Directionally solidified eutectic oxide composites have been attracting extensive attentions as materials for very high temperature structural applications because of their unique properties, such as high melting point, excellent thermal and chemical stability in oxidizing atmospheres, microstructural stability up to temperatures very close to the melting point, and superior high-temperature strength retention and creep resistance^[1,2,3]. Among them, Al₂O₃- based binary and ternary eutectics in Al₂O₃-ZrO₂-Y₂O₃ system are the most widely studied materials in the past years. It is reported that the binary Al₂O₃/Y₃Al₅O₁₂(YAG) eutectic composite fabricated by Bridgman method shows extremely stable microstructures, which remains un-growth after heat-treatment at 1973 K for 1000 h in air^[1]. For binary Al₂O₃/YAG eutectic composite, a flexural strength of 2 GPa at ambient temperature was reported by Pastor, et al. and this strength could remain practically constant up to 1700 K^[4]. The highest flexure strength of ternary Al₂O₃-YAG-ZrO₂ eutectic, determined by three- point bend testing, was (4.6±0.1) GPa, which was approximately four times higher than the best results measured in bulk non-oxide ceramics, such as SiC and Si₃N₄^[5]. For the fracture toughness, a value as high as 8 MPa∙m^1/2, has been reported for the laser melt-grown ternary Al₂O₃- YAG-ZrO₂ eutectic by Zhang, et al.^[6]. All these studies demonstrated the outstanding mechanical properties of directionally solidified eutectic ceramics.

More recently, research hotspots have spread to the development of new oxide binary compounds, such as Al₂O₃/RE₃Al₅O₁₂ (RE=Er and Yb), to further improve the mechanical properties or to expand the potential application fields of directionally solidified eutectic oxide composites. Nakagawa and Waku, et al.^[7] developed unidirectionally solidified Al₂O₃/Er₃Al₅O₁₂ (ErAG) binary eutectic and found that the composite possessed excellent thermally stability while strength at room temperature could be maintained up to 2073 K. Martinez, et al.^[8] reported that Al₂O₃/ErAG binary system exhibited excellent creep resistance property up to 1550 ℃, which was 3 or 4 orders of magnitude better than 15° off c-axis sapphire. Mesa, et al.^[9,10] successfully prepared Al₂O₃/ErAG eutectic rods by directional solidification, using the laser-heated floating zone method. The bend strength of these rods reached the maximum value of 2.7 GPa, which could be remained constant up to temperature about 1300 K. Besides the above excellent thermal stability and mechanical properties at high temperatures, it is also believed that rare earth ions in the crystals may emit radiation in narrow bands. These comprehensive properties make directionally solidified eutectics promising candidates that can meet the thermo- structural challenges in thermofotovoltaic (TPV) converter applications^{[11,12,13,14,15,16]}. The selective thermal emission results illustrated that Al₂O₃/ErAG showed intense emission band and could match with the GaSb photovoltaic cells^[17] and Al₂O₃/Yb₃Al₅O₁₂(YbAG) showed an intense selective emission band at 1 μm, which was compatible with the silicon photovoltaic cells^[18,19].

All of the above-mentioned researches have clearly illustrated directionally solidified Al₂O₃/ErAG and Al₂O₃/YbAG eutectics as promising materials for both high temperature structural and functional applications. However, studies on directionally solidified Al₂O₃/REAG eutectic ceramics, especially for Al₂O₃/YbAG eutectic, are still limited. Further studies on the solidification parameters optimization, microstructures evolution control and thermo-mechanical properties tailoring are still necessary. In this work, directionally solidified Al₂O₃/ErAG and Al₂O₃/YbAG eutectic composites were prepared using the optical floating zone melting method. This method can provide a stable and big melting zone at the liquid/solid interface, which is the key point for the growth of crack-free and low stress eutectic samples and is beneficial to attain eutectic rods with uniform and homogeneous microstructures. The composition and microstructure characteristics (phase distributions, selected orientations, and interfacial relationships) of the as- received eutectics were investigated specifically. The mechanical properties, including Vickers hardness and fracture toughness were evaluated and discussed combined with the microstructures.

Commercially available powders RE₂O₃ (RE= Er and Yb) (Rare-Chem. Hi-Tech. Co., Ltd., Huizhou, China) and Al₂O₃ (Sinopharm Chemical Reagent Co., Ltd., Shanghai, China) were used to fabricate the Al₂O₃/ErAG and Al₂O₃/YbAG eutectic composites. The powders were mixed according to the binary eutectic compositions with n(Al₂O₃) : n(Er₂O₃)=81:19^[20] and n(Al₂O₃) : n(Yb₂O₃)= 81.5 : 19.5^[21], respectively. The starting powders were ball milled for 24 h in a Si₃N₄ jar, with Si₃N₄ balls and ethanol as media. The obtained slurry was dried at 60 ℃ for 24 h and then passed through a 75 μm (200-mesh) sieve, and then synthesized at 1550 ℃ for 6 h. The as-received powders were then cold pressed under a pressure of about 30 MPa for 15 min in a die steel mold, followed by cold isostatic pressing at 280 MPa for 15 min. The cold pressed bars were then sintered at 1550 ℃ for 2 h. The directional solidification experiments were conducted by an optical floating zone furnace equipped with four 3 kW xenon arc lamps as heat sources. The directional solidification process was conducted in argon atmosphere with a polycrystalline alumina rod as seed crystal. The directional solidification growth speed was set at 10 mm/h.

Phase composition of the as prepared directionally solidified eutectics was determined using X-ray diffractometer (D/max-2400, Rigaku, Tokyo, Japan). Microstructures of the longitudinal sections were observed with a scanning electron microscope (Supra 35, Zeiss, Oberkochen, Germany) equipped with an electron- dispersive spectrometer (EDS) and backscatter electron detector (BSED). The average eutectic spacing was estimated according to the arithmetic mean value of about 200-400 eutectic lamellar spacings which were crossed by 15 straight lines in the SEM images. The reconstructed 3D microstructures were investigated via a high-resolution X-ray microscope (Xradia Versa XRM-500, Zeiss, Oberkochen, Germany). The Vickers hardness (H_V) and the indentation fracture toughness (K_IC) were measured on polished longitudinal cross-sections, using a microhardness tester (432SVD, Wolrert, Shanghai, China), under loads (P) of 3, 5, 10, 30, and 50 N with a dwell time of 15 s. Five indentations were performed for each load and the hardness was quantified taking the average lengths of the indentation diagonals (d) using the following equation:

The indentation fracture toughness (K_IC) was calculated based on Anstis’ model in Eq. (2), where c is the length of cracks emerging from indentation corners and E is the composite Young’s modulus^[22]:

Fig.1 shows the optical micrographs of the as received directionally solidified Al₂O₃/ErAG and Al₂O₃/YbAG eutectics. The as-grown eutectic rods are both with an average diameter in about 11 mm and length in about 90 mm, of which Al₂O₃/ErAG eutectic rod is pink and Al₂O₃/YbAG eutectic is blue. The surfaces of the rods are dense and smooth without any obvious pores while some slight skin-core structures could be observed in the transverse cross sections of both the eutectics. Samples cut from the end of these rods with a growth distance about 80-90 mm were chosen for the following phase composition, microstructure, and mechanical properties characterization.

The XRD patterns of the as-prepared directionally solidified Al₂O₃/ErAG and Al₂O₃/YbAG eutectics are shown in Fig. 2(a, b), respectively. It can be seen that both eutectics contain two phases: Al₂O₃ and RE₃Al₅O₁₂ phases, without diffraction peaks of other phases detected, which means the as-prepared eutectics are composed of Al₂O₃ and RE₃Al₅O₁₂.

The SEM morphologies of the transverse section of directionally solidified eutectics are exhibited in Fig. 3(a, b), respectively. A homogeneous and voids-free microstructure could be observed with only two phases in each sample. The light aeras are the ErAG and YbAG phases and the dark aeras are the Al₂O₃ phase in Fig. 3. The three-dimensional interpenetrating network of Al₂O₃ and REAG phases, which is a typical microstructure characteristic in directionally solidified eutectics and commonly referred to as “Chinese Script” microstructure, could be identified in both samples. It should be noticed that the size of the eutectic phases in these two eutectics are distinctly different. The average interlamellar spacing of as-prepared Al₂O₃/ErAG and Al₂O₃/YbAG eutectics are measured to be 6.9 and 17.2 μm, respectively.

The crystallographic relationships of Al₂O₃ and REAG phases of the as-grown eutectics were investigated by electron backscattering diffraction (EBSD) method. Also, we chose the end parts of the rods with a growth distance of 80-90 mm for EBSD observation. Fig. 4 show the EBSD maps and corresponding inverse pole figures (inserts) of Al₂O₃/ErAG (a,b) and Al₂O₃/YbAG (c,d). For Al₂O₃/ErAG, it is seen that the preferred growth directions of Al₂O₃ is <$10\bar{1}0$>. What should be mentioned is that there are two different colors in Fig. 4(a), which reveal two twin-related variants of Al₂O₃ and correspond to the same <$10\bar{1}0$> orientation, since the [$10\bar{1}0$] and [$0\bar{1}10$] directions are not strictly equivalent and cannot be identified by electron diffraction for Al₂O₃.

This phenomenon has also been observed and reported in the previous work on Al₂O₃/YAG eutectic^[23]. The preferred orientation of ErAG is revealed to be <111> in Fig. 4(b). For Al₂O₃/YbAG, similar to Al₂O₃/ErAG, direction <$10\bar{1}0$> was selected as the preferred growth orientation for Al₂O₃ (Fig. 4(c)), while for YbAG more than three grains with different orientations could be detected in Fig. 4(d) and no obvious preferential growth directions were established during the solidification process. We do not have an exact explanation about the phenomenon in the preferred orientation selection among Al₂O₃/REAG (RE=Y, Er, and Yb) at present. One possible reason is the surface energy discrepancy among different surfaces in REAG or the competition among different directions in REAG during the directional solidification process varied with the rare earth atoms. So, it needs a longer time or growth distance for the preferred orientation selection in YbAG phase in Al₂O₃/YbAG eutectic.

Fig. 5 depicts the pole figures of Al₂O₃/ErAG corresponding to the inverse pole figures in Fig. 4(a, b). Fig. 5(a) shows the {0001} pole figure of Al₂O₃ and Fig. 5(b) shows the {211} pole figure of ErAG, which clearly demonstrates that the orientation relationship between these two phases is {0001} Al₂O₃//{211} ErAG. According to the results in Fig. 4 and Fig. 5, the crystallographic orientation relationship in the as-prepared Al₂O₃/ErAG eutectic ceramic (at a growth distance of 80-90 mm) is as follows:

<$10\bar{1}0$> Al₂O₃//<111> ErAG;

{0001} Al₂O₃//{211} ErAG.

Table 1 lists the above growth directions and orientation relationships of Al₂O₃/ErAG together with the previously reported results of some eutectic ceramics for comparison^{[24,25,26,27,28]}. We can see that the growth directions of Al₂O₃ phase presented in Table 1 are quite similar although the preparation technology and eutectic system are different. The main divergence is the growth directions of REAG phase, which could be <111>, <101> and <$2\bar{1}0$>. For example, the crystallographic orientation relationship for Al₂O₃/ErAG identified by Mazerolles, et al. turned out to be <$10\bar{1}0$> Al₂O₃//<101> ErAG^[25]. It is worth mentioning that in REAG, [111] and [101] are two directions perpendicular to each other in {211} plane and compete during the directional solidification process^[29]. Similarly, both the growth directions of <111> and <101> for YAG have been detected in Al₂O₃/YAG eutectics^{[25,26,27,28]}. We suppose that the competitive growth behavior between these two directions could be influenced by eutectic systems, solidification method, solidification parameters and so on. The detailed grain selection process and competitive growth mechanism for Al₂O₃/ErAG and Al₂O₃/ YbAG eutectic ceramics will be investigated in our future work.

To characterize the distribution of Al₂O₃ and REAG phases in the directionally solidified composites more intuitively, we further reconstructed the 3D-tomographs of these two eutectics using X-ray tomography (XRT) technology. Fig. 6(a-f) illustrates the 3D-XRT images and the spatial distribution of ErAG and Al₂O₃ phases in Al₂O₃/ErAG as well as YbAG and Al₂O₃ phases in Al₂O₃/ YbAG eutectic, respectively. The 3D-XRT images clearly show the continuous network of the eutectic phases in the interior of the eutectics clearly, which is commonly referred as ‘Chinese Script’ microstructure. For example, in Fig. 6(a-c), ErAG (the cobalt red phase) and Al₂O₃ (the gray phase) are interpenetrated with each other and form a geometrical 3D network. Also, utilizing the 3D-tomographs, we can calculate the volume fraction of the eutectic phases. For Al₂O₃/ErAG eutectic, the measured volume fraction of Al₂O₃ and ErAG are 50.5% and 49.5%, respectively, which are close to their theoretical values (Al₂O₃ 51.2% and ErAG 48.8%). While for Al₂O₃/YbAG eutectic, the measured volume fraction of Al₂O₃ and YbAG are 53.5% and 46.5%, which are also consistant with the theoretical values of Al₂O₃ (55.3%) and YbAG (44.7%), respectively.

Fig. 7 presents the Vickers hardness and fracture toughness of the as prepared directionally solidified Al₂O₃/ErAG and Al₂O₃/YbAG eutectics. The Vickers hardness of Al₂O₃/ErAG and Al₂O₃/YbAG eutectics which almost remained unchanged with the variation of loads and the values tested under a load about 50 N are (13.5±0.4) and (12.8±0.1) GPa, respectively. An E-value of 327 GPa was taken for Al₂O₃/ErAG^[15] and of 340 GPa for Al₂O₃/ YbAG^[19] to calculate the fracture toughness. The fracture toughness results of Al₂O₃/ErAG and Al₂O₃/YbAG are (3.0±0.2) and (3.2±0.1) MPa·m^1/2, respectively. In Table 2, we also summarized the Vickers hardness and fracture toughness of some eutectic ceramics for comparison. Since the fracture toughness and hardness data for Al₂O₃/ErAG and Al₂O₃/YbAG prepared by optical floating zone method are not available in literatures so far, we chose the results of Al₂O₃/ErAG and Al₂O₃/YbAG prepared by laser floating zone method for comparison. Oliete, et al.^[18,19] prepared Al₂O₃/ErAG and Al₂O₃/YbAG eutectic coatings by laser floating zone method, and the reported Vickers hardness and fracture toughness, which were tested following the same testing methods using Eq. (1) and (2), were (14.8-14.9) GPa and (1.8-2.2) MPa·m^1/2, respectively. Mesa, et al.^[9] also prepared Al₂O₃/Er₃Al₅O₁₂ eutectic rods using laser-heated floating zone method with different grown speed (25-1500 mm/h). Their results indicated that the Vickers hardness increased slightly with growth rate, up to a maximum of 16 GPa, while the fracture toughness was about 1.9 MPa·m^1/2. That means that the as-prepared Al₂O₃/ErAG and Al₂O₃/YbAG eutectic ceramics exhibit slightly lower hardness and obviously enhanced fracture toughness comparing with previous works. It should be mentioned that the mechanical properties of directionally solidified eutectic ceramics, such as Vickers hardness and fracture toughness, are closely related to the preparation method and eutectic microstructure. The lamellar spacings, measured from transverse cross-section SEM micrographs for Al₂O₃/ErAG and Al₂O₃/ YbAG in Ref. [19] are (0.74±0.1) and (0.71±0.1) mm, respectively, which are much smaller than those of as-prepared Al₂O₃/ErAG and Al₂O₃/YbAG (6.9 and 17.2 μm, respectively). It is expected that this discrepancy in mechanical properties is mainly due to the difference in eutectic microstructures. Furthermore, we made some comparison with Al₂O₃/Y₃Al₅O₁₂ eutectic ceramic, which was prepared using the same optical floating zone method at a growth speed of 20 mm/h^[24]. The Vickers hardness and fracture toughness of Al₂O₃/ YAG are 13.5 GPa and (3.1±0.3) MPa·m^1/2, respectively, which are close to the values of as-prepared Al₂O₃/ErAG and Al₂O₃/YbAG eutectics.

In this work, directionally solidified Al₂O₃/ErAG and Al₂O₃/YbAG eutectic rods with uniform and homogeneous microstructures were prepared using the optical floating zone melting method. The as-grown eutectic rods are both with an average diameter in about 11 mm and length in about 90 mm, of which Al₂O₃/ErAG eutectic rod shows pink color and Al₂O₃/YbAG eutectic rod is blue. The microstructure characteristics of the as-received eutectics show that the crystallography of Al₂O₃/ErAG eutectic ceramic (at a growth distance of 80-90 mm from the seed) is: <$10\bar{1}0$> Al₂O₃//<111> ErAG and {0001} Al₂O₃//{211} ErAG. The volume fraction of Al₂O₃ and ErAG in Al₂O₃/ErAG are 50.5% and 49.5%, respectively, while for Al₂O₃/YbAG eutectic, the volume fraction of Al₂O₃ and YbAG are 53.5% and 46.5%, respectively. The Vickers hardness of Al₂O₃/ErAG and Al₂O₃/YbAG are determined as (13.5±0.4) and (12.8±0.1) GPa, respectively. The fracture toughness results of Al₂O₃/ErAG and Al₂O₃/YbAG are (3.0±0.2) and (3.2±0.1) MPa·m^1/2, respectively. The hardness values are slightly lower compared with previously reported Al₂O₃/ErAG and Al₂O₃/YbAG eutectic ceramics while the fracture toughness values show obvious enhancement comparing with previous works, which highlight the reliable mechanical properties and potential structural applications of the as-prepared Al₂O₃/ErAG and Al₂O₃/ YbAG eutectic ceramics.