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).     

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.     

Microstructure and mechanical properties of directionally solidified Al2O3/GdAlO3 eutectic ceramic prepared with horizontal high-frequency zone melting

Directionally solidified eutectic oxide ceramics are very promising as a next-generation structural material for ultrahigh-temperature applications, above 1600?°C, owing to their outstanding properties of high corrosion resistance, oxidation resistance, high fracture strength and toughness, and high hardness. Herein, Al₂O₃/GdAlO₃ eutectic ceramic was prepared with horizontal high-frequency induction zone melting (HIZM), and the effects of the processing parameters on the eutectic microstructure and mechanical properties were investigated. The results indicated that the directionally solidified Al₂O₃/GdAlO₃ eutectic ceramic was composed only of the Al₂O₃ phase and GdAlO₃ phase penetrating mutually, and the Al₂O₃ phase was the substrate in which the GdAlO₃ phase was embedded. As the solidification rate increased from 1 to 5?mm/h, the eutectic microstructure underwent a transformation from an irregular pattern to a relatively regular “rod” or “lamellar” pattern, and the eutectic spacing constantly decreased, reaching a minimum value of 0.5?μm. The eutectic ceramic hardness and fracture toughness at room temperature increased continuously, reaching 23.36?GPa and 3.12?MPa?m^1/2, which were 2.3 times and 2.5 times those of the sintered ceramic with the same composition, respectively. Compared with the samples obtained from vertical high-frequency induction zone melting, the orientation of eutectic phases along the growth direction decreased significantly, and the size uniformity of the GdAlO₃ phase became poorer in the samples prepared with HIZM at the same solidification rate; nevertheless, the hardness and fracture toughness of the samples increased by 11% and 63%, respectively.     

Insight into the complex coupled growth behavior of Al2O3/YAG eutectic ceramic based on the evolutions of microstructure and crystallographic texture

Clarifying the dependence of microstructure and crystallographic texture on growth kinetics is critical concerns for mechanical properties prediction in complex faceted eutectic growth. In this study, we present the comprehensive investigations of microstructure and crystallographic texture characteristic in directionally solidified Al₂O₃/YAG eutectic that rely on its coupled growth mechanism. The growth mechanism of eutectic spacing is also well depicted by the Magnin-Kurz (MK) model associated with morphological criteria for branching in faceted phase. As increasing the solidification rate (10 µm/s∼200 µm/s), the regularization tendency for irregular three-dimensional interpenetrated (TDI) structure in geometric category is stronger. Whereas, the enhanced kinetic undercooling increases the misorientation and interfacial energy, which is attributed to the fact that the growth kinetics is more dominant than the minimum interface energy in this work. Additionally, the fluctuation tendency of eutectic spacing and crystallographic orientation with the solidification rate is diametrically opposite. The colony structure consisting of coupled irregular eutectic is formed by the planar interface instability, and its formation mechanism is revealed by the misorientation distribution. This comprehensive research contributes to the design of high-performance eutectic ceramics considering the influence of microstructural features and crystallographic growth modes.     

Microstructure and mechanical properties of Al2O3/Y3Al5O12/ZrO2 hypereutectic directionally solidified ceramic prepared by laser floating zone

Al₂O₃/Y₃Al₅O₁₂/ZrO₂ directionally solidified ceramic has been considered as a promising candidate for ultrahigh temperature structural materials due to its excellent performance even close to its melting point. In this work, laser floating zone (LFZ) solidification experiments were performed on Al₂O₃/Y₃Al₅O₁₂/ZrO₂ hypereutectic with the solidification rates between 2 μm/s and 30 μm/s. The full eutectic lamellar microstructure is obtained with hypereutectic composition. The solid/liquid interface morphology is investigated. The microstructure characteristic is discussed based on the solid/liquid interface. The variation of lamellar spacing with different compositions and solidification rates was reported and discussed by considering an irregular eutectic growth model. The maximum hardness and fracture toughness are 19.06 GPa and 3.8 MPa m^1/2, respectively. The toughening mechanism of ZrO₂ is discussed based on the scenario of the crack propagation pattern.     

Processing,microstructure and performance of Al2O3/Er3Al5O12/ZrO2 ternary eutectic ceramics prepared by laser floating zone melting with ultra-high temperature gradient

Directionally solidified Al₂O₃/Er₃Al₅O₁₂(EAG)/ZrO₂ ternary eutectic/off-eutectic composite ceramics with high density, homogeneous microstructures, well-oriented growth have been prepared by laser floating zone melting at different solidification rates from 4 to 400 µm/s. Uniform and stable melting zone is obtained by optimizing temperature field distribution to keep continuous and stable eutectic growth and prevent from cracks and defects. The as-solidified composite ceramic exhibits complexly irregular eutectic structure, in which the eutectic spacing is rapidly refined but dotted ZrO₂ number inside Al₂O₃ phase is decreased as increasing the solidification rate. The formation mechanism of ZrO₂ distributed inside Al₂O₃ matrix is revealed by examining the depression of solid/liquid interface. Furthermore, after heat exposure 1500 °C for 200 h, the eutectic microstructure only shows tiny coarsening, which indicates it has excellent microstructural stability. As increasing the ZrO₂ content, the fracture toughness can be improved up to 3.5 MPa m^1/2 at 20.6 mol% ZrO₂.     

Fabrication of dense material having homogeneous GdAlO3–Al2O3 eutectic-like microstructure with off-eutectic composition by consolidation of the amorphous

Off-eutectic microstructures generally have both coarse crystals of rich component and ordinary eutectic microstructures. This paper shows a new method to form a dense bulk material having homogeneous eutectic-like microstructure with off-eutectic compositions. Mixture of Gd₂O₃ and Al₂O₃ powders with the off-eutectic composition was melted and quenched rapidly to form the amorphous phase. The amorphous film was pulverized. The dense bulk material could be fabricated by the consolidation of the amorphous powder using spark plasma sintering method. Field emission scanning electron microscope (FE-SEM) observation of the material showed homogeneous fine eutectic-like microstructure without coarse crystals. This is the first case that such material was successfully prepared.     

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.     

Solidification mechanism and microstructure evolution of Al2O3-ZrO2 ceramic coating prepared by combustion synthesis and thermal explosion spraying

The solidification mechanism and microstructure of the hypoeutectic Al₂O₃-ZrO₂ (Al₂O₃:72 mol%) ultra-fined ceramic coating prepared by combustion synthesis and rapid plate cooling method were analyzed by the heat transfer process and dynamic characteristics. The rapid solidification process inhibited the transformation from the t-ZrO₂ to m-ZrO₂ at low temperatures. The growth rate of the solid-liquid interface to form the amorphous and nano-crystalline Al₂O₃-ZrO₂ was about 65.7 mm/s and 13.7 mm/s, respectively. The mechanism formation of both the amorphous and nano-crystalline areas were analyzed using rapid solidification models. When the growth rate reduced to about 8.23 mm/s, large quantities of nanosized eutectic structures was identified by SEM in the pseudo-eutectic area. The interphase spacing of the eutectic structures was 40–100 nm. In addition, some typical divorced eutectic structures appeared at this area. After that, micron dendrites (0.2–0.6 µm) took the main part when the growth rate decreased to about 3.67 mm/s for such a hypoeutectic Al₂O₃-ZrO₂ binary system. The nano-crystalline area showed the highest nanohardness (22 GPa). This paper may provide new guidance to prepare high performance Al₂O₃-ZrO₂ ceramics both in experiment and theory.     

Laser additive manufacturing of melt-grown Al2O3/GdAlO3 eutectic ceramic composite: Powder designs and crack analysis with thermo-mechanical simulation

Directed energy deposition method is an efficient one-step laser additive manufacturing technology to achieve eutectic ceramic composite with high property and ultra-fine microstructures. In this paper, melt grown dense Al₂O₃/GdAlO₃(GAP) eutectic ceramic composites have been directly fabricated from the spherical powder reconstructed by an optimized spray granulation method. Effects of the powder size distribution, feeding rate, and heat treatment on the morphology and microstructure of the as-solidified eutectic ceramic composites have been investigated. Results show that the powder fluidity plays an important role in the heat conduction of the laser process. Finer powder (imperfect spherical powder with diameter less than 10 µm) gives rise to the disturbance of molten pool. Moreover, this powder greatly aggravates the phenomenon of powder’ sticking on surface of the specimen, which subsequently induces the leading growth of coarse dendrite-like GAP primary phases (higher interface temperature of dendrite tip for GAP phase) and sintered eutectic phases at the specimen edge. The finite element modeling (FEM) method is used to analysis the coupled thermal dynamic during the process after verification with infrared thermal image. It shows that longitudinal maximum principal stress exhibits a steep gradient at the edges of the as-solidified ceramic, making the specimen susceptible to cracking along the deposited direction at the first few layers. By optimizing the feedstock powder characteristics and the directed energy deposition process, completely solidified cylindrical and thin-walled Al₂O₃/GAP eutectic ceramic composites with the maximum dimensions of ? 4 × 95 mm³ and 10 × 4 × 44 mm³ have been successfully fabricated. The solidified specimens present smooth glossy surface and fine microstructures with the average eutectic spacing of 0.31 µm. The average micro-hardness and fracture toughness of 15.16 ± 0.29 GPa and 4.3 ± 0.09 MPa·m^1/2 have been obtained, respectively.     

Optical absorption and selective thermal emission in directionally solidified Al2O3-Er3Al5O12 and Al2O3-Er3Al5O12-ZrO2 eutectics

Al₂O₃-Er₃Al₅O₁₂ and Al₂O₃-Er₃Al₅O₁₂-ZrO₂ eutectic ceramic rods were directionally solidified using the laser floating zone technique at several growth rates. Binary eutectic microstructure consisted in a three-dimensional interpenetrated network of the eutectic phases whereas the ternary eutectic showed a geometrical microstructure at low growth rates and a nanofibrillar pattern at high rates. The microstructure size was strongly dependent on the growth rate, decreasing when the processing rate increased. The optical absorption was measured in the samples at room temperature and Judd–Ofelt analysis was used to model the optical absorption of the Er³⁺ ions. Thermal emission of the eutectic rods was studied at temperatures up to 1600 °C. An intense narrow emission band at 1.55 μm matching with the sensitive region of the GaSb photoconverter was obtained. The intensity of the selective emission band is larger for the binary eutectic than for the ternary compound and increases as the microstructural size decreases.     

In situ fabrication and microstructural evolution of Al2O3-ZrO2 nanoeutectic ceramics by a novel pulse discharge plasma assisted melting method

The present work concentrated on a novel one-step pulse discharge plasma melting method for in situ fabricating Al₂O₃-ZrO₂ binary eutectic in a short processing time. Al₂O₃-ZrO₂ eutectic exhibited well-aligned fibrous and colonial structure with a minimum lamellar spacing of ～250?nm at the growth rate of ～176.0?μm/s. The lamellar spacing was well consistent with the inverse-square-root dependence on the growth rate as $\lambda =2.90\times v^{?1/2}$, and the constant of proportionality of 2.90 was close to 3.32 predicted by the JH eutectic theory. The formation mechanism of Al₂O₃-ZrO₂ eutectic on primary faceted-alumina crystals was also proposed. The Al₂O₃-ZrO₂ eutectic originated from the faceted Al₂O₃ primary crystals, which played the leading role in the coupled growth process. The interfacial fracture behavior across the coarse intercolony regions was explored, and the fracture toughness of Al₂O₃-ZrO₂ eutectic ceramics was comparable to those achieved by traditional directional solidification.     

Formation of GdAlO3–Al2O3 composite having fine pseudo-eutectic microstructure

GdAlO₃ and Al₂O₃ powders were mixed and pulverized using ball mills. The prepared powder was sintered by SPS at 1450 °C without holding time. SEM observation of the sintered specimen showed a eutectic-like microstructure. This is called ‘pseudo-eutectic’ in this research. The microstructure formed from a powder pulverized by a tumbling ball mill for one week was much finer than that by a planetary ball mill for 5 and 10 h. The fine homogeneous eutectic-like (pseudo-eutectic) microstructures could be formed at both eutectic and off-eutectic compositions. In case of crystallization from a melt of eutectic components, homogeneous eutectic microstructures can be formed only at restricted compositions very close to the eutectic one. Coarse primary crystals generally exist in the eutectic microstructure at off-eutectic compositions. The pseudo-eutectic microstructures can be formed at any compositions because a mixing ratio of the starting powders can be varied.     

Solidification of Al2O3–YAG eutectic composites with off-metastable eutectic composition from undercooled melt produced by melting Al2O3–YAP eutectics

It has been reported that solidification of the Al₂O₃–YAG equilibrium eutectic structure follows melting of the Al₂O₃–YAP metastable eutectic structure. Since the exothermic heat due to solidification was consumed by the endothermic heat due to melting, a fine and uniform eutectic structure was obtained. However, the composition of the Al₂O₃–YAG eutectic structure is restricted to the metastable eutectic composition. In this paper, Al₂O₃–YAG eutectic compacts with an off-metastable eutectic composition were prepared by the addition of Al₂O₃ particles to Al₂O₃–YAP eutectic particles with diameters less than 20 μm. In compositions ranging from 18.5 mol%Y₂O₃ to 13.5 mol%Y₂O₃, dense Al₂O₃–YAG eutectic compacts were formed without any Al₂O₃ segregation. The flexural strength and the fracture toughness remained almost unchanged with the increase in the Al₂O₃ phase. The addition of Al₂O₃ particles to the Al₂O₃–YAP eutectic particles enabled the matrix phase to change from the YAG phase to the Al₂O₃ phase.     

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.     

Orientations and seed type effect on Al2O3-YAG-ZrO2 eutectic microstructure solidified from the melt by the micro-pulling down technique

Al₂O₃-YAG-ZrO₂ eutectic ceramic rods of 5 mm in diameter were grown by micro-pulling down technique. The seeding and the solidification rate affect microstructure, morphology, crystallography, and thermal stress of the solidified ceramics. The ternary eutectic grown through zirconia (111) seed had inhomogeneous and irregular cellular microstructures. At the stationary stable regime, the microstructure spacing (λ) depends on the pulling rate (v). Under solidification rate of 0.5 mm.min^?1, the rods grown by using eutectic poly-crystal, (100), (111) YAG, and c(0001), A(1?210), M(10?10) sapphire seeds, the YAG and ZrO₂ phases are oriented along the <100> direction parallel to the growth direction. The zirconia (111) seeding X-ray diagram eutectic presents additional peaks and the monoclinic ZrO₂ phase appears at the solidification rate of 1 mm.min-1. The rods grown through ZrO₂ seeding are more stressed than those solidified by using eutectic, YAG and sapphire seeds, respectively.     

Formation mechanism and process optimization of nano Al2O3-ZrO2 eutectic ceramic via laser engineered net shaping (LENS)

Periodic banded structure has great influence on the microstructure and fracture toughness of eutectic ceramic produced by LENS system. Formation Mechanism of periodic banded structure and influence regularity from laser power, scanning speed, feeding rate and feeding proportion were studied. X-ray diffractometer analysis and scanning electron microscopy observation were carried out for sample phase composition and microstructure respectively. Microhardness was measured by vivtorinox hardness tester, and surface fracture toughness was calculated by indentation crack length. The results show that periodic banded structure is divided into organizational coarsening zone and divorced eutectic zone. Due to the influence of upper cladding layer heat, the eutectic structure in the interlayer bonding zone is obviously coarsening in organization coarsening zone. When laser power is lower, Al₂O₃ phase takes the lead in nucleation and grows up quickly, forming a discrete block structure of Al₂O₃ particles in divorced eutectic area. When laser power is higher, ZrO₂ phase takes the lead in nucleation and forms a discrete block structure of ZrO₂ particles. Al₂O₃-ZrO₂ ceramics with the shape of thin-walled and cylinder were manufactured under the optimized process conditions, whose independent nucleation disappears and the thickness of periodic banded structure reduces to 10 µm. The eutectic spacing of prototype structure reaches 120–130 nm, has uniform microstructure and good surface morphology. The average micro-hardness is 18.59 GPa. Surface fracture toughness increases to 6.52 MPa m^1/2.     

Formation of Al2O3–GdAlO3 eutectic ceramics with a fine anisotropic structure in a flash event

An Al₂O₃–GdAlO₃ (GAP) composite with a fine eutectic microstructure was obtained through local melting and rapid solidification during a flash event. An AC field of 1 kV/cm at a frequency of 1 kHz was applied to a calcined Al₂O₃–GAP body at a furnace temperature of 1400°C to induce the flash event. The Al₂O₃–GAP body exhibited local melting through the current path. Solidification proceeded with the maximum cooling rate of about 140°C/s just after the power supply was turned off. Using this flash treatment, a refined eutectic structure with rodlike GAP phases in the Al₂O₃ matrix was obtained within 1 min. The interphase spacing of the obtained structure was approximately 170 nm, comparable with the finest Al₂O₃–GAP eutectic material obtained in previous studies. A flash event and subsequent rapid cooling can contribute to forming fine eutectic ceramics with a relatively low furnace temperature and short processing time.