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.     

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.     

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.     

Mechanical properties of highly textured porous Ni-YSZ and Co-YSZ cermets produced from directionally solidified eutectics

It is well known that several ceramic materials develop an usual; and sometimes unique; combination of properties as a result of mixing different phases with similar expansion coefficients. Sometimes they are elastically stiff, have low thermal expansion coefficients, and are resistant to chemical attack. As this paper will show, their mechanical properties are also enhanced.Nanoindentation is used to measure the mechanical properties for each phase of NiO-YSZ and CoO-YSZ eutectics produced by the laser floating zone technique, and also the analogues Ni-YSZ and Co-YSZ cermets produced by reduction from the eutectic precursors. The different tests have been performed at 100 nm, in order to obtain an imprint lower than the size of the secondary phase and extract the hardness and Young's modulus. Moreover, several tests have been performed at 2000 nm of indentation range to obtain the response of each material. The different imprints have been visualized by Atomic Force Microscopy.     

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.     

Preparation and microstructure of large-sized directionally solidified Al2O3/Y3Al5O12 eutectics with the seeding technique

A large-sized Al₂O₃/Y₃Al₅O₁₂ (YAG) eutectic single crystal is successfully prepared with the external seed by a modified Bridgman furnace. The microstructure, crystallography and interface structure of the large-sized Al₂O₃/YAG eutectic are well investigated. It is found that the longitudinal eutectic microstructure shows large length-to-width phase ratios. The crystallographic orientation relationship of the as-obtained large-sized Al₂O₃/YAG eutectic is consistent with that of the seed. The epitaxial solidification of the binary eutectic occurs, and the dominating of the seed is not lost in the long-range growth. The observed Al₂O₃/YAG interface structure is studied by near-coincidence site lattice (NCSL) theory. The volume strain for the NCSL is very low (0.02), which suggests that the interfaces have locally low interfacial energies. This small volume strain might be the reason for stable induced-growth along the seed.     

Effect of an abrupt change in pulling rate on microstructures of directionally solidified Al2O3-Er3Al5O12 eutectic and off-eutectic composite ceramics

Directionally solidified microstructures of Al₂O₃-Er₃Al₅O₁₂ eutectic and off-eutectic in situ composite ceramics were explored under abrupt-change pulling rate conditions. Corresponding temperature distributions and interface locations were studied. In eutectic composition, fluctuation of eutectic spacing occurred when the pulling rate increased abruptly. A gradually increase or abrupt increase in eutectic spacing was observed when the pulling rate decreased abruptly. In hypoeutectic and hypereutectic compositions, formation of the primary phases were suppressed when the pulling rate increased abruptly from 10?µm/s to 100?µm/s, while primary phases precipitated when the pulling rate decreased abruptly from 100?µm/s to 10?µm/s. The interface altitude decreased after the pulling rate increased abruptly, but increased after the pulling rate decreased abruptly. The liquid composition restriction (around the eutectic composition) at the eutectic interface plays an important role in the suppression of the primary dendrite and coupled eutectic oxides can be obtained in off-eutectic compositions even under higher solidification rate conditions.     

Ruby micro-piezospectroscopy in GdAlO3/Al2O3(/ZrO2), Er3Al5O12/Al2O3(/ZrO2) and Y3Al5O12/Al2O3(/ZrO2) binary and ternary directionally solidified eutectics

The fluorescence from (naturally present) Cr³⁺ impurities was used to measure the residual stress in the alumina phase of six melt-grown ceramic eutectic composites associating gadolinum aluminum perovskite (GAP), erbium aluminum garnet (EAG) or yttrium aluminum garnet (YAG) with α-alumina and cubic zirconia. Such measurements are reported for the first time in the GAP containing eutectics.In the usual hydrostatic assumption, we conclude to a residual compression in the range of ～70–400 MPa depending on the sample composition. The validity of the hydrostatic assumption is questioned when a microscope is used for the measurements.     

On the growth and structure of Al2O3-Y3Al5O12-ZrO2:Y solidified eutectic

Structural features of Al₂O₃-YAG-ZrO₂:Y eutectic plates grown by the EFG process have been studied. X-ray tomography shows that all three phases are continuous along the whole sample, suggesting a fully coupled ternary eutectic growth. However the growth of alumina and YAG is well described by a classical binary coupled growth, in spite of the facetted structure of their growth interface. Colonies are observed at high growth rate and have been related to the chemical rejection of zirconium ions at the solid-liquid interface, possibly due to a slight off-stoechiometry of the raw material.     

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.     

Mechanical properties of directionally solidified Al2O3/Y3Al5O12 eutectic ceramic prepared by optical floating zone technique

Al₂O₃/Y₃Al₅O₁₂(YAG) directionally solidified eutectic (DSE) crystal was prepared by optical floating zone technique. Al₂O₃/YAG DSE consists of continuous entangled Al₂O₃ and the YAG forming a three-dimensional networks structure. The volume fraction of porosity is ultra-low (0.013%) and the average equivalent diameters of most pores (>84%) are smaller than 4?μm. The Al₂O₃/YAG DSE shows excellent high-temperature elastic stiffness. The Young’s modulus at 1500?°C maintains more than 85% of the value at room temperature. Bending strength exhibits excellent retention up to high temperature as well. High-temperature ball indentation testing shows plastic deformation involving dislocations and twinning, which predominantly occur in Al₂O₃ phase, while the YAG phase is stable. Evaluation on H_v/E index predicts Al₂O₃/YAG DSE with moderate capability to accommodate damages. Our results highlight Al₂O₃/YAG DSE as excellent high-temperature structural materials.     

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.     

Comparative study of Yb:Lu3Al5O12 and Yb:Lu2O3 laser ceramics produced from laser-ablated nanopowders

We present a comparative study of two Lu-based oxide ceramics doped with Yb³⁺ ions, namely Yb:Lu₃Al₅O₁₂ (garnet) and Yb:Lu₂O₃ (sesquioxide), promising for thin-disk lasers. The ceramics are fabricated using nanopowders of 3.6 at.% Yb:Lu₂O₃ and Al₂O₃ produced by laser ablation: Yb:Lu₃Al₅O₁₂ – by vacuum sintering at 1800 °C for 5 h with the addition of 1 wt% TEOS as a sintering aid, and Yb:Lu₂O₃ – by vacuum pre-sintering at 1250 °C for 2 h followed by Hot Isostatic Pressing at 1400 °C for 2 h under Ar gas pressure of 207 MPa. The comparison includes the structure, Raman spectra, transmission, optical spectroscopy and laser operation. The crystal-field splitting of Yb³⁺ multiplets is revealed for Lu₃Al₅O₁₂. A continuous-wave (CW) Yb:Lu₃Al₅O₁₂ ceramic microchip laser generates 5.65 W at 1031.1 nm with a slope efficiency of 67.2%. In the quasi-CW regime, the peak power is scaled up to 8.83 W. The power scaling for the Yb:Lu₂O₃ ceramic laser is limited by losses originating from residual coloration and inferior thermal behavior.     

Strain measurement of the directionally solidified eutectic Al2O3/Y3Al5O12 (YAG) ceramic by neutron diffraction

The eutectic Al₂O₃/Y₃Al₅O₁₂ (YAG) ceramic has been reported to be composed by single-crystalline Al₂O₃ and YAG, the microstructure of which is characterized by the three dimensionally entangled two single-crystalline composites. Therefore, Laue diffraction and high-resolved energy-dispersive neutron diffraction (time-of-flight method) techniques were employed to measure residual strain precisely. It was found that the YAG phase was in tension and the Al₂O₃ phase was in compression with strains in the range of ∼10⁻⁴ at room temperature through comparing the lattice spacings of the sintered YAG and sintered Al₂O₃ as the references of strain-free materials.     

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.     

Directionally solidified Al2O3/(Y0.2Er0.2Yb0.2Ho0.2Lu0.2)3Al5O12 eutectic high-entropy oxide ceramics with well-oriented structure,high hardness,and low thermal conductivity

Directionally solidified Al₂O₃/(Y_0.2Er_0.2Yb_0.2Ho_0.2Lu_0.2)₃Al₅O₁₂ eutectic high-entropy oxide ceramics (HEOCs) were successfully prepared with an optical floating zone furnace. The Al₂O₃/(Y_0.2Er_0.2Yb_0.2Ho_0.2Lu_0.2)₃Al₅O₁₂ eutectic HEOCs were pure phases with uniform distribution of rare-earth elements. The preferred growth orientation relationships were <10−10 > {0001}Al₂O₃ // <110 > {211}(Y_0.2Er_0.2Yb_0.2Ho_0.2Lu_0.2)₃Al₅O₁₂. The indentation fracture toughness and Vickers hardness were 6.8 ± 0.9 MPa·m^1/2 and 16.1 ± 0.3 GPa, which were higher than that of Al₂O₃/Y₃Al₅O₁₂ eutectic ceramics. The room temperature bending strength was 333 ± 42 MPa. Crack bridging, deflection and bifurcation were the main toughening mechanism. Hardness and reduced modulus mapping results illustrated that the hardness of (Y_0.2Er_0.2Yb_0.2Ho_0.2Lu_0.2)₃Al₅O₁₂ was close to that of Al₂O₃. Thermal expansion coefficient of Al₂O₃/(Y_0.2Er_0.2Yb_0.2Ho_0.2Lu_0.2)₃Al₅O₁₂ eutectic HEOCs was very similar to that of Al₂O₃/Y₃Al₅O₁₂ but thermal conductivity was as low as 4.9 Wm⁻¹ K⁻¹ due to strong lattice distortion. These results suggest that high-entropy Al₂O₃/(Y_0.2Er_0.2Yb_0.2Ho_0.2Lu_0.2)₃Al₅O₁₂ eutectic ceramics are promising candidates for structural components application in gas turbine engines.     

Luminescent properties of doped amorphous and polycrystalline Y3Al5O12-Al2O3

Transparent polycrystalline ceramics (TPCs) are crystalline materials with single-crystal-like transparency, which, however, have to rely on fabrication processes with a relatively high cost. Here, we produced lab-scale TPCs based on the typical refractory Y₂O₃-Al₂O₃ system, through full congruent crystallization of the parent glass prepared by aerodynamic levitation melting method. Doping of the glass and TPCs by rare-earth (RE) ions (Ce³⁺, Tb³⁺, Nd³⁺, and Yb³⁺) and transition-metal (TM) ions (Cr³⁺) results in strong visible and near-infrared (NIR) photoluminescence with high quantum yield. The dominance of Stark splitting of the emission band for RE and TM ions in the TPCs as compared with that of the glass confirms crystallization of the parent glasses.     

Fabrication and characterizations of Tb3Al5O12-based magneto-optical ceramics

Transparent terbium aluminum garnet (TAG)-based ceramics were fabricated by vacuum pre-sintering and hot isostatic pressing (HIP) posttreatment from the co-precipitated TAG powders with different stoichiometric ratios. After component optimization, the transparent ceramics with TAG single-phase and attractive optical quality were obtained. The in-line transmittance of optimal Tb_(1+x)3(Al_0.996255Si_0.003745)₅O_{12.0093625+3x/2} (x = −.004, −.002) ceramics (1.7-mm thick) pre-sintered at 1700°C for 20 h with HIP posttreatment at 1700°C for 3 h under 176-MPa Ar reaches 82.6% at the wavelength of 1064 nm. With increasing terbium components, the secondary phase TAP appears in ceramics, which significantly degrades the optical quality of TAG-based ceramics. The Verdet constant of the TAG-based ceramics at 632.8 nm is about −181 rad T⁻¹ m⁻¹ at room temperature, which is about 33% higher than that of the TGG single crystals (−134 rad T⁻¹ m⁻¹).