Eutectic Ceramics Research Articles

Chemical Degradation of the Ternary Al2O3–YAG–ZrO2 Eutectic Ceramic by Molten CMAS

Chemical Degradation of the Ternary Al2O3–YAG–ZrO2 Eutectic Ceramic by Molten CMAS

Unveiling crystallographic texture in laser floating zone melted Al2O3/YAG eutectic ceramic by seed-crystal inducing

This study demonstrated the unique crystallographic features in Al2O3/YAG eutectic by seed-crystal inducing in laser floating zone melting (LFZM) process for crystallographic orientation control. The as-fabricated eutectic ceramics induced by different single crystalline seeds were mainly composed of randomly oriented Al2O3 faceted particles and irregular eutectic, as a result of the initial uncoupled growth at the junction of seed and eutectic. Specifically, along the direction of directional solidification, the eutectic ceramics grew stably with a single crystallographic orientation relationship with low planar disregistry (2 %∼6 %). The stability of phase interface with minimum interfacial strain and better ionic charge balance in Al2O3/YAG eutectic has a greater effect on the regulation of the eutectic growth orientation than the constraint of the single crystalline seed.

The effect of fibers addition on the microstructure and mechanical properties of ZrO2 fibers toughened Al2O3/ZrO2 (Y2O3) solidified ceramics

Fiber toughening is considered an effective way to strengthen the toughness of traditional sintered ceramics. Herein, ZrO2 fibers toughened Al2O3/ZrO2 (Y2O3) directionally solidified eutectic ceramics (DSECs) were prepared with the high-frequency induction heating zone melting (IHZM). The effects of ZrO2 fibers content on the phase composition, microstructure, and mechanical properties of the eutectic ceramics were studied. The findings revealed that DSEC consists of α-Al2O3 and ZrO2 phases, presents a typical colony microstructure. The phase spacing in the colony decreased with the addition of fibers, reaching a minimum value of 1.99 ± 0.25 μm at 20 wt%, which was nearly 40 % lower than that without the fiber’s addition. The hardness of DSEC increased gradually with the addition of fibers. Its highest value reaches 18.42 ± 0.55 GPa when the fiber content is 20 %. The fracture toughness of DSEC first increased and then decreased, reaching a maximum of 5.43 ± 0.22 MPa·m1/2 at 10 wt%, which was 1.5 times higher than that of the fibers-free eutectic ceramic. The improvement of fracture toughness was primarily due to the fibers toughening. In addition, the crack deflection, pinning, bridging, and branching had a significant inhibitory effect on the crack propagation.

Microstructure evolution and mechanical property of melt-grown Al2O3/GdAlO3/ZrO2 eutectic ceramics prepared by laser directed energy deposition

Highly dense Al2O3/GdAlO3/ZrO2 eutectic ceramics are one-step additively manufactured by laser directed energy deposition technique under different scanning speed to investigate the inherent response relationship between processing parameter, microstructure, and mechanical property. During the layer-wise deposition process, planar-cellular transition occurs near the bottom of the molten pool, leading to the transformation of irregular eutectic structure into eutectic colony structure lengthened along the building direction, accompanied by the refinement of microstructure dimension. With the increase of scanning speed, the eutectic spacing decreases, and the initially irregular “Chinese script” eutectic ultimately transforms into hexagonally arranged rod-like eutectic structure induced by lamellar instability. The rod spacing is 1.136 times larger than the lamellar spacing. The mechanical property of the laser 3D-printed Al2O3/GdAlO3/ZrO2 eutectic ceramic is manifested as isotropic. The hardness increases with the refinement of the microstructure, while the fracture toughness decreases as the eutectic morphology transforms into rod-like structure. The averaged hardness and fracture toughness are 16.27 GPa and 3.39 MPa‧m1/2, respectively.

Optimization of growth theory of the directionally solidified alumina based eutectic ceramics

Directionally solidified Al2O3/Y3Al5O12 (YAG), Al2O3/YAG/ZrO2, Al2O3/GdAlO3 (GAP), Al2O3/GAP/ZrO2 and Al2O3/ZrO2 eutectic/non eutectic ceramics were prepared with laser zone melting. Their microstructure, phase component and shape were investigated. The results indicated that the molar ratio of Zr4+ to rare earth ions (Y3+, Gd3+) in the ZrO2 phase of the ternary eutectic was closed to 2:1. The free energy of chemical reaction was the key to determine the precipitation order of phase. The growth of the firstly-precipitated phase could be divided into the formation of ordered Bravais lattice composed by the oxygen ions, the diffusion of metal cations in this lattice and the change of phase morphology. The unfilled cation gap might be a fast channel for the ion transport. On the same growth plane, the nucleation of the later-precipitated phase began only after the firstly-precipitated phase was fully formed. The triangular was the main ion arrangement way for the stable nucleation and growth of phases. The effects of the precipitation temperature, the phase size, the eutectic type and the growth rate on the microstructure were discussed. This improved theory provided a more reasonable explanation for the formation of rod-shaped, layered, triangular, Chinese script (CS) and cellular microstructures, which might be of significance to regulate the morphology of microstructures.

Effects of Laser Power on Molten Pool Morphology, Microstructure, and Mechanical Properties of Al2O3‐ZrO2 Eutectic Ceramics Shaped by Laser Powder Bed Fusion

Effects of Laser Power on Molten Pool Morphology, Microstructure, and Mechanical Properties of Al2O3‐ZrO2 Eutectic Ceramics Shaped by Laser Powder Bed Fusion

Making high-performance Al2O3-3YSZ ceramics with eutectic/polycrystalline structure by hot-pressing eutectic powders

In this communication, the Al2O3-3YSZ ceramic (AYC) was prepared by hot-pressing the eutectic powders. A continuous transition from raw powders mixture to synthesized eutectic powders to sintered eutectic bulk was traced and investigated. The AYC well preserves the structure and phase composition of the original Al2O3-3YSZ eutectic powders (AYEP), which possess a eutectic/polycrystalline composite structure. The average interphase spacing of eutectic region in AYC reaches 106 nm. The strength, toughness, and hardness of AYC specimen are as high as 768 ± 23 MPa, 8.17 ± 0.30 MPa m1/2, and 19.04 ± 0.45 GPa, which far surpasses other solidified and sintered Al2O3-based eutectic ceramics. The excellent mechanical properties can be attributed to the synergistic toughening mechanisms of intergranular/transgranular fracture, t-m phase transformation and residual stress. These results clearly demonstrate the feasibility of sintering eutectic powders method to fabricate Al2O3-based eutectic ceramics concurrent with fine structure and large dimension.

Morphology and formation mechanism of cracks in Al2O3–ZrO2 eutectic ceramics fabricated via laser powder bed fusion

AbstractThe cracks in Al2O3–ZrO2 eutectic ceramics produced by laser powder bed fusion (LPBF) have a significant impact on their practical applications in various industries. In order to understand the factors influencing crack formation, a systematic study of the characterization and propagation of cracks in single‐track, multi‐track, and bulk samples by varying the process parameters has been carried out in this research. The results showed that parallel cracks can be healed by reducing the scanning spacing and scanning length. Additionally, it was found that using a modest scanning speed and a shorter length can minimize the accumulation of thermal stress, resulting in the suppress crack formation. Based on this conclusion, a crack‐free Al2O3–ZrO2 eutectic ceramic sample was finally obtained under the optimized parameters with the power of 100 W, the scanning speed of 100 mm/s, the hatch spacing of 100 μm, the scanning length of 3 mm, and the layer thickness of 50 μm. Additionally, three typical microstructures, including eutectic, cellular, and dendritic structures, were identified in the LPBF‐fabricated Al2O3–ZrO2 samples. The cellular microstructure showed improved crack inhibition capability due to the deflection and pinning effects. Cracks expand more easily in dendritic and eutectic microstructures, where anisotropy is more prominent.

Insights into the applicability of laser energy density in directly preparing Al2O3-based eutectic ceramics by laser additive manufacturing

Laser additive manufacturing (LAM) has become a promising technique to directly fabricate high-performance Al2O3-based eutectic ceramics, but the applicability of laser energy density (LED), a comprehensive index commonly used to regulate the LAM process of metallic parts, in preparing brittle ceramic samples with narrower laser processing windows is not clear yet. In this paper, Al2O3/GAP/ZrO2 eutectic ceramics are prepared by LAM to investigate the effects of LED on forming quality and melting dimensions of the as-deposited eutectic samples. The results indicate that the reliability of using LED as a design parameter depends on the way how it is changed. There is a response relationship between the LED and experimental results when individually adjusting laser power P or scanning speed v, but abnormal phenomena are detected when simultaneously changing P and v. It is found that the melting dimensions obtained under higher LED can be unexpectedly smaller. Moreover, there is a significant difference in the melting dimensions despite using the same LED. In addition, 50 layers thin-wall structured Al2O3/GAP/ZrO2 eutectic specimens prepared under a constant LED present significant differences in the forming quality. It is revealed that the contribution of P to LED is much greater than that of v, which is not consistent with the same influence degree reflected in the calculation formula. Thus, there are significant limitations to utilizing LED to regulate the LAM process of brittle ceramics in consideration of the sensitivity of their forming quality to changes in laser parameters.

Microstructural stability and high temperature strength of directionally solidified Al2O3/Er3Al5O12/ZrO2 eutectic ceramics

Al2O3/Er3Al5O12(EAG)/ZrO2 directionally solidified eutectic ceramics (DSECs) were prepared with high frequency induction heating zone melting (IHZM), and heat treated at high temperature. Their phase components, microstructures, flexural and tensile strengths were investigated. The results indicated that the microstructures were composed of the mutually interpenetrated Al2O3, EAG and ZrO2 (stabilized by Er3+ ion) phases. As the growth rate increased from 2 mm/h to 20 mm/h, the phase size decreased and the irregular microstructure became regular. When the heat treatment temperature was up to 1400 °C, the ionic diffusion was triggered and the phase began to grow. Under the action of surface energy and special microstructure, the growth of phase stopped after it reached a certain threshold. The sample with smaller phase size exhibited higher strength at room temperature, which was dependent on the crack propagation resistance. However, the speed of strength reduction was faster. The formation of surface groove, microcrack and malignant growth phase was responsible for this rapidly decreasing of strength. The flexural and tensile strengths of eutectic ceramic with larger phase size remained at 671.6 MPa and 359.45 MPa after long-term heat treatment at 1500 °C, respectively. The smaller difference between the original phase size and the maximum coarsened phase size was beneficial to avoid the occurrence of the above mentioned harmful factors and was the source of strength stability.

Seed-crystal-induced directional solidification toward Al2O3/(Y0.2Er0.2Yb0.2Ho0.2Lu0.2)3Al5O12/ZrO2 ternary eutectic ceramics

The present work successfully prepared a novel Al2O3/(Y0.2Er0.2Yb0.2Ho0.2Lu0.2)3Al5O12/ZrO2 (or Al2O3/(5RE0.2)AG/ZrO2 in short) ternary eutectic ceramic with the seed-crystal-induced technique by directional solidification. ZrO2 was a cubic structure and was stabilized by all oxides of Y2O3, Er2O3, Yb2O3, Ho2O3, and Lu2O3. The three phases interpenetrated each other and ZrO2 was highly dispersed. Al2O3 inherited well the direction of the c-sapphire seed. The orientation relationship among the three was determined to be [0001]Al2O3 || [100](5RE0.2)AG || [110]ZrO2, (11–20)Al2O3 || (004)(5RE0.2)AG || (002)ZrO2. The three interfaces were all semi-coherent and showed higher interfacial misfit strain energy due to the solid solution effect of (5RE0.2)AG. The novel Al2O3/(5RE0.2)AG/ZrO2 ternary eutectic ceramic showed improved mechanical properties compared to other Al2O3-based eutectic ceramics. The solid solution effect was the main reason for its enhanced hardness and elastic modulus. Several toughening mechanisms including crack bridging, bifurcation, deflection, and screening can be found in the Al2O3/(5RE0.2)AG/ZrO2 ternary eutectic ceramic. The micro-scale suture-like deflection was newly found in the microstructure, which contributed to its fracture toughness.

Effect of the Al2O3 content on the microstructure evolution of flash-sintered Al2O3–8YSZ ceramics

Systematic explorations of complex insulator–conductor composites may help in better comprehending the nature of flash sintering. In this study, flash sintering of alumina–8 mol% yttria stabilized zirconia (Al2O3–8YSZ) binary system with different mixing molar ratios was investigated. The results indicate that Al2O3 content significantly affects the flash-sintering behavior of Al2O3–8YSZ ceramics. The onset temperature of flash sintering increased with the increase in the Al2O3 content under 900 V cm−1. For the Al2O3 content above 20 mol%, Al2O3–8YSZ multiphase ceramics with eutectic morphology could be obtained at 800 °C-700 V⋅cm−1-120 s. Eutectic structures with different morphologies were obtained by adjusting the content of Al2O3. Formation of the eutectic structures is related to the Joule heating and defects introduced by the electric field. Flash sintering is a promising and novel sintering technology for the rapid fabrication of high-performance oxide eutectic ceramics.

Effect of laser focus shift on the forming quality, microstructure and mechanical properties of additively manufactured Al2O3–ZrO2 eutectic ceramics

Melt-grown Al2O3–ZrO2 eutectic ceramics have attracted extensive attention for harsh environment applications. In this work, Al2O3–ZrO2 eutectic ceramic is additively manufactured via one-step laser powder bed fusion (LPBF). The effect of laser focus shift on the relative density, phase formation, microstructure evolution, and mechanical properties was systematically investigated. The as-fabricated specimens are dominated by α-Al2O3 and t-ZrO2. The m-ZrO2 phase increases with increasing the laser defocus shift. The cellular-like structure formed when convection dominated at focal plane, while colony eutectic structure occurred when eutectic growth dominated at higher defocus shift. The deformation of cellular-like structure and needle-like ZrO2 into concave cells and dendritic structures is principally determined by the evolution of solid/liquid (S/L) interfaces. With the formation of needle-like or dendritic ZrO2 ductile phase, the crack propagation is prevented, leading to progressively enhanced fracture toughness with a maximum value of 8.04 MPa m1/2.

Microstructure and mechanical properties of ZrO2 fiber toughened Al2O3/ZrO2(Y2O3) solidified ceramics prepared with high frequency induction zone melting

ZrO2 fiber toughened and non-fiber toughened Al2O3/ZrO2(Y2O3) directionally solidified eutectic ceramics (DSECs) were prepared via high frequency induction zone melting method, and the effects of both the fibers and the growth rate on the microstructure and mechanical properties were investigated. The results showed that both the fiber toughened and the non-fiber toughened DSECs were composed of α-Al2O3, c-ZrO2 and m-ZrO2, and their microstructures were all typical colony structure. The phase spacing in intracolony (λ) for these two kinds of DSECs gradually decreased with the increase of growth rate (ν), they satisfied λ = 10.31ν−1/2 and λ = 13.65 v−1/2, respectively. And the minimum values of λ were 2.33 ± 0.25 µm and 2.99 ± 0.17 µm, respectively, i.e., that is ZrO2 fiber decreased the phase spacing in intracolony about 22%. The fracture toughness for these two DSECs increased continuously with the growth rate, reached the maximum values of 5.72 ± 0.20 MPa m1/2 and 3.95 ± 0.23 MPa m1/2 at 30 mm/h, respectively, i.e., the former was about 1.45 times of the latter. At the same growth rate, the fracture toughness of ZrO2 fiber toughened DSEC were obviously higher than those of DSEC without fiber. When they encounter fibers with high elastic modulus during their propagation, the cracks will undergo deflection, bifurcation, or bridging, and the fibers will undergo debonding and pulling-out under the external stress, both of which consume a large amount of crack propagation energy and achieve toughening effect.

Regulation of Y2O3 addition on structure and properties of Al2O3/ZrO2(Y2O3) directionally solidified eutectic ceramic

Al2O3/ZrO2(Y2O3) directionally solidified eutectic ceramics (DSECs) with different Y2O3 addition were prepared. Three polymorphs of ZrO2: m-ZrO2, t-ZrO2 and c-ZrO2 appeared and disappeared in order with the increase of Y2O3 addition, the effects of Y2O3 addition amount on the mass fraction of ZrO2 polymorphs were quantitatively analyzed. The gradual decrease of area fraction and average size of colony was attributed to the increase of constitutional undercooling region, the variation of interphase spacing in the colonies was explained via selection mechanism of dendrite tip. The maximum hardness and fracture toughness were 18.35 GPa and 8.55 MPa m1/2 obtained at 9 mol% and 3 mol% Y2O3 addition, respectively. The hardness was determined by the mass fraction of m-ZrO2 phase, the area fraction of intercolony and the residual compressive stress, and the toughening mechanisms were composed of microcrack toughening, t-m transformation toughening and residual stress toughening.

Large-size complex-structure ternary eutectic ceramic fabricated using laser powder bed fusion assisted with finite element analysis

Melt-grown oxide eutectic ceramics are well known for their excellent mechanical properties and microstructural stabilities at ultra-high temperatures. Laser powder bed fusion (LPBF) provides a novel technology for manufacturing materials with large sizes and complex structures. However, crack controlling is a serious challenge in this process owing to the intrinsic brittleness of ceramic materials. In this study, LPBF method is applied to fabricate Al2O3/GdAlO3/ZrO2 ternary eutectic ceramics assisted by a finite element method (FEM) to investigate an optimized process and innovatively reveal the crack formation mechanism and propagation behavior via a single track, a single layer, and multilayers. Temperature and thermal stress distributions under different process parameters are obtained. The maximum tensile stress σy (in front of and behind the edge of the molten pool) perpendicular to the scanning direction induces a longitudinal crack, the tensile stress σx (at the side rim of the molten pool) parallel to the scanning direction induces a transverse crack, and the shearing stress on the XY plane promotes the deflection of the transverse crack on one side of the single track. In the multilayer, the maximum principal stress occurs at the margin zone of the deposition. The maximum principal stress increases at the first three layers and then decreases after the fourth layer. Based on the simulation result, the as-solidified Al2O3/GdAlO3/ZrO2 eutectic ceramics with multiple shapes, such as cylindrical, cubic with round corners, triangular prism, and quadrangular, with dimensions of 30 mm (width or diameter) × 30 mm (length or diameter) × 1 mm (height) and ultra-fine eutectic microstructures (eutectic spacing of approximately 88 nm) are fabricated by LPBF, providing a significant potential to overcome the bottleneck for fabricating large-size and complex-structure ceramics directly in a single-step process. This work provides theoretic foundation and technical support for preparation of ultra-high temperature solidified oxide eutectic ceramics with ultra-fine microstructures and complex structures directly, which has great application potential in fields as aerospace, machinery, and chemical industry.

One-step preparation of melt-grown Al2O3/GdAlO3/ZrO2 eutectic ceramics with large size and irregular shape by directed energy deposition

One-step preparation of melt-grown Al2O3/GdAlO3/ZrO2 eutectic ceramics with large size and irregular shape by directed energy deposition

Nanocrystalline Alumina-Zirconia-Based Eutectic Ceramics Fabricated with High-Energy Beams: Principle, Solidification Techniques, Microstructure and Mechanical Properties.

Nanocrystalline alumina-zirconia-based eutectic ceramics fabricated with high-energy beams and composed of ultrafine, three-dimensionally entangled, single-crystal domains are a special category of eutectic oxides that exhibit exceptionally high-temperature mechanical properties, such as strength and toughness as well as creep resistance. This paper aims to provide a comprehensive review on the basic principles, advanced solidification processes, microstructure and mechanical properties of alumina-zirconia-based eutectic ceramics, with particular attention to the status of the art on a nanocrystalline scale. Some basic principles of coupled eutectic growth are first introduced based on previously reported models, followed by concise introduction of solidification techniques and the control strategy of solidification behavior from the processing variables. Then, the microstructural formation of nanoeutectic structure is elucidated with regard to different hierarchical scales, and mechanical properties such as hardness, flexural and tensile strength, fracture toughness and wear resistance are discussed in detail for a comparative study. Nanocrystalline alumina-zirconia-based eutectic ceramics with unique microstructural and compositional characteristics have been produced with high-energy beam-based processes, and in many cases, promising improvements in mechanical performance have been reported as contrasting with conventional eutectic ceramics.

Can orientations of directionally solidified dual-phase Al2O3/YAG eutectics be induced by single-phase sapphire seeds?

• The orientations of the dual-phase directionally solidified Al 2 O 3 /Y 3 Al 5 O 12 (YAG) eutectic ceramics were induced successfully by various single-phase sapphire seeds. They show special crystallographic orientation relationships (COR I-IV). • The directions of Al 2 O 3 in the eutectics were governed by the seeds and the YAG in each induced eutectics had a specific growth direction endowed by the Al 2 O 3 in the as-induced eutectics or the sapphire seed. • Based on calculated planar lattice misfits and total interfacial strain energies of four CORs, which indicated that the constraint of the sapphire seed makes the YAG grow following the rule of minimizing the interfacial strain energy. Directionally solidified dual-phase Al 2 O 3 /Y 3 Al 5 O 12 (YAG) eutectic ceramics (DSECs) typically exhibit strong anisotropy. To improve their properties, various single-phase sapphire seeds, including r -axis [11_02], m -axis [101_0], c -axis [0001], and a -axis [112_0], were used as seeds to induce the orientation of the Al 2 O 3 /YAG DSECs. The results showed that Al 2 O 3 in the eutectics could be governed by the sapphire seeds. The YAG in each induced eutectic had a specific growth direction endowed by Al 2 O 3 in the as-induced eutectics or the sapphire seed. Herein, we calculated the planar lattice misfits and interfacial strain energies of four crystallographic orientation relationships based on the constructed lattice models. It was elucidated the constraint of the sapphire seed caused YAG to grow following the rule of minimizing the interfacial strain energy. This revealed the reason why Al 2 O 3 /YAG DSECs orientation can be successfully induced. These results may provide a novel method for the design of high-performance eutectic ceramic materials.

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.