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.     

Microstructure, growth mechanism and mechanical property of Al2O3-based eutectic ceramic in situ composites

Directionally solidified Al₂O₃-based eutectic ceramic in situ composites with inherently high melting point, low density, excellent microstructure stability, outstanding resistance to creep, corrosion and oxidation at elevated temperature, have attracted significant interest as promising candidate for high-temperature application. This paper reviews the recent research progress on Al₂O₃-based eutectic ceramic in situ composites in State Key Laboratory of Solidification Processing. Al₂O₃/YAG binary eutectic and Al₂O₃/YAG/ZrO₂ ternary eutectic ceramics are prepared by laser zone melting, electron beam floating zone melting and laser direct forming, respectively. The processing control, solidification characteristic, microstructure evolution, eutectic growth mechanism, phase interface structure, mechanical property and toughening mechanism are investigated. The high thermal gradient and cooling rate during solidification lead to the refined microstructure with minimum eutectic spacing of 100 nm. Besides the typical faceted/faceted eutectic growth manner, the faceted to non-faceted growth transition is found. The room-temperature hardness H_V and fracture toughness K_IC are measured with micro-indentation method. For Al₂O₃/YAG/ZrO₂, K_IC = 8.0 ± 2.0 MPa m^1/2 while for Al₂O₃/YAG, K_IC = 3.6 ± 0.4 MPa m^1/2. It is expectable that directionally solidified Al₂O₃-based eutectic ceramics are approaching practical application with the advancement of processing theory, technique and apparatus.     

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.     

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.     

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.     

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.     

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.     

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.     

One-step additive manufacturing and microstructure evolution of melt-grown Al2O3/GdAlO3/ZrO2 eutectic ceramics by laser directed energy deposition

Synchronized-powder-feeding-based laser directed energy deposition (LDED) has great application potential for the rapid fabrication of large-scale composite ceramics with complex shapes. In this study, near-full-density Al₂O₃/GdAlO₃/ZrO₂ ternary eutectic ceramics with different shapes and smooth surfaces were directly prepared by using an improved LDED device. Spherical ceramic powders with eutectic composition and good flowability were obtained by centrifugal spray drying. The microstructure characteristics and microstructure evolution of the rapidly solidified 3D-printed eutectic ceramic were systematically elucidated. In particular, the formation mechanism of the observed periodic banded structures was revealed through a unique laser partial remelting technique. The result indicated that the appearance of the banded structure is attributed to the drastic abnormal coarsening of the nanoscale microstructures adjacent to the molten pool. On the basis these results, a physical model was proposed to illustrate the microstructure evolution of the 3D-printed Al₂O₃/GdAlO₃/ZrO₂ eutectic ceramic.     

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.     

Ultrafast formation of Al2O3–Y3Al5O12 eutectic ceramic by flash sintering

In this study, a dense Al₂O₃–Y₃Al₅O₁₂ (YAG) ceramic was synthesized by flash sintering a powder mixture of Al₂O₃ and Y₂O₃ in less than 150 seconds at a furnace temperature of 1350°C. The resultant ceramic has a well-defined eutectic structure consisting of alternating Al₂O₃ and YAG layers. The hardness and fracture toughness of the ceramic were measured to be 18.5 GPa and 4.3 MPa.m^1/2, respectively. These values are comparable to those of similar eutectic ceramics made by directional solidification techniques. The results suggest a new method for making high-performance eutectic ceramics, which could be applied in other systems.     

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.     

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.     

In-situ microcantilever deflection to evaluate the interfacial fracture properties of binary Al2O3/SmAlO3 eutectic

This study aims to elucidate the impact of interfacial microstructure and properties on fracture toughness in binary Al₂O₃/SmAlO₃ eutectic and its polycrystalline samples via in-situ microcantilever deflection tests. Al₂O₃-SmAlO₃ eutectic with semi-coherent heterointerfaces exhibits superior interfacial toughness of 2.85 MPa?m^1/2 relative to randomly oriented interfaces in Al₂O₃/SmAlO₃ polycrystals. SmAlO₃ grain boundaries (GBs) have the lowest toughness of 1.97 MPa?m^1/2 among all the constituents, i.e. bulk and interfaces. The toughness of eutectic Al₂O₃-SmAlO₃ interfaces is intermediate between polycrystalline Al₂O₃-Al₂O₃ GBs and SmAlO₃-SmAlO₃ GBs. Fracture toughness values of both polycrystalline and eutectic Al₂O₃-SmAlO₃ samples obtained by mesoscopic indentation measurements are in the range of 4.2 MPa?m^1/2to 4.7 MP?m^1/2, which exceeds all constituent components. Much of the overall fracture toughness of the bulk samples derives from microstructural and geometric energy dissipation mechanisms.     

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.     

Directly fabricated Al2O3/GdAlO3 eutectic ceramic with large smooth surface by selective laser melting: Rapid solidification behavior and thermal field simulation

Selective laser melting (SLM), a novel approach for one-step melting and solidifying ceramic powder beds layer by layer without post-process of degreasing and sintering, has been developed to directly prepare highly dense (>95 %) Al₂O₃/GdAlO₃(GAP) eutectic composite ceramics with large smooth surfaces. Compact net-shaped plates with the maximum size of 73 × 24 × 5 mm³ are obtained by different strategies of laser pre-heating and multi-tracks’ deposition without any binders. Combined with the finite element thermodynamic coupling simulation results, it is proved that the stress between the substrate and depositions during SLM can be greatly reduced by the step-up preheating, and thus effectively improving the ceramic forming quality. The macro-morphology, microstructure evolution, rapid solidification behavior and mechanical properties of the SLM-ed eutectic ceramics are systematically investigated at different laser processing parameters. The microstructure transforms from ultra-fine irregular eutectic to complex regular eutectic with the increase of the scanning rate. The average eutectic spacing, and solidification rate has an approximately linear relationship consistent with the Jackson-Hunt (JH) model. The microhardness and fracture toughness can reach 17.1 ± 0.2 GPa and 4.5 ± 0.1 MPa·m^1/2, respectively. The results indicate that SLM method is a highly effective technique for fabricating high-performance net-shaped structural composite ceramics.