Communication pubs.acs.org/crystal
Interpenetrating Network-Structured Al2O3−Y3Al5O12 Eutectic Composite Grown by Containerlessly Directional Solidification Process Xiaoguang Ma,†,‡ Jianqiang Li,*,† Zhijian Peng,‡ Bingqian Ma,† Xiaoyu Li,† Wei Pan,§ and Longhao Qi§ †
National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology, Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, P. R. China ‡ School of Engineering and Technology, China University of Geosciences, Beijing 100083, P. R. China § State Key Laboratory of New Ceramic and Fine Processing, Tsinghua University, Beijing 100084, P. R. China S Supporting Information *
ABSTRACT: Directionally solidified (DS) Al2O3/YAG eutectic composites with microstructures of three-dimensionally continuous networks were prepared using containerlessly directional solidification through an aerodynamic levitator. The morphological evolution of DS eutectics was studied at a large growth rate range. The DS eutectics present irregular “Chinese script” morphologies at growth rates of 9 μm/s, transforming into complex regular eutectic morphologies with growth rates increasing to 68 μm/s. At a superhigh crystal growth rate of 800 μm/s, the DS eutectics showed regular lamellar morphologies. The indentation hardness and Young’s modulus of DS Al2O3/YAG eutectic composites at growth rates of 68 μm/s are 22.3 and 338 GPa, respectively. Bridgman,1 laser heated floating zone (LHFZ),10 micropulling down (μ-PD),11 or edge-defined film-fed growth (EFG)12 methods. Arai et al. prepared Al2O3 single crystal through an advanced directional solidification thechnique using aerodynamic levitation (ADL).13 Prevous work demonstrated that the ADL technique, as an approach to a containerless condition, could dramatically reduce unexpected nucleation introduced from container walls, and realize deep undercooling of melts.14−17 However, fabricating Al2O3/YAG eutectic with the ADL process has not been reported to date, largely contributing to the fact that the Al2O3/YAG eutectic melt prefers to solidify as the Al2O3−YAP (yttrium−aluminum−perovskite) metastable system owing to the sluggishness of YAG nucleation.18,19 In this paper, we successfully fabricated the Al2O3/YAG eutectic avoiding metastable solidification during containerless solidification via the improved aerodynamic levitation, which was named the levitation directional solidification (LDS) process. The growth mechanism of the Al2O3/YAG eutectic within a broad crystal growth rate ranging from 9 to 800 μm/s was studied systematically. The mixture of commercial powders of Y2O3 (99.99%) and Al2O3 (99.99%) in the binary (18.5 at% Y2O3, 81.5 at% Al2O3) eutectic compositions were homogeneously mixed using wet ball milling with ethanol and then vacuum-dried. The raw oxide powers were then pressed into columnar rods, and were sintered at 1000 °C for 10 h to obtain a certain strength. After
T
he eutectic phases grow from the melt and self-organize into various microstructures during directional solidification. Through this bottom-up approach, some directional solidification eutectic (DSE) ceramic oxides, such as Al2O3/ GdAlO3,1 Tb3ScAl3O12/TbScO3,2 SrTiO3/TiO2,3 and Al2O3/ Y3Al5O12 (YAG),4−7 were fabricated and present potential for electronic, magnetic, optical, or high temperature structural applications. Waku et al. prepared the DS Al2O3/YAG eutectic with a three-dimension continuous network composed of single crystal Al2O3 and single crystal YAG without any amorphous phase at interface.1 The as-prepared Al2O3/YAG eutectic presents excellent flexural strength of 360−500 MPa from room temperature up to about 2100 K.1,4 The compression creep strength at 1873 K is about 13 times higher than that of sintered composite with the same composition.1,4 In addition, the Al2O3/YAG eutectic shows neither grain growth nor weight gain even after heating at 1973 K in air for 250 h.1,4 Due to its excellent high temperature mechanical properties, DS Al2O3/ YAG eutectic has recently been considered one of the most promising structural materials for the applications in oxidizing environment.4,5 However, the reports about DS Al2O3/YAG eutectic mainly focused on its outstanding mechanical properties and the thermal and microstructural stability as compared with conventional composites and monolithic ceramics.4−7 Studies about the growth mechanism of the DS Al2O3/YAG eutectic are limited because their morphological evolution needs a much higher undercooling degree ΔT associated with the high entropy of the oxide melting phase.8,9 At present, several directional solidification techniques have been developed to prepare DSE ceramic oxides, such as the © XXXX American Chemical Society
Received: July 17, 2015 Revised: October 27, 2015
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DOI: 10.1021/acs.cgd.5b01013 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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sintering, a small piece of approximately 80 mg was cut down from the columnar rods, and premelted into a sphere using a CO2 laser in an aerodynamic levitator as shown in Figure 1.
Figure 2. SEM morphologies of (a) the spherical DS Al2O3/YAG eutectic sample, (b) enlarged part of the free surface marked in (a), (c) enlarged part of the internal fracture surface marked in (a), and (d) XRD pattern of the cross-section perpendicular to the growth direction of the DS Al2O3/YAG eutectic at the growth rate of 400 μm/s.
Figure 1. Setup of an aerodynamic levitator.
The spherical sample was levitated by O2 flow and slowly remelted. The bottom part of the sphere was intentionally kept unmelted to seed unidirectional solidification. By decreasing the laser power with a controlled speed, the crystal began to grow from the seeds to the top of the sample. The growth rate is calculated from the length of the sample along the growth direction divided by the time for the complete solidification process (i.e., growth process from bottom to top). The solidification process was monitored with the charged-coupleddevice (CCD), and the radiance temperature was measured by a pyrometer. The solidified samples were ground and polished into wafers perpendicularly to the solidification direction. The microstructure and components of the transverse cross sections were determined by scanning electron microscopy (JSM-7001F), energy dispersed spectroscopy (EDS Link-Isis), and X-ray diffraction (XRD, Smartlab 9 kW, Cu Kα). The Al2O3−YAG eutectic spacing (λ) was estimated from interfacial length (interface between Al2O 3−YAG) per unit area.20 The calculation formula was as follows: λ = 2d2/L, where d is the length (about 100 μm) of a selected square image in SEM photographs of the transverse section, and L is the total interfacial (interface between Al2O3−YAG) length obtained by using an image analyzer.20 Hardness and Young’s modulus of the samples were measured by the nanoindenter (MTS-XP) with the force and displacement of 650 mN and 1400 nm, respectively. Ten indentations were carried out per sample using a Berkovich-type diamond indenter. The indentation morphologies were observed by an atomic force microscope (AFM) (NanoscopeIII, Digital Instruments, Woodbury, NY). The indenter load and displacement were continuously and simultaneously recorded during loading and unloading in the indentation process. The hardness and Young’s modulus were analyzed by the loading date based on the Oliver-Pharr model.21 Figure 2 presents the typical microstructure and the XRD pattern of the cross-section perpendicular to the growth direction of the DS eutectic at the growth rate of 400 μm/s. The XRD pattern result indicates that the DS eutectic consists of only two phases, i.e., α-Al2O3 (corundum) and YAG (yttrium aluminum garnet) phases, without any other crystalline phases or amorphous phases. The DS eutectic shows strong
orientations of Al2O3 (113) and YAG (444) along the growth directions which suggests that the Al2O3 (113) and YAG (444) are preferential growth directions during the directional solidification process. As shown in Figure 2b,c and EDS (Supporting Information Figure S1), both the external free surface and internal fracture surface consist of three-dimensionally continuous and complexly entangled α-Al2O3 (the gray) and Y3Al5O12 (the white). This indicates that this threedimensional network microstructure runs through the whole eutectic sample. As the LDS technique avoids unexpected nucleation introduced from container walls, the two phases of the DS Al2O3/YAG eutectic completely grow from the bottom of the sample and may form single crystals. Figure 3a−c illustrates the eutectic morphologies of DS Al2O3/YAG eutectic at the growth rates of 9 μm/s, 68 μm/s, and 800 μm/s, respectively. According to the Jackson-Hunt theory based on entropy of fusion, the eutectic will grow in a faceted−faceted manner if both phases process high entropies of fusion, typically, ΔS/R > 5, where ΔS is the entropy of fusion and R is the gas constant.7,22 For the Al2O3/YAG system, both compositions have high entropies of fusion: ΔSAl2O3/R is 5.8 and ΔSYAG/R is 14.7.7 The DS Al2O3/YAG eutectic with the growth rate of 9 μm/s presents the irregular “Chinese script” eutectic morphology, namely, triangular or tetragonal shapes, owing to the crystalline anisotropy of crystal growth during faceted−faceted growth. When the growth rate increases to 68 μm/s, the DS Al2O3/YAG eutectic morphology transforms into a complex regular eutectic morphology which consists of an entangled curved lamellar-like and rod-like microstructure. YAG is influenced by a large interface-kinetic effect during the crystal growth because of its highly complex garnet structure.23,24 The transform of eutectic morphology may occur because that YAG tends to form a nonfaceted phase at the growth rate of 68 μm/s. The complex regular morphology resembles the one formed in the Al2O3/GdAlO3 eutectic, in which GdAlO3 grows in a nonfaceted manner that may infer the nonfaceted growth manner of YAG.12 When the growth rate increases to 800 μm/s, the DS Al2O3/YAG eutectic presents the regular lamellar eutectic colonies which are B
DOI: 10.1021/acs.cgd.5b01013 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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Figure 3. Morphological evolution of DS Al2O3/YAG eutectic composites at the growth rates of (a) 9 μm/s, (b) 68 μm/s, and (c) 800 μm/s; (d) high magnification of eutectic morphology of (c); and (e) relationship between the interlamellar spacing and growth rates.
Figure 4. Hardness and reduced modulus of the DS Al2O3/YAG eutectic composite at the growth rate of about 68 μm/s (a), and the typical indentation morphology (b).
were made under the maximum load of 650 mN. The typical indentation morphology is shown in Figure 4b. The hardnessdisplacement and modulu-displacement curves were shown Figure S2−S3 in the Supporting Information. The hardness and reduced modulus obtained from all of the nanoindentations were stable around 22.3 and 338 GPa, respectively. Moreover, all of the load−displacement curves recorded during indentation tests overlapped very well. The hardness is much higher than that of the Al2O3/YAG eutectic with a similar eutectic spacing prepared by Bridgman (16.3 GPa)6 and double side laser zone remelting process (18.6 GPa).27 The outstanding hardness may be attributed to the regular eutectic morphology containing entangled curved rod-like and lamellar-like microstructure. This indicates that the LDS process will become a promising method for directional solidification of oxide eutectic ceramics. In summary, the DS Al2O3/YAG eutectic with the threedimensional networked microstructures consisting of entangled single crystal α-Al2O3 and single crystal Y3Al5O12 was first prepared by the LDS process. The DS eutectic presents the irregular “Chinese script” eutectic morphology at the crystal growth rate of 9 μm/s. The morphology transforms into a
separated by coarse cellular morphology as shown in Figure 3c. To the best of our knowledge, this eutectic structure was never found in the Al2O3/YAG binary eutectics in previous reports. Figure 3d illustrates high magnification of the lamellar eutectic morphology of Figure 3c. The lamellar eutectic indicates a coupled growth mechanism which suggests a planar solid− liquid interface. The eutectic structure change to lamellar morphology from rod morphology also occurs in the Al2O3− ZrO2 eutectic with increasing growth rates.25,26 The result indicates that Al2O3/YAG eutectic may transform into a nonfaceted−nonfaceted growth manner under a superhigh growth rate of 800 μm/s. As shown in Figure 3e, the interlamellar spacing decreases with the increasing growth rate because the rapid growth rate and high kinetic undercooling inhibit the solute exchanged at the solid−liquid interface. Moreover, the interlamellar spacing (λ) and the growth rate (ν) generally conformed to the relation λ ∼ ν−1/2, as derived from the Jackson-Hunt model.10 The constant of proportionality equals 8.7, if λ has the dimensions μm and v μm/s. Figure 4 shows the hardness and reduced modulus of the DS Al2O3/YAG eutectic composite at the growth rate of about 68 μm/s obtained by nanoindentation tests. Ten indentations C
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(16) Yu, J. D.; Arai, Y.; Masaki, T.; Ishikawa, T.; Yoda, S.; Kohara, S.; Taniguchi, H.; Itoh, M.; Kuroiwa, Y. Chem. Mater. 2006, 18, 2169− 2173. (17) Ma, B. Q.; Li, J. Q.; Xu, Z.; Peng, Z. J. Appl. Energy 2014, 132, 568−574. (18) Yasuda, H.; Mizutani, Y.; Ohnaka, I.; Ohnaka, I.; Sugiyama, A. Mater. Trans. 2001, 42, 2124−2130. (19) Yasuda, H.; Ohnaka, I.; Mizutani, Y.; Sugiyama, A.; Morikawa, T.; Takeshima, S.; Sakimura, T.; Waku, Y. Sci. Technol. Adv. Mater. 2004, 5, 207−217. (20) Mizutani, Y.; Yasuda, H.; Ohnaka, I.; Maeda, N.; Waku, O. J. Cryst. Growth 2002, 244, 384−392. (21) Oliver, W. C.; Pharr, G. M. J. Mater. Res. 1992, 7, 1564−1583. (22) Hunt, J. D.; Jackson, K. A. Trans. Metall. Soc. AIME 1966, 236, 843. (23) Nagashio, K.; Kuribayashi, K. Acta Mater. 2001, 49, 1947−1955. (24) He, G.; Liu, G. H.; Yang, Z. C.; Guo, S. B.; Li, J. T. Ceram. Int. 2014, 40, 15265−15271. (25) Echigoya, J.; Takabayashi, Y.; Suto, H.; Ishigame, M. J. Mater. Sci. Lett. 1986, 5, 150−152. (26) Ando, T.; Shiohara, Y. J. Am. Ceram. Soc. 1991, 74, 410−417. (27) Yu, J. Z.; Zhang, J.; Su, H. J.; Song, K.; Liu, L.; Fu, H. Z. J. Inorg. Mater. 2012, 27, 843−848.
complex regular eutectic morphology containing entangled curved rod-like and lamellar-like microstructure at the high growth rate of 68 μm/s. When the crystal growth rate increases to 800 μm/s, the DS eutectic presents the regular lamellar eutectic morphology which indicates a nonfaceted−nonfaceted growth manner. The indentation hardness of DS Al2O3−YAG eutectic composite at the growth rate of about 68 μm/s is 22.3 GPa which is superior compared with that by other processes.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.5b01013. EDS analysis of the DS Al2O3/YAG eutectic sample at growth rate of 400 μm/s, hardness-displacement and modulus-displacement curves of Point 1 (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Telephone: 86-10-82544953. Fax: 8610-82544953. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work is financially supported by the National Natural Science Foundation of China (Grant Nos. 51471158, 51274182, 51474061), Beijing Natural Science Foundation (Nos. 2152032, 2112039) National Key Technology R&D Program of China (No. 2012BAA03B03). This research is also funded by State Key Laboratory of New Ceramic and Fine Processing (Tsinghua University).
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REFERENCES
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DOI: 10.1021/acs.cgd.5b01013 Cryst. Growth Des. XXXX, XXX, XXX−XXX
