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.     

Microstructure and mechanical properties of Al2O3/Y3Al5O12/ZrO2 ternary eutectic materials

Directionally solidified fibers and rods have been grown from the ternary Al₂O₃/Y₃Al₅O₁₂/ZrO₂ system using micro-pulling-down method. Fiber diameter could be varied 0.3 mm–2 mm at pull-rates ranging 6–900 mm/h and 500 mm in length. The ternary eutectic fibers had homogeneous colony patterned eutectic microstructures. The interlamellar spacing λ exhibited an inverse-square-root dependence on the growth speed v according to λ = 8 × v^−1/2, where λ has the dimension of μm and v is in μm/s. The tensile strength was recorded 1730 MPa at 25 °C and 1100 MPa at 1200 °C for a fiber crystals grown at a growth speed of 900 mm/h. Eutectic rods having 5 mm of diameter and up to 80 mm in length were also successfully grown by the micro-pulling-down method. The eutectic rods showed 1400 MPa of mechanical strength by compressive mode at 1500 °C with homogeneous colony microstructures.     

Thermal emission properties of Al2O3/Er3Al5O12 eutectic ceramics

The mechanical properties and thermal stability of the Al₂O₃/Er₃Al₅O₁₂ (EAG) eutectic ceramics have been investigated at very high temperature. The emissive properties of this eutectic ceramics have also been measured and its possibilities of application to an emitter have been discussed. The present eutectic ceramic has excellent high-temperature strength characteristics, showing that tensile yielding stress is approximately 300 MPa at 1650 °C and superior thermal stability at 1700 °C in an air atmosphere. The present material shows strong selective emission bands at wavelength 1.5 μm due to Er³⁺ ion. The emission bands of this material are nearly coincident with the sensitive region of GaSb PV cell, therefore, the Al₂O₃/EAG eutectic ceramic can be regarded as one of the promising emitter materials in TPV systems.     

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.     

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 of the directionally solidified ternary eutectic ceramic Al2O3/MgAl2O4/ZrO2

The directionally solidified Al₂O₃/MgAl₂O₄/ZrO₂ ternary eutectic ceramic was prepared via induction heating zone melting. Smooth Al₂O₃/MgAl₂O₄/ZrO₂ eutectic ceramic rods with diameters of 10 mm were successfully obtained. The results demonstrate that the eutectic rods consist of Al₂O₃, MgAl₂O₄ and ZrO₂ phases. In the eutectic microstructure, the MgAl₂O₄ and Al₂O₃ phases form the matrix, the ZrO₂ phase with a fibre or shuttle shape is embedded in the matrix, and a quasi-regular eutectic microstructure formed, presenting a typical in situ composite pattern. During the eutectic growth, the ZrO₂ phase grew on non-faceted phases ahead of the matrix growing on the faceted phase. The hardness and fracture toughness of the eutectic ceramics reached 12 GPa and 6.1 MPa·m ^1/2, respectively, i.e., two times and 1.7 times the values of the pre-sintered ceramic, respectively. In addition, the ZrO₂ phase in the matrix reinforced the matrix, acting as crystal whiskers to reinforce the sintered ceramic.     

Directionally solidified Al2O3-ME3Al5O12 (ME: Y,Er and Yb) eutectic coatings for thermophotovoltaic systems

Selective emitters for thermophotovoltaic systems consisting of directionally solidified Al₂O₃-ME₃Al₅O₁₂ (ME: Y, Er and Yb) eutectic coatings on Al₂O₃ substrates were produced and characterizated. Coatings were deposited by dip-coating on cylindrical substrates. After sintering, a continuous-wave CO₂ laser was used to produce the surface resolidification. The optimization of the processing parameters yielded dense eutectic coatings with good adhesion to the substrate and with 90–200 µm in thickness. All coatings were free of voids and showed a eutectic microstructure consisting of a three dimensional interpenetrated network of Al₂O₃ and ME₃Al₅O₁₂. The mechanical properties of the coatings (hardness and fracture toughness) were evaluated by indentation techniques. Thermal emission was studied by heating the rods with a CO₂ laser at temperatures between 1000 and 1400 °C. Selective emission was observed in Er³⁺ and Yb³⁺ based coatings and attributed to the electronic transitions of the rare earth ions. Er³⁺-coatings showed the best emission properties as selective emitters for thermophotovoltaic converters.     

Mechanical properties up to 1900 K of Al2O3/Er3Al5O12/ZrO2 eutectic ceramics grown by the laser floating zone method

Directionally solidified Al₂O₃–Er₃Al₅O₁₂–ZrO₂ eutectic rods were processed using the laser floating zone method at growth rates of 25, 350 and 750 mm/h to obtain microstructures with different domain size. The mechanical properties were investigated as a function of the processing rate. The hardness, ∼15.6 GPa, and the fracture toughness, ∼4 MPa m^1/2, obtained from Vickers indentation at room temperature were practically independent of the size of the eutectic phases. However, the flexural strength increased as the domain size decreased, reaching outstanding strength values close to 3 GPa in the samples grown at 750 mm/h. A high retention of the flexural strength was observed up to 1500 K in the materials processed at 25 and 350 mm/h, while superplastic behaviour was observed at 1700 K in the eutectic rods solidified at the highest rate of 750 mm/h.     

Longitudinal cross-section microstructure of growth striation in Al2O3/Y3Al5O12/ZrO2 directionally solidified eutectic ceramic prepared by laser floating zone

Periodic growth striations with a width of 380 μm are observed in Al₂O₃/Y₃Al₅O₁₂/ZrO₂ ternary directionally solidified eutectic ceramic prepared by laser floating zone. It is found that the microstructure of the growth striations exhibits disparate growth characteristics. The direct experimental evidence of the formation of the microstructure of the growth striations is obtained through examining the solid–liquid interface. The lamellar coarsening of the growth striations is related to the nucleation of Al₂O₃ particles and their engulfment by the extended Y₃Al₅O₁₂ phase. The mechanism accounting for this phenomenon is explained by considering eutectic growth behavior under mutative condition induced by oscillatory convection.     

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 diagram of the Al2O3–ZrO2–La2O3 system

The phase diagram of the Al₂O₃–ZrO₂–La₂O₃ system was constructed in the temperature range 1250–2800 °C. The liquidus surface of the phase diagram reflects the preferentially eutectic interaction in the system. Three new ternary and two new binary eutectics were found. The minimum melting temperature is 1665 °C and it corresponds to the ternary eutectic LaAlO₃ + T-ZrO₂ +  La₂O₃·11Al₂O₃. The solidus surface projection and the schematic of the alloy crystallization path confirm the preferentially congruent character of phase interaction in the ternary system. The polythermal sections present the complete phase diagram of the Al₂O₃–ZrO₂–La₂O₃ system. No ternary compounds or regions of remarkable solid solution were found in the components or binaries in this ternary system. The latter fact is the theoretical basis for creating new composite ceramics with favorable properties in the Al₂O₃–ZrO₂–La₂O₃ system.     

Growth of Al2O3/ZrO2(Y2O3) eutectic rods by the laser floating zone technique: effect of the rotation

The laser floating zone technique has been applied to the growth of Al₂O₃/ZrO₂(Y₂O₃) rods in the eutectic composition to reveal the effect of forced convection induced by rotation on the rod microstructure. A systematic experimental study of this effect has been carried out combining different source rod and/or eutectic rod rotation (0–200 rpm) and travelling speeds (10–1500 mm/h) in an axial thermal gradient close to 6 × 10⁵ C/m. The results indicate that the rotation is useful to achieve a more homogeneous temperature distribution, especially in thick rods but it has a limited effect in the change of the solidification front shape. The forced convection in the floating zone caused by rotation slightly flattens the solidification interface enhancing the homogeneity of the phase distribution across the sample. However, it introduces several new microstructural features like banding and phase coarsening that can deteriorate the mechanical behaviour of the rods. On the other hand, rods above 1.6 mm in diameter cannot be grown without cracks, even with fast eutectic rod rotation. Rotation does not change the pulling rate threshold (50 mm/h) at which the transition from coupled to dendritic and cellular growth morphology takes place.     

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.     

Microstructure and mechanical properties of Al2O3–YSZ and Al2O3–YAG directionally solidified eutectic plates

Plates of Al₂O₃–YSZ and Al₂O₃–YAG eutectic composition with a thickness from 0.1 to 1 mm were prepared by directional solidification using a diode laser stack. The melt processed regions of plates exhibited colony microstructure consisting of finely dispersed phases. Due to the curved shape of the melted pool, the growth rate depends on the distance to the surface plate, decreasing from top to bottom. In this way, the microstructure characteristic length changes as a function of the distance to the plate surface. Vickers indentations and piezo-spectroscopy measurements were done on longitudinal and transverse cross-sections of the samples at different depths. From these measurements, we concluded that the Vickers hardness (H_V), indentation fracture toughness (K_IC) and residual stresses (σ_h) of the plates were mainly independent from the distance to the surface. The mean values that we obtained in the Al₂O₃–YSZ plates were H_V = 16 GPa, K_IC = 4.2 MPa m^1/2 and σ_h = −0.33 GPa, and in the Al₂O₃–YAG plates were H_V = 16 GPa, K_IC = 2.0 MPa m^1/2, and σ_h = −0.1 GPa. These values are similar to those found in directionally solidified eutectic rods.     

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.     

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.     

Effect of cooling rate on the microstructure and mechanical properties of melt-grown Al2O3/YAG/ZrO2 eutectic ceramic

Melt-grown Al₂O₃/YAG/ZrO₂ ternary eutectic samples were solidified by quenching and with controlled cooling rates of 10, 50, and 250 °C/min, respectively. Effect of cooling rate on microstructure and mechanical properties were examined. With the increase of cooling rate, three classical characteristic microstructures are obtained and developed from colony structure to dendrite structure and to cell structure. In the quenching process, the sample consists of lamellar eutectic cells and its fracture toughness increases to 4.13 ± 0.8 MPa m^1/2. The microstructure transitions with the cooling rate are attributed to instability of the solid–liquid interface. In this work, the interface instability is analyzed to explain the microstructure evolutions in terms of undercooling and characteristic lengths of solute diffusion and capillarity effect.     

Corrosion of a unidirectionally solidified Al2O3/YAG eutectic composite in a combustion environment

Owing to their remarkable higher creep resistance, some oxides eutectic composites those fabricated by unidirectional solidification are prime candidates for structural components used in a severe corrosive environment at high temperatures. In this paper, the possibility of an Al₂O₃/YAG eutectic composite for high-temperature application, where the materials would be exposed to combustion gases, was investigated. The Al₂O₃/YAG eutectic composite was stable at 1700 °C in an atmosphere of oxygen/water vapor (O₂/H₂O), showing only slight changes in microstructure, volume and flexural strength after an exposure for 200 h. Thus, Al₂O₃/YAG eutectic composite is among the most promising ceramics for structural applications at high temperatures.