upon the solidification rate.     

High temperature deformation of non-directionally solidified Al2O3/YAG/ZrO2 eutectic bulk ceramic

Excellent high temperature mechanical properties of melt-grown Al₂O₃-based eutectics have previously been demonstrated in samples prepared by directional solidification methods. In this study, the deformation behaviour of melt-grown Al₂O₃/YAG/ZrO₂ eutectic bulk prepared by a non-directional solidification method was investigated by means of compressive tests in a temperature range of 1200–1700 °C. The non-directionally solidified eutectic bulk ceramic has a colony structure and is polycrystalline. It begins to show ductility and has a compressive strength of 320 MPa at 1500 °C, which is much higher than that of the sintered ceramic with the same composition. However, its plastic deformability is insufficient, even at 1700 °C (just below the melting point of 1715 °C), and cracking occurs during compressive deformation.     

Dislocation mechanism of deformation and strength of Al2O3–YAG single crystal composites at high temperatures above 1500°C

A new unidirectionally solidified eutectic Al₂O₃–YAG composite has recently been fabricated by accurately controlling the unidirectional solidification. The eutectic composite has a new microstructure, in which single crystal Al₂O₃ and single crystal YAG are three-dimensionally and continuously connected and finely entangled without grain boundaries. The dislocation structure is observed in both single crystal Al₂O₃ and YAG in the plastically deformed specimens in the tensile and compressive tests at high temperatures for the Al₂O₃–YAG single crystal composite, showing that the plastic deformation occurred by dislocation motion. The Al₂O₃–YAG single crystal composite fabricated has the following properties: (1) the flexural strength at room temperature can be maintained up to just below melting point (about 1830°C), (2) the compressive flow stress at 1600°C and a strain rate of 10⁻⁴/s is about 13 times higher than that of sintered composites of the same composition.     

Rapid fabrication of bulk graded Al2O3/YAG/YSZ eutectics by combustion synthesis under ultra-high-gravity field

A rapid and simple way of producing bulk graded Al₂O₃/YAG/YSZ ternary eutectics was investigated. The combustion reaction between Al/Fe₂O₃/Y₂O₃/ZrO₂ led to the formation of molten mixtures consisting of Al₂O₃/YAG/YSZ, and the subsequent separation of the ceramic melt from the iron melt was realized under ultra-high-gravity field, followed by the solidification of the ceramic melt. The as-solidified ceramic ingot sank into the iron melt, where an instantaneous isostatic pressure about 2 MPa was exerted on the around of the ceramic ingot resulting in an enhanced degree of densification. Microstructure analysis demonstrated that densified ceramic was composed of Al₂O₃/YAG/YSZ ternary eutectics. The phase composition, morphologies, hardness and fracture toughness of the eutectic product changed gradually along the direction of high gravity field. The maximum density of the eutectic ceramic was 97.32%, correspondingly, the maximum value of the Vickers hardness and fracture toughness reached 17.82 GPa and 5.51 MPa m^1/2, respectively.     

Investigation of the solidification behavior of Al2O3/YAG/YSZ ceramic in situ composite with off-eutectic composition

Directionally solidified Al₂O₃/YAG/YSZ ceramic in situ composite is an interesting candidate for the manufacture of turbine blade because of its excellent mechanical property. In the present study, two directionally solidified hypoeutectic and hypereutectic Al₂O₃/YAG/YSZ ceramic in situ composites are prepared by laser zone remelting, aiming to investigate the solidification behavior of the ternary composite with off-eutectic composition under high-temperature gradient. The results show that the composition and laser scanning rate significantly influence the solidification microstructure. The ternary in situ composite presents ultra-fine microstructure, and the eutectic interspacing is refined with the increase of the scanning rate. The Al₂O₃/YAG/YSZ hypoeutectic ceramic displays an irregular hypoeutectic network structure consisting of a primary Al₂O₃/YAG binary eutectic and fine Al₂O₃/YAG/YSZ ternary eutectic. Only at low scanning rate, homogeneous ternary eutectic-like microstructures are obtained in the hypoeutectic composition. Meanwhile, the Al₂O₃/YAG/YSZ hypereutectic ceramic shows homogeneous eutectic-like microstructure in most cases and the eutectic interspacing is finer than the ternary eutectic. Furthermore, the formation and evolution mechanism of the off-eutectic microstructure of the ternary composite are discussed.     

Microstructure control,competitive growth and precipitation rule in faceted Al2O3/Er3Al5O12 eutectic in situ composite ceramics prepared by laser floating zone melting

Microstructure control and competitive growth of Al₂O₃/Er₃Al₅O₁₂ eutectic/off-eutectics are explored over wide ranges of solidification rates and compositions. Gradual transformation phenomenon of microstructure morphology from complete eutectic to eutectic + coarse Er₃Al₅O₁₂ phase and to eutectic + Er₃Al₅O₁₂ dendrite is observed and the corresponding influence factors are evaluated. Competitive growth between single-phase Al₂O₃ (or Er₃Al₅O₁₂) dendrite and eutectic is analyzed and coupled growth zone is mapped through comparing interface temperatures of different patterns of microstructures. The complete eutectic microstructure could be obtained at Al₂O₃/Er₃Al₅O₁₂ hypoeutectic (Al₂O₃-17.5 mol% Er₂O₃) under fast solidification rate and the onset growth rate (?0.94 × 10⁴ μm/s) estimated from the measured eutectic spacing (?150 nm) fits well with the result calculated on the basis of competitive growth (?1.27 × 10⁴ μm/s). Transformation of microstructure from irregular eutectic to regular eutectic and probable adjustment mechanism of eutectic spacings are discussed when the eutectic spacings refined from micron-scale (<10 μm) to nano-scale (?20 nm).     

Microstructural refinement and mechanical response of Al2O3–ZrO2 eutectics fabricated by a novel pulse discharge plasma assisted melting method

In the present work, microstructural refinement and mechanical response of Al₂O₃–ZrO₂ eutectics fabricated by a pulse discharge plasma assisted melting (PDPAM) method were investigated. The solidified microstructure evolves from polygonal eutectic colonies into irregular cellular colonies with increasing the superheating temperature of the melt from 1820 °C to 1900 °C. The average eutectic spacing inside the colonies decreases from 1.80 ± 0.10 μm to 0.25 ± 0.06 μm, and the coarse inter-colonial structure is refined, which is attributed to the increase in undercooling temperature. High-temperature microstructural stability of Al₂O₃–ZrO₂ eutectics is improved significantly as contrasted with the as-sintered ceramics. Besides, the load dependence of Vickers hardness for Al₂O₃–ZrO₂ eutectics is investigated.     

Mechanical properties of melt-grown Al2O3–ZrO2(Y2O3) eutectics with different microstructure

The effect of the microstructure on the mechanical properties of Al₂O₃–ZrO₂(Y₂O₃) eutectic ceramic oxides was studied. Rods processed by the laser-heated floating zone method with three different microstructures were obtained as the growth rate increased: a homogeneous dispersion of irregular ZrO₂ lamellae within the Al₂O₃ matrix, colonies with a core containing a dispersion of submicron ZrO₂ lamellae or rods surrounded by a thick intercolony region, and elongated cells formed by a dispersion of very fine ZrO₂ lamellae and separated by thin intercellular boundaries. The average flexure strength (close to 1.6 GPa) of the eutectics made up of a homogeneous dispersion of ZrO₂ lamellae was outstanding, and they also presented an excellent Weibull modulus (12.9) when the microstructure was homogeneous throughout the sample. Banding did not affect the average strength but degraded the Weibull modulus. In general, the flexure strength decreased as the size of the main morphological features of the microstructure (colony or cell diameter) increased. The thickness of the intercellular boundaries increased with the Y₂O₃ content and, above a critical value, reduced dramatically the strength by activating a new failure mechanism based on the coalescence of the pores and shrinkage cavities concentrated at the intercellular boundaries.     

Stabilization of zirconia lamellae in rapidly solidified alumina–zirconia eutectic composites

In-situ fabrication of ceramic eutectic composites by rapid solidification of eutectic drops is a cheap and quick method compared to directional solidification or to multi-step fabrication methods of fiber reinforced/layered materials for high temperature use. Binary eutectic composites with a homogeneous periodic microstructure have been obtained by directional solidification of eutectic melts for many years, but typical solidification velocities used in directional solidification are limited to the range of cm/hour or, more recently, up to 15 mm/min. The present study aims to determine the effects of faster solidification rates on the structure of the alumina–zirconia binary composites obtained at higher growth rates by rapid solidification from eutectic melts in air or vacuum. A binary composite with zirconia stabilized in the high-temperature tetragonal form is presented. The stabilization of the tetragonal phase has not been observed before in bulk crystalline pellets of binary Al₂O₃–ZrO₂ eutectic composites.     

Preparation,microstructures, and mechanical properties of directionally solidified Al2O3/Lu3Al5O12 eutectic ceramics

Al₂O₃/Lu₃Al₅O₁₂ (LuAG) directionally solidified eutectic (DSE) ceramics with two solidification rates were prepared utilizing optical floating zone (OFZ) technique. The microstructures (eutectic morphology, preferred growth direction and interface orientation) of Al₂O₃/LuAG were characterized, and the mechanical properties (Vickers hardness and fracture toughness) were compared with those of Al₂O₃/REAG (RE = Y, Er, and Yb). Results show that Al₂O₃/LuAG with solidification rate of 30 mm/h has established preferred growth direction in both Al₂O₃ and LuAG phases with cellular eutectic structures. While Al₂O₃/LuAG with solidification rate of 10 mm/h only shows preferred growth direction in Al₂O₃ phase and presents degenerate irregular eutectic microstructures. Besides, Al₂O₃/LuAG exhibits higher hardness compared with Al₂O₃/REAG (RE = Y, Er, and Yb). In addition, a special attention is focused on the relations among rare earth ionic radius, eutectic microstructures, and mechanical properties of these DSE ceramics. It is demonstrated that a smaller rare earth ionic radius could lead to larger eutectic interspacing as well as higher Vickers hardness of DSE Al₂O₃/REAG, revealing the possibility and feasibility of microstructure control and mechanical properties optimization for DSE Al₂O₃/REAG ceramics by tailoring the rare earth elements.     

Directionally solidified CeO2 (or GDC)/CoO eutectic ceramics as cermet precursors for SOFCs anodes: Microstructure cross-over

Rods of CeO₂ and gadolinium-doped CeO₂ (GDC)-CoO eutectics were prepared by directional solidification using a laser heated floating zone (LFZ) technique. The microstructure has been studied as a function of the growth rate from V = 10 to 750 mm/h. Regular eutectic microstructures are obtained except for the highest growth rate. The interspacing follows the λ²V = C law with C = 4.1(3) × 10⁻¹⁷ and 2.6(3) × 10⁻¹⁷ m³/s for CeO₂-CoO and GDC-CoO eutectics, respectively. A cross-over between fibrous and lamellar eutectic microstructures was observed depending on the growth rate. The crystallography of the eutectics was studied by Electron Backscatter Diffraction (EBSD). The growth directions [1 1 0]_GDC ∼ //[110]_CoO, and the interfacial planes (200)_GDC//(111)_CoO, were identified. Solubility of Co in the ceria matrix was determined by Energy Dispersive X-ray (EDX) Spectroscopy after Co was leached out from the matrix. Co solubility in ceria at 1650 °C was found to be less than 1 mol%.     

Microstructures and mechanical properties of directionally solidified Al2O3/GdAlO3 eutectic ceramic by laser floating zone melting with high temperature gradient

Directionally solidified Al₂O₃/GdAlO₃ eutectic ceramic rods with high densities and low solidification defects are prepared by laser floating zone melting at solidification rate from 2 to 200 μm/s. The microstructure evolution, eutectic growth behavior and mechanical properties are investigated. At low solidification rates (<30 μm/s), the eutectic rods present a homogeneous irregular eutectic microstructure, whereas cellular microstructure containing regular lamella/rod structure is developed at higher solidification rates. The relationship is established between the eutectic interphase spacing and solidification rate, which follows the Magnin-Kurz eutectic model. The Vickers hardness (15.9–17.3 GPa) increases slightly with decreasing interphase spacing, but the fracture toughness (4.08 MPa m^1/2) shows little dependence with the solidification rate. Different crack propagation mechanisms are revealed among the indentation cracks. The flexural strength at ambient temperature reaches up to 1.14 GPa for the eutectic grown at 100 μm/s. The fracture surface analysis indicates that the surface defects are the main crack source.     

Phase diagrams of the systems Al2O3-ZrO2-Ln(Y)2O3 as a source of multiphase eutectics for creating composite structural and functional materials

Phase equilibria in the ternary systems Al₂O₃-ZrO₂-Ln₂O₃ (Ln = La, Nd, Sm, Gd, Er, Yb and Y) were investigated by identical methods in all range of concentrations and 1250-2700 °C temperature range using differential and derivative in solar furnace thermal analysis in controlled medias, X-ray diffraction phase analysis, local X-ray spectral analysis, chemical analysis, electron and optical microscopy, petrography. The phase diagrams of the systems studied are presented as isothermal at 1650 °C sections and melting diagrams. The interaction in the systems is characterized by the absence of ternary compounds and regions of appreciable solid solutions based on the binary compounds and components. Only narrow regions of ternary solid solutions were discovered at high temperatures by CALPHAD method and because of existing small solubility on the base of ZrO₂ in the binary bounding system Al₂O₃-ZrO₂. The phase equilibria in the systems are determined by zirconia as the most stable compound. Solidification in the systems is completed in eutectic reactions. The established interaction regularities allowed to forecast interaction and phase diagrams construction in systems with other lanthanides (Ce, Pr, Pm, Eu, Tb, Dy, Ho, Tm, Lu). New 13 quasibinary and 26 ternary eutectics were found for the first time. Their temperatures rise from 1660 °C for the La₂O₃ system to 1840 °C for the Lu₂O₃ system. Fluorite-type phases equilibrate with garnet-type phases for the systems from Tb₂O₃ to Lu₂O₃. In the systems from Pr₂O₃ to Gd₂O₃ they equilibrate with perovskite-type phases and β-Al₂O₃. On the base of microstructure investigations it was established that three-phase alumina-rich eutectics crystallizes according to the mechanism of cooperative growth. It opens up possibilities to obtain composite materials using directional solidification method.