Alumina-based Directionally Solidified Eutectic Ceramics: Microstructure, Control Strategies and Environmental Stability

doi:10.15541/jim20250440

Abstract

Abstract:

With the rapid advancement of aviation industry, high-temperature structural materials used in aero-engines are encountering increasingly severe service environments and performance challenges. To meet exacting requirements of future technologies, turbine inlet temperature of advanced aero-engines with a thrust-to-weight ratio exceeding 10 is projected to surpass 1500 ℃. Al₂O₃-based directionally solidified eutectic ceramics are regarded as ideal and key candidate materials for next-generation extreme working conditions, particularly for long-term use in high-temperature oxidative environments, owing to their melting points exceeding 1700 ℃, outstanding high- temperature stability, and inherent oxidation resistance. This paper provides a comprehensive review of microstructural characteristics of Al₂O₃-based directionally solidified eutectic ceramics, with particular emphasis on their morphological features, crystallographic relationships and interfacial structures. Subsequently, discussion focuses on strategies for microstructure control, including optimization of processing parameters, optimal selection of eutectic components, and implementation of innovative approaches such as high-entropy design. Furthermore, a comprehensive analysis is conducted to evaluate stability of these materials under typical high-temperature service conditions, including thermal exposure, oxidative environments, and chemically corrosive media, with particular emphasis on their chemical stability characteristics. Based on this comprehensive review, key scientific issues and core technical challenges in current research are identified. Future research directions are also proposed to establish a theoretical foundation and provide technical guidance for engineering applications of these materials in advanced aero-engine systems.

Key words: Al₂O₃-based eutectic ceramic, directional solidification, microstructure control, microstructure stability, environmental stability, review

CLC Number:

TQ174

ZHOU Cui, LI Jie, SUN Luchao, SU Haijun, WANG Jingyang. Alumina-based Directionally Solidified Eutectic Ceramics: Microstructure, Control Strategies and Environmental Stability[J]. Journal of Inorganic Materials, 2026, 41(7): 899-914.

Figures/Tables 14

Fig. 1 Typical microstructure of Al2O3-based directionally solidified eutectic ceramics[28-30] (a1-c1) Three-dimensional network structure of (a1) YAG/Al2O3[28-29], (b1) Er3Al5O12 (EAG)/Al2O3[30] and (c1) Yb3Al5O12 (YbAG)/Al2O3[30]; (a2-c2) SEM morphologies of transverse section of (a2) YAG/Al2O3[28-29], (b2) EAG/Al2O3[30] and (c2) YbAG/Al2O3[30]

Fig. 2 Microstructure of top surface of LPBF-fabricated sample[38] (a) Needle-like ZrO2 settlement in the center of the molten pool; (b) Imperfectly needle-like ZrO2; (c) Colony eutectic with nano spherical ZrO2 structure; (d) Dendritic ZrO2 located in colony eutectic structure; (e) Needle-like structure ZrO2 located in colony eutectic structure; (f) Aggregation, growth, and elongation of nano spherical ZrO2

Table 1 Crystallographic relationships in Al2O3-based directionally solidified eutectic ceramics fabricated via various methods[21,25,44 -53]

Eutectic system	Preparation method	Crystallographic relationship
Eutectic system	Preparation method	Growth direction	Interface relationship
YAG/Al₂O₃^[44]	MLSPM	<$10 \bar{1} $> YAG // <$10 \bar{1} 0$> Al₂O₃	(121) YAG // (0001) Al₂O₃
YAG/Al₂O₃^[25]	OFZM	<011> YAG // <$1 \bar{1} 00$> Al₂O₃	($2 \bar{1} 1$) YAG // (0003) Al₂O₃
YAG/Al₂O₃^[45]	LFZM	<111> YAG // <$10 \bar{1} 0$> Al₂O₃	($11 \bar{2}$) YAG // (0001) Al₂O₃
YAG/Al₂O₃^[46]	Bridgman	<101> YAG // <$1 \bar{1} 00$> Al₂O₃	($2 \bar{1} 1$) YAG // (0001) Al₂O₃
YAG/Al₂O₃^[47]	Bridgman	<110> YAG // <$11 \bar{2} 0$> or <$01 \bar{1} 0$> Al₂O₃	($\bar{1} 12$) YAG // (0001) Al₂O₃
REAG/Al₂O₃^[48]	OFZM	<110> or <$2 \bar{1} 0$> REAG // <$10 \bar{1} 0$> Al₂O₃	(121) REAG// (0001) Al₂O₃ (RE=Y, Yb, Er, Dy)
YAG:Ce³⁺/Al₂O₃^[49]	LFZM	<$ \bar{1} 10$> YAG:Ce³⁺ // <$1 \bar{1} 00$> Al₂O₃	(220) YAG:Ce³⁺ // ($11 \bar{2} 0$) Al₂O₃
YAG/Al₂O₃/ZrO₂^[48]	OFZM	<001> YAG // <$10 \bar{1} 0$> Al₂O₃ // <001> ZrO₂	(001) YAG // (0001) Al₂O₃ // (100) ZrO₂
YAG/Al₂O₃/ZrO₂^[50]	OFZM	<001> YAG // <$1 \bar{1} 00$> Al₂O₃ // <001> ZrO₂	(001) YAG // ($11 \bar{2} 0$) Al₂O₃ // (001) ZrO₂
YAG/Al₂O₃/ZrO₂^[21]	μ-PD	<100> YAG // <$0 \bar{1} 10$> Al₂O₃ // <100> ZrO₂	($0 \bar{3} 1$) YAG // (0003) Al₂O₃ // (010) ZrO₂
ZrO₂/Al₂O₃^[51]	CZ/OFZM	<010>/<001> ZrO₂ // <0001>/<$01 \bar{1} 0$> Al₂O₃	(100) ZrO₂ // ($2 \bar{1} \bar{1}0$) Al₂O₃
ZrO₂/Al₂O₃^[52]	CZ/OFZM	<001> ZrO₂ // <$0 1 \bar{1}0$> Al₂O₃	(100)/(010) ZrO₂ // ($2 \bar{1} \bar{1}0$)/ (0001) Al₂O₃
ZrO₂/Al₂O₃^[53]	LHFZ	<011> ZrO₂ // <0001> Al₂O₃	(200) ZrO₂ // ($\bar{2}110$) Al₂O₃

Fig. 3 High-resolution transmission electron microscope images of interface in Al2O3-based directionally solidified eutectic ceramics[25,50] (a) YAG/Al2O3[25]; (b-d) YAG/Al2O3/ZrO2[50]

Fig. 4 Schematic illustrations of lattice mismatch in GAP/Al2O3 directionally solidified eutectic ceramics[42] (a) (101) GAP // (0001) Al2O3; (b) (010) GAP // (0001) Al2O3

Table 2 Eutectic systems, preparation methods, λv1/2 and microstructures of some Al2O3-based eutectic ceramics[10,19 -20,22,46,49,61 -68]

Eutectic system	Preparation method	C=λ·v^1/2/(μm^1.5·s^-0.5)	Microstructure
YAG/Al₂O₃^[46]	OFZM	11.6	Chinese script
YAG/Al₂O₃^[46]	Bridgman	44.7	Chinese script/Cellular structure
YAG/Al₂O₃^[22]	EBFZM	6.7	Chinese script/Lamellar structure
YAG/Al₂O₃^[61]	LHFZ	10	Chinese script
YAG/Al₂O₃^[19]	μ-PD	10	Chinese script
YbAG/Al₂O₃^[62]	LFZM	11.5	Chinese script
EAG/Al₂O₃^[10]	LHFZ	11.6	Chinese script
Al₂O₃/YAG:Ce^3+[49]	LFZM	22.0	Chinese script/Cellular structure
YAG/Al₂O₃/ZrO₂^[63]	μ-PD	8	Chinese script/Geometric structure
YAG/Al₂O₃/ZrO₂^[64]	μ-PD	8	Chinese script
YAG/Al₂O₃/ZrO₂^[65]	LFZM	12.4	Chinese script/Cellular structure
YAG/Al₂O₃/ZrO₂^[66]	LFZM	14.7	Chinese script/Cellular structure
YAG/Al₂O₃/ZrO₂^[20]	μ-PD	6.5	Chinese script
YAG/Al₂O₃/ZrO₂^[67]	EBFZM	5.3	Chinese script/Cellular structure
ZrO₂/Al₂O₃^[68]	Laser 3D printing	1	Chinese script/Cellular structure

Fig. 5 Transversal sections of Al2O3-based directionally solidified eutectic ceramics fabricated via laser zone melting method[75]

Fig. 6 Crystallographic orientation electron back scatter diffraction (EBSD) images and corresponding inverse pole figures of YAG/Al2O3 eutectic ceramics obtained at various distances from polycrystalline Al2O3 seed[77] (a-d) Al2O3 phase; (a1-d1) YAG phase

Fig. 7 EBSD images and corresponding inverse pole figures of YAG/Al2O3 eutectic ceramics after growing 10 mm induced by different single crystal seeds[79] (a-d) Al2O3 phase; (a1-d1) YAG phase

Fig. 8 Typical EBSD images and corresponding inverse pole figures of (Y0.2Gd0.2Ho0.2Er0.2Yb0.2)3Al5O12/Al2O3 eutectic ceramics at different positions from seed crystal[88]

Fig. 9 (a1-d3) Surface scanning electron microscope (SEM) images of Al2O3-based directionally solidified eutectic ceramics subjected to various heat treatment temperatures and durations[93]; (e, f) Surface SEM images of Al2O3-based eutectic ceramics sintered at (e) 1450 and (f) 1500 ℃[94]

Fig. 10 Microstructural evolution of GAP/Al2O3 eutectic ceramics fabricated under various growth rates before and after heat treatment at 1773 K[96] (a1-a4) 2 μm/s; (b1-b4) 16 μm/s; (c1-c4) 50 μm/s; (d1-d4) 100 μm/s

Fig. 11 SEM images of the YAG/Al2O3 interface after 10 h of corrosion at 1800 ℃ in different atmospheres[100]

Fig. 12 Ca elemental mappings of the cross-section of (Y0.2Gd0.2Ho0.2Er0.2Yb0.2)3Al5O12/Al2O3 eutectic ceramics after CMAS corrosion at 1500 ℃ for different durations[102] (a) 4 h; (b) 50 h; (c) 100 h; (d) 200 h

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