Eutectic Spacing Research Articles

Microstructure, Crystallographic Orientation and Mechanical Property in AlCoCrFeNi2.1 Eutectic High-Entropy Alloy Under Magnetic Field-Assisted Directional Solidification

Microstructure, crystallographic orientation and mechanical properties in the AlCoCrFeNi2.1 eutectic high-entropy alloy (EHEA) formed using magnetic field-assisted directional solidification were studied experimentally. An aligned eutectic structure formed using magnetic field-assisted directional solidification. Both eutectic lamellar spacing and eutectic cellular spacing decreased with the increasing growth speed and the application of a magnetic field. The crystallographic orientation relationship between FCC phase and BCC (B2) phase in AlCoCrFeNi2.1 EHEAs was investigated and only one orientation relationship, {110}BCC//{111}FCC〈1$$\overline{1}$$2〉BCC//〈11$$\overline{2}$$〉FCC was found. Significantly, the magnetic field-assisted directionally solidified AlCoCrFeNi2.1 EHEA possessed the highest elongation to failure (ef) relative to the same EHEA series reported so far. The ef reached about 45 pct with an ultimate tensile strength of ~ 1058 MPa. This enhancement in mechanical properties can be attributed to the microstructural modification caused by the directional solidification and/or the magnetic field, highlighting an effective pathway to achieve superior mechanical properties in EHEAs.

Improvement of microstructure and tensile properties of Sn–Bi–Ag alloy by heterogeneous nucleation of β-Sn on Ag3Sn

Different weight fractions (0, 0.5, 1.0, 2.0, and 3.0 wt%) of sliver (Ag) were used to dope Sn–Bi eutectic alloys in this study. The microstructure and tensile and thermal properties of Sn–Bi eutectic alloys doped with Ag were investigated. The results indicate that the addition of Ag increases the solidification temperature and reduces the fusing latent heat of alloy, but it has no significant effect on the melting point of the alloy. According to the electron backscattered diffraction (EBSD) analysis and calculation of the disregistry between ε-Ag3Sn and β-Sn, ε-Ag3Sn can be used as the heterogeneous nucleation substrate of β-Sn, thus refining lamellar eutectic spacing. The orientation relationship between the ε-Ag3Sn phase and the β-Sn phase was as follows: [ 0 2‾1 ]Ag3Sn//[ 0 1 2 ]β-Sn, (0 1 2)Ag3Sn//(‾1 2‾1) β-Sn, and [‾1 2 0 ]Ag3Sn//[ 0 1 2 ]β-Sn, (2 1‾1)Ag3Sn//(‾1 2‾1)β-Sn. The tensile strength of the eutectic Sn–Bi alloy with addition of 1.0 wt% Ag reached 72 MPa, which was 20% higher than that of the original alloy. The analysis showed that Ag doping increase the tensile strength of the eutectic Sn–Bi alloy, because of fine grain strengthening and second phase strengthening mechanism

Effects of Growth Rate on Eutectic Spacing, Microhardness, and Ultimate Tensile Strength in the Al–Cu–Ti Eutectic Alloy

In the present work, dependences of lamellar spacing (λ), microhardness (HV), and ultimate tensile strength (σUTS) on growth rate (V) were investigated in the Al–Cu–Ti (Al–33 wt % Cu–0.1 wt % Ti) eutectic alloy. For these purposes, the Al–Cu–Ti eutectic alloy was solidified at a constant temperature gradient of 6.45 K mm–1 with a wide range of growth rates, from 8.58 to 2038.65 µm/s. Then, the values of λ, HV, and σUTS were measured on the directionally solidified Al–Cu–Ti specimens. The dependences of λ, HV, and σUTS on V in the Al–Cu–Ti eutectic alloy were experimentally obtained using regression analysis. The results obtained in the present work were compared with the similar experimental results in the literature.

Effects of solidification thermal parameters and Bi doping on silicon size, morphology and mechanical properties of Al-15wt.% Si-3.2wt.% Bi and Al-18wt.% Si-3.2wt.% Bi alloys

The use of bismuth (Bi) in aluminum (Al) industries is recognized as prime importance because of possible recycling as alloying element in special applications such as those enhancing machinability. The effects of Bi interaction on solidification progress as well as on the resulting microstructure of hypereutectic Al-Si alloys, i.e. the morphology and size of Si crystals are rare information. As such, the present research work investigates the characteristic parameters of eutectic and primary Si with addition of Bi for both ternary Al-15 wt.%Si- 3.2 wt.%Bi and Al-18 wt.%Si- 3.2 wt.%Bi alloys samples directionally solidified (DS) under various cooling rates. The alloy containing 15 wt.%Si showed that Bi acts in the suppression of the growth of primary Si crystals for samples solidified at cooling rates ranging from 2.0 to 31 K/s. For lower rates, a minor fraction of five-fold branched Si was recognized in transverse specimens. A mixture of globular-fibrous eutectic Si is shown to occur for eutectic cooling rates higher than 5 K/s. The Al-18 wt.%Si- 3.2 wt.%Bi alloy is characterized by a mixed structure of eutectic flaky Si and primary angular Si with intersecting spines at nearly 90º all over the entire range of experimental cooling rates (i.e., from 0.4 to 40 K/s). Despite the intricacy of the formed microstructures, it was possible to establish relationships between tensile properties and eutectic Si spacing in order to compare the properties between the tested alloys. Tensile strength and elongation decrease with increasing alloy Si content.

Simulation of three-dimensional eutectic growth multi-phase field based on OpenCL parallel

Based on the muti-component eutectic multi-phase field model of S. G. Kim, W. T. Kim, T. Suzuki et al. [J. Cryst. Growth 261, 135–158 (2004)], the high-performance computing method of hardware and software architecture of OpenCL + graphics processing unit (GPU) was studied. Taking CBr4–C2Cl6 as an example, the evolution process of large-scale three-dimensional eutectic structure growth is realized by concurrent execution of multiple processes and multiple threads on two heterogeneous platforms of AMD and NVIDIA, respectively. The effects of different initial lamellar spacing and flow on eutectic lamellar morphology were also studied. The results show that, with the increasing of eutectic lamellar spacing, the morphology of eutectic lamellar changes are as follows: the symmetrically steady-state perpendicular to the interface is based on growth, the slightly oscillating state is unstable, and the large oscillating state is unstable; Under the condition of forced convection, the symmetrically alternating growth pattern of the original eutectic lamellar was broken, the melt flow led to the change of eutectic growth morphology, and the eutectic lamellar grew in the opposite direction of the flow. At the same computing scale, compared with the serial algorithm on the central processing unit platform, the acceleration ratio on the single GPU on the heterogeneous platform reaches 24.3 times and 21.6 times respectively, which improves the computing efficiency. At the same time, with its strong floating-point computing power to obtain more accurate simulation results and achieve the dual needs of computational efficiency and portability, it has also proven to solve the problems of a large amount of calculation, low efficiency and limited to qualitative research existing in the traditional phase-field models.

The role of eutectic colonies in the tensile properties of a Sn–Zn eutectic solder alloy

The growth of eutectic colonies in Sn–Cu, Sn–Zn and Sn–Ag–Cu eutectic alloys has already been reported in the literature. However, relationships between this kind of microstructure and mechanical properties remain undetermined for solders. The use of water-cooled copper (Cu) and AISI 1020 low-C steel molds and the eutectic Sn-9 wt.%Zn alloy make it possible to address this matter. The samples grown in the Cu mold demonstrated higher solidification rates than those developed in the low-C steel mold. Overall, the microstructure is constituted by Zn-lamellae embedded in a Sn-rich matrix. The Zn lamellae are not only uneven in thickness but also irregularly perforated. Due to Cu dissolution into the alloy, a small fraction of Cu5Zn8 intermetallic particles formed during solidification of the Sn-9 wt.%Zn alloy in the Cu mold. The contamination with Cu appears to be responsible for the improvement in the distribution of Zn-lamellae. The decrease in spacing between broken lamellae measured from SEM images, as well as a higher number of Zn particles per area, explain such occurrence. Ductility and tensile strength of different samples could allow the establishment of relationships among properties vs. eutectic colony spacing. For the Cu mold, the motion of Cu towards the alloy as well as higher solidification rates, allowed microstructures to be formed combining 60% of strain to fracture and 52 MPa of ultimate tensile strength. These achievements are mainly due to the finest spacings of both the eutectic colony (λc = 36 μm) and the Zn lamellae (λL=0.9 μm), besides homogeneous distribution of Cu across the resulting microstructure.

Directionally Solidified Al–Cu–Si–Fe Quaternary Eutectic Alloys

Directional solidification of eutectic alloys attracts considerable attention because of microhardness, tensile strength, and electrical resistivity affected by eutectic structures. In this research, solidification processing of Al–Cu–Si–Fe (Al–26 wt % Cu–6.5 wt % Si–0.5 wt % Fe) quaternary eutectic alloy by directional solidification is examined. The alloy was prepared by vacuum furnace and directionally solidified in Bridgman-type equipment. During the directional solidification process, the growth rates utilized varied from 8.25 to 164.80 μm/s. The Al–Cu–Si–Fe system showed a eutectic transformation, which resulted in the matrix Al, lamellar Al2Cu, plate Si, and plate Al7Cu2Fe phases. The eutectic spacing $${{\lambda }_{{{\text{A}}{{{\text{l}}}_{{\text{2}}}}{\text{Cu}}}}}$$ between lamellae of Al2Cu, as well as—$${{{\lambda }}_{{{\text{Si}}}}}$$, between plates of Si phase,—was measured. Additionally, the microhardness, tensile strength, and electrical resistivity of the studied alloy were determined using directionally solidified samples, and the experimental relationships between them were obtained. It was found that the microhardness, tensile strength, and electrical resistivity were affected by both eutectic spacing and the growth rate.

Effects of Bi Addition on Si Features, Tensile Properties and Wear Resistance of Hypereutectic Al-15Si Alloy

Bismuth (Bi) alloying in Aluminum (Al) alloys is considered of importance in special applications requiring improvements in the alloy machining. Despite that, the influence of Bi either on evolution of the solidification or on resulting microstructure of hypereutectic Al-Si alloys is still to be determined. The present research work is devoted to these requirements. In the present study, Al-15wt.% Si hypereutectic alloy is modified with 1.0 wt.% Bi. The effects of this addition on the morphology of the phases forming the microstructure and on the tensile/wear properties are investigated. Various samples characterized by distinct cooling rates are generated by transient directional solidification of the ternary Al-Si-Bi alloy. Microstructure is examined by optical microscopy and scanning electron microscopy (SEM) analyses, segregation by energy-dispersive X-ray spectroscopy (EDS) whereas wear and tensile properties have been characterized by standardized tests. The experimental variations of the eutectic spacing are compared to each other based on their trends. It is found that the addition of minor Bi content (~1 wt. %) permitted tensile strength and ductility of 195 MPa and 14%. Furthermore, wear resistance is improved by up to 20% due to the Bi addition. The reasons for that will be outlined.

Microstructure and mechanical properties of directionally solidified Al2O3/YAG binary eutectic ceramic prepared with induction heating zone melting

In order to reveal the dependence of the microstructure and mechanical properties of directionally solidified eutectic ceramic (DSEC) on the growth rate so as to further improve its performance, Al2O3/Y3Al5O12 (YAG) DSEC with diameter Φ10.16 was prepared via induction heating zone melting (IHZM) method, and the effects of the growth rate on the phases’ distribution, eutectic spacing, hardness and fracture toughness of the DSEC were investigated. XRD patterns show that the crystallinity of the eutectic ceramic significantly improved after directional solidification. The DSEC displays a typical irregular eutectic microstructure like mosaic, YAG phase formed the matrix in which Al2O3 phase embedded. The eutectic constant is calculated λv1/2 = 8.944, the microstructure evolution during solidification progress is shown via nucleating and growth mode. The hardness and fracture toughness of the DSEC are 1.89 times and 1.42 times those of the sintered ceramic, respectively. The relationship between the hardness, fracture toughness and the growth rate are expressed as Hv=13.4+7.72v1/4 and KIC=2.24+2.48v1/4 via Hall-Petch equation.

Research on Microstructure Evolution of Al–Al2Cu Eutectic by Regional Melting under Directional Solidification

AbstractAl–Al2Cu eutectic is prepared by regional melting through vacuum directional solidification furnace in an experiment. The effects of different pulling rates, heating power, and transition acceleration on the microstructure evolution of Al–Al2Cu eutectic under directional solidification are analyzed. Four constant growth rates of 3, 6, 9, and 15 mm min−1 are selected for the pulling rate. The primary Al2Cu phase will precede the coupled eutectic phase. The morphology of the irregular faceted primary Al2Cu phase is coarse and the feature of growth is transformed from faceted to non‐faceted as the pulling rate increases. The microstructure at three different heating powers is studied. With the increase of heating power, the primary Al2Cu phase in the alloy decreases, the eutectic structure increases, and the eutectic spacing of the alloy is obviously refined. The pulling rate is changed from 3 to 6 mm min−1, 6 to 9 mm min−1, and 9 to 15 mm min−1, respectively, in directional solidification for transition acceleration. The increase of the pulling rate will change the morphology of the primary Al2Cu phase and produce a partial rod eutectic structure. The lamellar eutectic spacing obtained by transition acceleration is smaller than that obtained by the same constant growth rate.

On the Growth Dynamics of Nearly-Locked Grain in the Three-Phase In-Bi-Sn Eutectic System

Solidification microstructures are significantly affected by the anisotropy of crystal/crystal interphase energy. A recent experimental work on a three-phase eutectic system by the authors suggested that two distinguishable eutectic grains, i.e., quasi-isotropic and locked, form when crystal/crystal interphase energies contain negligible and strong anisotropy, respectively (Mohagheghi and Şerefoglu in Acta Mater 2018, vol. 151, pp. 432–42, 2018). In two-phase eutectic systems, in addition to these two grain types, another class of eutectic grain called nearly-locked (NL) was reported. In order to investigate the existence of the NL grain in three-phase eutectic systems, real-time directional solidification (DS) and rotating directional solidification (RDS) experiments are performed on thin samples of In-Bi-Sn eutectic alloy. It is found that NL grains also form in three-phase eutectics and they contain some characteristic features of both quasi-isotropic and locked grains. The anisotropy is strong enough to tilt the lamellar pattern with respect to the thermal gradient axis, as in the case of locked grains; however, the NL grains also retain some of the characteristic features of the quasi-isotropic grains, such as λ-diffusion, systematic eutectic spacing adjustment, and recovery mechanisms. As a result, these grains tend to form a relatively uniform ABAC-type growth pattern, similar to quasi-isotropic grains. Using the equilibrium shapes extracted from the interphase traces of RDS patterns, the γ plot of the anisotropic interphase, which contains 2 twofold smooth and distinct minima, is determined.

Phase separation of AlCoCrFeNi2.1 eutectic high-entropy alloy during directional solidification and their effect on tensile properties

AlCoCrFeNi2.1 nominal eutectic high entropy alloy (EHEA) was prepared by directional solidification at different withdrawal rates (v = 5 − 100 μm/s). The directionally solidified samples exhibit superior mechanical properties. When the withdrawal rate is 5 μm/s, the solidified microstructure consists of primary CoCrFe FCC phase and eutectic CoCrFe/AlNi lamellar phase, and the fracture strength is 696 MPa. As the withdrawal rate increase, the size of primary CoCrFe FCC phase and eutectic lamellar spacing decreased obviously. When the withdrawal rate is 100 μm/s, the solidified microstructure is nearly full eutectic lamellar phase, and the fracture strength is 1012 MPa. Meanwhile, the elongation of the sample prepared at 5 μm/s and 100 μm/s is 33.3% and 14.5%, respectively. In order to reveal the relationship between phase formation and mechanical properties of EHEA, phase diagram of AlCoCrFeNi2.1 is simplified as AlNi–CoCrFe binary phase diagram. The results indicate that directional solidification is a suitable method to balance the strength and ductility of AlCoCrFeNi2.1 EHEA. When the solidified phase is nearly full eutectic soft/hard lamellar phase, the strain field shows wavy distributed during the tensile test.

Effect of scanning speed on the solidification process of Al2O3/GdAlO3/ZrO2 eutectic ceramics in a single track by selective laser melting

Abstract Al2O3/GdAlO3/ZrO2 ternary eutectic ceramics with fine microstructure were directly fabricated from mixed pure ceramic powders by selective laser melting. The specimen forming quality, molten pool morphology and microstructure characteristic were investigated as functions of scanning speed. Solidification defects such as cracks and pores were effectively suppressed when the scanning speed was 12 mm/min. The relative density of the as-solidified eutectic specimens decreased from 98.7% to 95.7% with increasing the scanning speed up to 48 mm/min. The melting in this study was governed by conduction mode, leading to a decrease tendency of both melting width and depth with the increase of the scanning speed. Different from ordinary cognition, the eutectic spacing in top zone of the molten pool first decreased and then increased with increasing the scanning speed from 6 mm/min to 48 mm/min. The transition point appeared at 12 mm/min, where the dominant factor affecting the solidification rate changed from the scanning speed to the value of the angle between microstructure growth direction and laser scanning direction.

Theoretical prediction and experimental comparison for eutectic growth of Al2O3/GdAlO3 faceted eutectics

Ultra-fine Al2O3/GdAlO3 eutectics with eutectic spacing of 193 ± 20 nm are directionally solidified by laser floating zone melting process. When the solidification rate increases from 2 μm/s to 400 μm/s, the morphology transition from “Chinese script” irregular eutectic to rod-like regular eutectic is found. To predict the growth behaviour of Al2O3/GdAlO3 eutectic, the solute diffusion coefficient DL, which exhibits significant influence on eutectic growth, is determined to be 6 × 10−10 m2/s. Then, the eutectic microstructures as a function of solidification rate are well predicted by modified Magnin-Kurz model and Jackson-Hunt model, both for “Chinese script” irregular eutectic and rod-like regular eutectic, respectively.

A Transition from Fine to Coarse Lamellar Eutectics in the Undercooled Ni-29.8 At. Pct Si Eutectic Alloy: Experiments and Modeling

In contrast to the classical eutectic growth models, according to which the eutectic spacing decreases invariably with undercooling, an anomalous transition from fine to coarse lamellar eutectics was found in the undercooled Ni-29.8 at. pct Si eutectic alloy. In this study, the growth kinetics, recalescence processes, and grain orientations were analyzed. A sharp increase of the growth velocity at an undercooling of about 100 K was found. The recalescence front transited in sequence from a diffuse one with tips, to a diffuse one without tips and then to a sharp one. The microstructures changed from a mixture of directional rod-shaped γ-Ni31Si12 grains and fine lamellar eutectics to solely coarse lamellar eutectics. Coarse lamellar eutectics were found to be formed by rapid solidification of primary directional rod-shaped Ni31Si12 intermetallic compound and subsequent epitaxial growth of secondary Ni2Si intermetallic compound, being consistent with the predictions of eutectic–dendritic and dendritic growth models. Coarse anomalous eutectics at low undercooling were formed by fragmentation of fine lamellar eutectics and their subsequent coarsening. At high undercooling, they were formed by decoupled-eutectic growth.

Microstructure and mechanical properties of Al2O3/Er3Al5O12/ZrO2 prepared by a high-frequency zone melting method

Oxide directionally solidified eutectic ceramics (DSECs) have excellent mechanical properties, oxidation resistance, and ablation resistance in an ultra-high temperature environment above 1600 °C. In this study, Al2O3/Er3Al5O12/ZrO2 ternary DSECs were prepared by high-frequency induction zone melting. Their diameters were considerably greater than those of samples prepared with other processing techniques under the conditions where the microstructures were uniform. The component, microstructure, and mechanical properties in these samples were investigated. The results indicate that these DSECs were composed of only Al2O3, Er3Al5O12, and ZrO2 phases. As the solidification rate increased, both the eutectic phase size and eutectic spacing decreased continuously, and the microstructure was refined. The relationship between the eutectic spacing and solidification rate satisfied the formula λ2ν ≈ 33.8 ± 0.49. A polygonal colony structure appeared at a growth rate of 6.7 μm/s, which was associated with the uneven distribution of the temperature field. The hardness and indentation fracture resistance increased marginally with an increase in solidification rate, achieving 17.6 ± 0.8 GPa and 5.0 ± 0.5 MPa·m1/2, respectively. The hardness of the Al2O3/Er3Al5O12/ZrO2 ternary DSEC was 18.3% less than that of the Al2O3/Er3Al5O12 binary DSEC owing to the reduced hardness of the ZrO2 phase added in the eutectic composition. The ternary DSEC's indentation fracture resistance was 88.4% greater than that of the binary eutectic ceramic owing to the phase transformation toughening of the ZrO2 phase.

Preparation of directionally solidified Al2O3/YAG/ZrO2 ternary eutectic ceramic with induction heating zone melting

Al2O3/YAG/ZrO2 directionally solidified eutectic ceramics (DSECs) were prepared via induction heating zone melting (IHZM), in which sintered ceramic rods were used as the preform. The DSECs consisted of Al2O3, Y3Al5O12 (YAG), and ZrO2 phases, similar to the sintered preforms. The microstructure of the DSECs was similar to a “sponge-net” structure, wherein the three phases exhibited coupled growth; the YAG phase made up the matrix, and the Al2O3 and ZrO2 phases formed the framework of the sponge-net. The eutectic inter-phase spacing (λ) gradually decreased with an increase in the growth rate (v), and this was in accordance with a constant λ2v of about 100. The difference in the microstructures of the DSECs prepared via IHZM and a natural-cooling solidified eutectic ceramic (NCSEC) was attributed to the different nucleation and growth modes. The hardness and fracture toughness of the DSECs were 16.1 GPa and 2.98 MPa m1/2, respectively, which were 1.82 and 1.27 times those of a sintered ceramic. The ZrO2 phase exhibited a toughening effect in a variety of ways such as crack bridging, crack deflection, crack branching, and crack pinning. The hardness and fracture toughness showed complex variations due to the size effect of the ZrO2 phase and defect reduction. The ZrO2 phase distribution effect improved the fracture toughness of the NCSEC and the DSECs prepared at high growth rates.

Near-eutectic Zn-Mg alloys: Interrelations of solidification thermal parameters, microstructure length scale and tensile/corrosion properties

Zn-Mg alloys are considered to have potential application in bone implants, since both metals are biocompatible and have biodegradable characteristics. Adding Mg to Zn can boost mechanical and corrosion resistances. However, the literature is very limited on quantifying the interrelation of solidification parameters, microstructural features and mechanical/corrosion properties of Zn-Mg alloys. The present study examines the interrelations of alloy Mg content, macrosegregation effects, morphology and scale of the matrix and eutectic phases, nature of intermetallics and tensile and corrosion properties of near-eutectic Zn-Mg alloys. The alloys samples are obtained by unsteady-state directional solidification resulting in a wide range of solidification thermal parameters and microstructures. We examine microstructural features of both dendritic and complex regular eutectic phases. It is shown that the eutectic exhibits a bimodal pattern with neighboring areas of coarse and fine lamellae. Experimental growth laws relating the primary, secondary and eutectic spacings to the solidification cooling rate and growth rate are proposed. Hall-Petch type equations are derived expressing tensile strength and elongation to dendritic and eutectic spacings. Electrochemical parameters determined by polarization curves during corrosion tests and SEM analyses of corroded areas have shown that the alloy having an essentially eutectic microstructure is associated with better corrosion resistance.

Microstructure and mechanical properties of directionally solidified Al2O3/GdAlO3 eutectic ceramic prepared with horizontal high-frequency zone melting

Directionally solidified eutectic oxide ceramics are very promising as a next-generation structural material for ultrahigh-temperature applications, above 1600 °C, owing to their outstanding properties of high corrosion resistance, oxidation resistance, high fracture strength and toughness, and high hardness. Herein, Al2O3/GdAlO3 eutectic ceramic was prepared with horizontal high-frequency induction zone melting (HIZM), and the effects of the processing parameters on the eutectic microstructure and mechanical properties were investigated. The results indicated that the directionally solidified Al2O3/GdAlO3 eutectic ceramic was composed only of the Al2O3 phase and GdAlO3 phase penetrating mutually, and the Al2O3 phase was the substrate in which the GdAlO3 phase was embedded. As the solidification rate increased from 1 to 5 mm/h, the eutectic microstructure underwent a transformation from an irregular pattern to a relatively regular “rod” or “lamellar” pattern, and the eutectic spacing constantly decreased, reaching a minimum value of 0.5 μm. The eutectic ceramic hardness and fracture toughness at room temperature increased continuously, reaching 23.36 GPa and 3.12 MPa•m1/2, which were 2.3 times and 2.5 times those of the sintered ceramic with the same composition, respectively. Compared with the samples obtained from vertical high-frequency induction zone melting, the orientation of eutectic phases along the growth direction decreased significantly, and the size uniformity of the GdAlO3 phase became poorer in the samples prepared with HIZM at the same solidification rate; nevertheless, the hardness and fracture toughness of the samples increased by 11% and 63%, respectively.

Halo formation in directionally solidified Al2O3-Er3Al5O12 off-eutectic in situ composite ceramics

Directionally solidified Al2O3-Er3Al5O12 binary off-eutectic composite ceramics have been fabricated by laser floating zone melting. Microstructure morphologies and halo formation behaviors were explored. At hypereutectic composition (Al2O3–22.5 mol% Er2O3), Er3Al5O12 primary phase formed but no Al2O3 halo structure has been detected at normal solidification rate (~200 μm/s). When the eutectic spacings have refined down to 100–150 nm, only thin layer of Al2O3 halo structure could form around the Er3Al5O12 primary phase. At hypoeutectic composition (Al2O3–13.5 mol% Er2O3), coarse Er3Al5O12 halo phase can form around the Al2O3 primary dendrite even at low solidification rate (50 μm/s). The formation behavior of halo structures has been analyzed on the basis of competitive growth between primary dendrite phase, halo phase and eutectic.