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.     

Growth of MgAl2O4/MgO eutectic crystals by the micro-pulling-down method and its characterization

MgAl₂O₄/MgO eutectic fibers and rods have been grown successfully by the micro-pulling-down method, and the microstructures and optical characterizations of grown crystals were performed. MgAl₂O₄/MgO eutectic fibers of 0.3–1 mm in diameter and about 500 mm in length, and the rods having 5 mm in diameter with approximately 60 mm in length have been grown with the 6–120 mm/h of growth speed. The eutectic fibers showed homogeneous microstructure in which MgO fiber/whisker aligned to the growth direction in the MgAl₂O₄ (spinel) matrix. The grown crystals looked semitransparence under naked eyes. Optical and orientational characterizations were performed. The second phase of MgO was easily removed by selective etching with hydrochloric acid, and then porous single crystalline bodies were obtained.     

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.     

Synthesis and characterization of a MgO-MgAl2O4-ZrO2 composite with a continuous network microstructure

Using analytically pure MgO, analytically pure Al₂O₃ and analytically pure ZrO₂ as raw materials, Mg_4.68Al_2.64Zr_1.68O₁₂ was prepared at 1993 K for 10 h, and then, a MgO-MgAl₂O₄-ZrO₂ composite with a continuous network was successfully obtained by controlling the cooling rate based on the in-situ decomposition reaction of Mg_4.68Al_2.64Zr_1.68O₁₂ at temperatures below 1887 K. The three phases of MgO, MgAl₂O₄ and ZrO₂ are highly dispersed in this continuous network microstructure, with ZrO₂ intertwined by MgO and MgAl₂O₄ and micropores with a size of less than 2 µm. Furthermore, the synthesis mechanism of Mg_4.68Al_2.64Zr_1.68O₁₂ is given as follows: first, MgAl₂O₄ is synthesized using the reaction: MgO(s)+Al₂O₃(s)=MgAl₂O₄(s) at temperatures below 1894 K; and then, Mg_4.68Al_2.64Zr_1.68O₁₂ is further prepared through MgO and ZrO₂ diffusion and dissolution into MgAl₂O₄ at temperatures above 1894 K, for example, at 1923 K or 1993 K in this work.     

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.     

Sintering,microstructure and mechanical properties of Al2O3–Y2O3–ZrO2 (AYZ) eutectic composition ceramic microcomposites

We report on how the mechanical properties of sintered ceramics (i.e., a random mixture of equiaxed grains) with the Al₂O₃–Y₂O₃–ZrO₂ eutectic composition compare with those of rapidly or directionally solidified Al₂O₃–Y₂O₃–ZrO₂ eutectic melts. Ceramic microcomposites with the Al₂O₃–Y₂O₃–ZrO₂ eutectic composition were fabricated by sintering in air at 1400–1500 °C, or hot pressing at 1300–1400 °C. Fully dense, three phase composites of Al₂O₃, Y₂O₃-stabilized ZrO₂ and YAG with grain sizes ranging from 0.4 to 0.8 μm were obtained. The grain size of the three phases was controlled by the size of the initial powders. Annealing at 1500 °C for 96 h resulted in grain sizes of 0.5–1.8 μm. The finest scale microcomposite had a maximum hardness of 19 GPa and a four-point bend strength of 282 MPa. The fracture toughness, as determined by Vickers indentation and indented four-point bending methods, ranged from 2.3 to 4.7 MPa m^1/2. Although strengths and fracture toughnesses are lower than some directionally or rapidly solidified eutectic composites, the intergranular fracture patterns in the sintered ceramic suggest that ceramic microcomposites have the potential to be tailored to yield stronger, tougher composites that may be comparable with melt solidified eutectic composites.     

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.     

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.     

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.     

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.     

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.     

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.     

Fabrication and mechanical properties of short ZrO2 fiber reinforced NiFe2O4 matrix composites

Short ZrO₂ fibers (ZrO_2(f)) reinforced NiFe₂O₄ ceramic composites were fabricated by cold pressing process. The phase composition, microstructure, mechanical properties and fiber/matrix interface of the composites were investigated by X-ray diffraction, scanning electron microscopy and mechanical testing machines. ZrO_2(f) show good thermodynamic and chemical compatibility with NiFe₂O₄ ceramic matrix and effectively enhanced the mechanical properties. The toughening mechanisms are fiber bridging, interfacial debonding, fiber pullout, phase transformation and the matrix constraint effect. By incorporation of 3 wt% fibers with the average length of 5～6 mm, the bending strength and fracture toughness of the composites reached 88.92 MPa and 4.62 MPa m^1/2, respectively, while the strength conservation ratio after thermal shock increased from 48.85% to 75.86%. The weak interface bonding built up between ZrO_2(f) and NiFe₂O₄ facilitates the reinforcing effects of the fibers to operate.     

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.     

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.     

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.     

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.     

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.     

Experimental phase relations between MgO-saturated magnesium-aluminate spinel (MgAl2O4) and CuOx-rich liquid

Experimental phase equilibrium data for the Cu-O-Al₂O₃-MgO system is required to improve the performance of MgAl₂O₄-containing refractories and slagging in non-ferrous smelting. In this work, the phase relations of MgAl₂O₄ in the Cu-O-Al₂O₃-MgO system were studied experimentally in air within a temperature range of 1100–1400 °C using the equilibration and quenching method. The chemical compositions of the phases in the quenched samples were determined using electron probe microanalysis (EPMA). Less than 1 wt% of Al₂O₃ or MgO were found in the oxide liquid phase, whereas the solid MgAl₂O₄ and MgO phases contained up to 23 wt% and 30 wt% of ‘Cu₂O’, respectively. Discrepancies between these results and the corresponding calculated values generated by the MTDATA 6.0 software and Mtox database Version 8.2 ranged from 3 wt% to 19 wt%. The results of this work indicate that the MgAl₂O₄ spinel is chemically stable in the presence of a CuO_x-rich liquid under the conditions studied.