Ceramic Eutectic Composites Research Articles

Ceramic eutectic composites based on B4C directionally reinforced by high-entropy (TiZrHfNbTa)B2 boride

Directionally solidified B4C/(Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)B2 eutectic high-entropy ceramics were prepared by the floating zone method based on the crucibleless zone melting of compacted powders using B4C and transition metal diboride (TiB2, ZrB2, HfB2, NbB2, TaB2) powders as initial materials. The structure of as-prepared composites consists of a B4C matrix uniformly reinforced by a single-phase high-entropy (Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)B2 diboride. The XRD analysis of the composites confirmed the presence of the following phases in their compositions: B4C and (Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)B2. The effect of the solidification rate on structural and mechanical characteristics of the B4C/(Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)B2 ceramics was revealed. It was found that an increase in the solidification rate leads to a decrease in the size of the reinforcing phase and an increase in hardness and fracture toughness from 36 to 39 GPa and 5.9–7.3 MPa·m1/2, respectively.

Unveiling the dynamic instability mechanism of microstructure transformation in faceted oxide eutectic composite ceramics

Microstructure control is a great challenge in the high-temperature gradient directional solidification of eutectic composite ceramics due to the complex solidification behavior. Herein, the microstructure transformation of faceted Al2O3/Er3Al5O12 thermal emission eutectic composite ceramics is explored over wide ranges of compositions (13.5 mol%–22.5 mol% Er2O3) and solidification rates (2–200 μm/s). Entirely coupled eutectics with primary phases suppressed are fabricated and the coupled zone is broadened in a wide range of 15.5 mol%–22.5 mol% Er2O3 at low solidification rates. The competitive growth between eutectic and dendrite is evaluated on the basis of the maximum interface temperature criterion. In addition, the mechanisms of irregular eutectic spacing selection and adjustment under different solidification rates are revealed based on Magnin–Kurz model. A successful prediction of lamellar to rod-like eutectics is achieved associated with the dynamic instability of lamellar eutectic and the corresponding enlarged coexistence region is mapped based on the interface undercooling. According to the well microstructure tailoring, the flexural strength of Al2O3/Er3Al5O12 eutectic composite ceramics has improved from 508 MPa up to 1800 MPa due to the refined eutectic spacing with low fluctuation. The eutectic composite ceramics show strong selective optical absorption and the intensity increases with the refining microstructure. The as-designed Al2O3/Er3Al5O12 composites with microstructural tailoring have great potential as integrations of structural and functional materials.

One-step additive manufacturing of TiCp reinforced Al2O3–ZrO2 eutectic ceramics composites by laser directed energy deposition

With the rapid and adaptable process properties, laser directed energy deposition (LDED) is increasingly becoming popular for the direct additive manufacturing of melt growth eutectic composite ceramics. In this research, LDED is used to fabricate TiCp-reinforced Al2O3–ZrO2 eutectic ceramics. The inhibitory mechanism of various ratios (0 wt % - 50 wt %) of TiCp on crack and porosity defects is investigated. In addition, a summary of the influence of TiCp on the microstructure and mechanical properties is provided. The results indicate that TiCp particles are uniformly distributed throughout the matrix, and the eutectic microstructure around TiCp exhibits an irregular eutectic transition as a result of a decrease in solidification rate. The mismatch of thermal expansion coefficient and elastic modulus between TiCp and Al2O3–ZrO2 eutectic matrix causes the cracks to be pinned and transgranular, which suppresses the cracks effectively. Meanwhile, doping TiCp particles enhances the molten pool impact and increases the molten pool temperature, which accelerates the gas escape rate, leading to the porosity decreasing from 6.37% to 0.29%. In comparison to Al2O3–ZrO2 eutectic ceramics, 50 wt% TiCp content results in the greatest flexural strength and fracture toughness, with increases of 34.18 and 13.42%, respectively. The greatest compressive strength is attained at 30 wt% TiCp doping, which is approximately 83.75% greater than eutectic ceramics.

Effect of defect distribution on thermal expansion coefficient of eutectic composite ceramics

Based on Eshelby’s equivalent inclusion theory, the four-phase model and the interaction direct derivative estimate, the prediction model of effective thermal expansion coefficient of composite containing multiple types of inclusion in anisotropic matrix was established. The effective thermal expansion coefficient of eutectic composite containing defects was calculated. And then the influence of defects and inclusions on the effective thermal expansion coefficient is discussed in detail. The results show that the influence of inclusions will be amplified by defects when there are multiple inclusions in the matrix. Therefore, the interaction direct derivative estimate cannot accurately predict the influence of defect distribution on thermal expansion coefficient of eutectic composite ceramics.

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·m1/2, respectively. The results indicate that SLM method is a highly effective technique for fabricating high-performance net-shaped structural composite ceramics.

Constitutive model of multiphase ceramics based on microcrack damage evolution

abstract The damage evolution of microcracks that dispersed in eutectic composite ceramics is an important factor leading to fracture failure. We propose a meso damage model (CMMC), which uses our proposed two-scale cellular model with microcracks, and fully considers the characteristics of the microstructure on the eutectic composite ceramics, thus providing a more objective structural modeling. In addition, there are two significant features existing in CMMC: One is to use the interaction direct derivative (IDD) estimate method to calculate the additional flexibility increment caused by a single open elliptical microcrack, which considers the spatial orientation of the microcrack. The other is that the additional flexibility tensor from the microcrack system is obtained by introducing probability density function and selecting appropriate microcrack growth criteria. Experiments on the related datasets show that CMMC achieves the consistent results compared with the experimental data. Furthermore, the effects of microcrack aspect ratio and microstructure parameters on the effective properties of multiphase ceramics were analyzed by using numerical method. • Damage evolution of microcracks in composite ceramics leading to fracture failure. • A meso damage model based on microcrack damage evolution is proposed. • Microcrack aspect ratio affects effective properties of multiphase ceramics. • Microstructure parameters influences effective properties of multiphase ceramics. • The additional flexibility tensor from the microcrack system is obtained.

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.

Rupture stress of eutectic composite ceramics with rod-shaped crystals

Eutectic composite ceramics has a wide range of applications in the aerospace industry due to its excellent mechanical properties. The rupture stress of the materials is a subject of considerable importance. Eutectic composite ceramics primarily consist of rod-shaped crystals, with a small amount of particles and preexisting defects dispersed throughout. Aligned nano-micron fibers are embedded within the rod-shaped crystals. Rupture stress of a eutectic composite ceramic depends on its fracture surface energy and preexisting defects. In this study, the equivalent fracture surface energy of a eutectic ceramic composite was calculated based on its additional fracture work. Next, the effects of the preexisting defects were considered. Then, a micromechanical model of the eutectic composite ceramic was established based on its microstructural characteristics. The defects were assumed to be lamellar, and the surrounding matrix was assumed to be transversely isotropic. Using this information, the rupture stress of the eutectic ceramic composite was predicted. A comparison of the theoretical and experimental results indicated that the predicted rupture stresses corresponded with the tested data.

Damage and fracture model for eutectic composite ceramics

Study of damage and fracture models to analyze the fracture mechanism of eutectic composite ceramics is of considerable importance because no accurate fracture models are available for these materials. Eutectic composite ceramics are composed of microcells with random direction. We present herein a model that predicts the damage and fracture of eutectic composite ceramics based on analysis of defect stability and the damage localization band. Firstly, given the microstructure of eutectic composite ceramics, a mesodomain and a microcell model are constructed. The local stress field in the mesodomain is then analyzed based on the interaction direct derivative estimate. Secondly, the stability of a defect around particles in a microcell is analyzed, and the stress intensity factor of an annular defect under the applied stress field and the residual stress field in the particle are calculated. The stress intensity factor of a defect is controlled by the residual stress when the defect extension is small. However, it is controlled by the applied stress when the defect extension is large. Finally, a model for the damage localization band at the crack tip is constructed based on the Dugdale–Barenblatt model. The residual intensity is the important factor affecting the length of the damage localization band. When the damage variables reach their largest value, the residual intensity and the length of the damage localization band attain their minimum value. This work provides the theoretical basis for further study of the damage mechanics of eutectic composite ceramics and guides the engineering applications of these materials.

Microstructure tailoring and thermal stability of directionally solidified Al2O3/GdAlO3 binary eutectic ceramics by laser floating zone melting

Directionally solidified Al2O3/GdAlO3 eutectic composite ceramics with low solidification defects are prepared by laser floating zone melting to tailor homogeneous microstructure and thermal stability performance under high temperature gradient. At low solidification rate, the solidification microstructure shows periodical growth striations in which coarse colony microstructure and homogeneous ‘Chinese script’ irregular eutectic structure are coexisted. The growth striation interval L and solidification rate V follow the relationship of L/V = (238.8 ± 2.4) s. By further optimizing laser processing parameters, the eutectic spacing is greatly refined to submicron scale and microstructure oscillation is eliminated at high solidification rates, and fully regular lamellar/rod eutectic structures are obtained by faceted eutectic transition. The rod-like eutectic spacing (λ) and solidification rate (V) follow the linear relationship of λ·V1/2 = 6.21 µm3/2 s−1/2. Furthermore, the as-solidified eutectic composite even with fine eutectic structure exhibits excellent thermal stability after heat exposure at 1500 °C for 250 h.

Mechanical properties of individual phases of ZrB2-ZrC eutectic composite measured by nanoindentation

Zr-based ceramics are of interest for applications in aerospace propulsion, owing to their high hardness and stability. Application of these ceramics requires better understanding of their mechanical behavior in small scales and at elevated temperatures. Here, we prepared a ZrB2-ZrC ceramic eutectic composite, consisting of self-assembled ZrC rods grown within a single-crystalline ZrB2 matrix, and studied the mechanical properties of the individual eutectic phases using nanoindentation at both room and elevated-temperatures. The Vickers hardness of the eutectic composite was measured at room temperature. These data provide insight into the understanding of the mechanical behavior of the individual phases of non-oxide ceramic eutectic composites for high-temperature applications.

The intrinsic strength of triangular symmetrical composite eutectic

Composite ceramic has the excellent properties at normal and high temperatures, especially when the structure of the composite eutectic is triangular symmetrical. Obviously, mechanical behavior and fracture properties of composite ceramic closely relates to the micro-structure of symmetrical triangular eutectic. In order to reveal the mechanical properties of eutectic composite ceramic, it is necessary to determine the intrinsic strength of triangular composite eutectic. Since the fiber and matrix of triangular symmetrical composite eutectic sharing a same covalent bond, the theoretical cohesion strength of symmetrical triangular eutectic was obtained by the combination-separation displacement of intrinsic bond. Basing on micro-structure plastic deformation before fracture of composite eutectic matrix, the dislocation pile-up model of eutectic composite ceramics was established. And then intrinsic bond fracture shear stress of triangular symmetrical composite eutectic was given by using the theory of dislocation pile-up. According to the macroscopic structure properties of triangular symmetrical composite eutectic and the distribution of stress field of composite eutectic, intrinsic strength of eutectic was obtained. The results shows that intrinsic strength of triangular symmetrical composite eutectic possessed clear size-dependence and the stress decreases with the increases of the diameter of fiber inclusions.

ZrB2–ZrCxN1−x Eutectic Composites Produced by Melt Solidification

Ceramic eutectics are naturally occurring in‐situ composites and can offer superior mechanical properties. Here, ZrB2–ZrCxN1−x quasi‐binary ceramic eutectic composites were produced by arc‐melting a mixture of ZrB2, ZrC, and ZrN powders in an N2 atmosphere. The arc‐melted ZrB2–ZrCxN1−x composites containing 50 mol% of ZrB2 (irrespective of the ZrC/ZrN ratio) showed rod‐like eutectic structures, where ZrCxN1−x single‐crystalline rods were dispersed in the ZrB2 single‐crystalline matrices. Multiple orientation relationships between the ZrCxN1−x rods and the ZrB2 matrices were observed, and one was determined as ZrB2 {} //ZrxN1−x {111} and ZrB2 < > //ZrCxN1−x < > . The rod‐like eutectic composites had higher hardness than the hypo‐ and hypereutectic composites and the 50ZrB2–40ZrC–10ZrN (mol%) eutectic composite showed the highest Vickers hardness (Hv) of 19 GPa.

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

Abstract The fluorescence from (naturally present) Cr 3+ 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.

The Mechanical-Stress-Field of Matrix in Eutectic Ceramic Composite

Based on the interaction between nano-fiber and eutectic interphase, forth-phase mode is used to get the mechanical stress field of matrix in eutectic composite ceramics. The effective flexibility increment tensor of eutectic ceramic composite is obtained by the volumetric average strain. The remote stress boundary condition of the eutectic composite ceramis is accounted for getting the mechanical stress field in matrix. The results show the mechanical stress field of the matrix is associated with the stiffness and the volume fractions of each component in eutectic composite ceramic , the shape of interphase and nano-fiber. The stresses in matrix will decrease due to the strong constraining effects of the eutectic interphase. The eutectic interphase make the eutectic composite ceramics strengthen.

TiB<sub>2</sub>-(Ti,W)C Eutectic Composite Ceramics Prepared by Combustion Synthesis under High Gravity

Large bulk solidified TiC-TiB2 eutectic composite ceramics were prepared by combustion synthesis under high gravity, and the WO3 was introduced into the combustion synthesis as one of oxidants in thermit to obtain the Ti-W-Cr-C-B liquid, so near-full-density TiB2-(Ti,W)C eutectic composite ceramics without the macrocracks were achieved. The ceramic matrix was mainly composed of TiB2-(Ti,W)C eutectic microstructures, and a few of Al2O3 and Al2O3-ZrO2 eutectic structures were also detected in between Ti-W carbides. Due to the introduction of the high gravity field, Stocks immigration of the immiscible liquids took place due to their density differences, resulting in float-up of oxide liquid and settle-down of Ti-W-Cr-C-B liquid, and the layered melt consisting of oxide liquid and Ti-W-Cr-C-B liquid was formed, finally, TiB2-(Ti,W)C eutectic composite grown from the melt. Due to the mutual solubility of W-Ti, the W atom diffused into the TiC, leading to the formation of (Ti,W)C solid solution as same as crystal lattice structure of TiC. The relative density, Vickers hardness and fracture toughness of the composite ceramics measured 98.4%, 26.4 GPa and 7.6±0.5 MPa•m1/2, respectively.

Large-Bulk Solidified Al<sub>2</sub>O<sub>3</sub>-ZrO<sub>2</sub>(Y<sub>2</sub>O<sub>3</sub>) Prepared by Self-Pressure Assisting Combustion Synthesis under High Gravity

By introducing self-pressure processing into combustion synthesis under high gravity, Al2O3-ZrO2(4Y) eutectic composite ceramics without shrinkage cavities and holes were achieved, and the self-pressure processing also made the macro-crack be controlled and the ceramic densification be promoted evidently. XRD results showed the eutectic ceramics were composed of α-Al2O3, t-ZrO2 and a few m-ZrO2 phases, and the volume fraction of the transformable t-ZrO2 increased with self-pressure force increasing. SEM images showed that with increasing self-pressure force, the rod-shaped eutectic colonies were refined and the volume fraction of the colonies increased. Meanwhile, the irregular ZrO2 crystals around eutectic colonies transformed to be fine t-ZrO2 spherical crystals gradually, and thickness of eutectic colonies decreased. With the changes in microstructures of the ceramics, the mechanical properties of the ceramics were improved greatly.

Bulk ultrahard composites in the eutectic TiB2-TiC system by SHS under high gravity

Eutectic TiB2-TiC composite ceramics were prepared by combustion synthesis under high gravity. XRD, SEM, and EDS results showed that TiB2-TiC composites were mainly composed of the eutectic microstructures of a TiC matrix in which a large number of fine TiB2platelet grains were dispersed uniformly; meanwhile, at the boundaries of the eutectic microstructures, discontinuously dispersed ɛ-carbides enriched in Ti atoms, and a few isolated irregular α-Al2O3 grains and Al2O3-ZrO2 colonies were observed. Because high-temperature chemical reaction results in fully liquid products, the application of high gravity induces the Stocks flow in the melts, which leads to the formation of layered melts consisting of liquid Ti-Cr-C-B melt and liquid oxides. Therefore, it is considered that TiB2-TiC composites grow through eutectic transformation far away from the equilibrium state. The results of properties measurements indicate that, with increasing mass fraction of B4C + Ti + C in combustion systems, the relative density and fracture toughness of TiB2-TiC composites are all among 97–99% and 6.5–7.1 MPa m1/2, respectively, and the Vickers hardness and flexural strength are increased gradually to the maximum values of 28.6 GPa and 615 MPa, respectively. The achievement of full-density TiB2-TiC composites benefited from the design of fully liquid SHS products and application of high-gravity field, a high hardness of the composite ceramics resulted from the absence of intermediate borides, the achievement of stoichiometric TiC phases is due to rapid solidification, whereas a high flexural strength of the composite ceramics benefited from the homogenization and refinement of the microstructures due to the rapid separation of the liquid oxides and the rapid coupled growth of TiB2-TiC.

Microstructural and Interface Analysis of Ceramic Eutectic Composites

Extended abstract of a paper presented at Microscopy and Microanalysis 2008 in Albuquerque, New Mexico, USA, August 3 – August 7, 2008

Growth Characteristic and Fracture Toughness of Laser Rapidly Solidified Al<sub>2</sub>O<sub>3</sub>/YAG/ZrO<sub>2</sub> Ceramic Eutectic <i>In Situ<i> Composite

Directionally solidified oxide ceramic eutectic composites with superior strength, oxidation resistance, creep resistance, structural stability and low sensitivity to crack at high temperature have aroused much attention in recent years, and various preparation techniques have been developed. In situ fabrication of ceramic eutectic composites by laser rapid solidification is a cheap and quick method compared to conventional multi-step fabrication methods of fiber reinforced composites for high temperature use. In this paper, Al2O3/YAG/ZrO2 ternary eutectics are rapidly prepared from melt by directional solidification using laser zone remelting technique, the growth characteristic and fracture toughness are investigated. The results show that: (1) Laser rapidly solidified Al2O3/YAG/ZrO2 ceramic eutectic in situ composite presents a fine interpenetrating network structure, in which Al2O3, YAG and ZrO2 phases are continually interconnected and finely coupled without pores, colonies and grain boundaries between interfaces. (2) Laser scanning rate and power density strongly affect the eutectic growth. With the processing parameters adjusted properly, the eutectic shows homogeneous and coupled lamellar microstructure. The characteristic dimensions of the microstructure are around 2~3 1m for Al2O3 and YAG phases, and around 0.2~1 1m for ZrO2 phases, respectively. (3) The hardness and fracture toughness of the rapidly solidified Al2O3/YAG/ZrO2 eutectic are 16.7 GPa and 8.0 MPa.m1/2, respectively.