Trans. Nonferrous Met. Soc. China 22(2012) s579-s584

HR-TEM and FIB-SEM characterization of formation of eutectic-like structure from amorphous GdAlO₃-Al₂O₃ system

Y. H. HAN¹, Y. HARADA¹, J. F. SHACKELFORD², Jaehyung LEE¹, K. KAKEGAWA³

1. School of Materials Science and Engineering, Yeungnam University, 214-1 Daedong, Gyeongsan, Gyeongbuk, 712-749, Korea;

2. Department of Chemical Engineering and Materials Science, University of California at Davis, CA, USA;

3. Graduate School of Engineering, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba, 263-8522, Japan

Received 21 May 2012; accepted 6 November 2012

Abstract: The crystallization process of the eutectic composition of GdAlO₃-Al₂O₃ from the amorphous phase prepared by rapid-quenching of melt that leads to the formation of a cantaloupe skin-like microstructure was investigated using focused ion-beam scanning electron microscopy (FIB-SEM) and high-resolution transmission electron microscopy (HR-TEM). The amorphous films were heat-treated at temperatures between 1000 °C and 1500 °C for up to 30 min to form the eutectic phases of GdAlO₃ and Al₂O₃. The GdAlO₃ and Al₂O₃ crystal phases that formed from the amorphous phase were identified by FIB-SEM and HR-TEM. Both components began to crystallize and grow from the amorphous phase separately at different temperatures. The formation process of these crystal phases was different from that of the ordinary eutectic microstructure solidified from the GdAlO₃-Al₂O₃ system. Therefore, the observed structure is termed “eutectic-like” for distinction. The microstructures formed from the amorphous phases at sufficiently high temperatures consisted of ultra-fine microstructures of individually crystallized components and were similar to ordinary eutectic microstructures. By heat-treating the amorphous films at 1500 °C for either 2 min, 8 min or 30 min, the ultra-fine components of GdAlO₃ and Al₂O₃ were found to crystallize following a eutectic-like stage after 8 min of heat treatment.

Key words: eutectic; amorphous; GdAlO₃-Al₂O₃; eutectic-like; HR-TEM; FIB-SEM; Crystallization

1 Introduction

One solution for realizing the higher operating temperatures of the next-generation turbo-blades operating at gas temperatures as high as 1600 °C is by way of an in-situ development of oxide-based ceramic composites by directed solidification of melt having a eutectic composition. The resulting structure is a single-crystal, three-dimensional lattice with no grain boundary, i.e., a material with coherent interfaces and no vitreous phase.

This material is known to exhibit a mechanical strength that is nearly constant up to the melting temperature unlike polycrystalline ceramics that gradually looses strength with temperature. It is also known that the rupture stress in this material can be increased by reducing the size of the constituent phases without reducing the creep strength. This is the case of perovskite-structured oxides having a high temperature plasticity in a eutectic system (melting at 1700-1900 °C) that combines alumina with a rare earth oxide having either a perovskite (XAlO₃; X: Gd, Eu) or a garnet (Y₃Al₅O₁₂; Y: Er, Dy, Yb) structure. Recently, eutectic ceramics have been considered to be a candidate not only as a heat-resistant material but also as a functional material in various other applications, such as thermophotovoltaic (TPV) generation [1-3] ^,and porous materials technology [4-6].

Although the formation process of the ordinary eutectic microstructures by cooling the eutectic melts has been well-known, the formation process of the eutectic-like microstructures from an amorphous phase has not so far been elucidated in our previous work [7,8].

The current study focuses on the focused ion-beam scanning electron microscopy (FIB-SEM) and high-resolution transmission electron microscopy (HR-TEM) characterization of the crystallization process of the GdAlO₃-Al₂O₃ binary eutectic system [9-12]. We characterized the ultra-fine eutectic microstructure processed both from the eutectic melt and from the amorphous phase.

2 Experimental

2.1 Sample preparation

The raw materials were the powders of Al₂O₃ (99%, Kanto Chemical, Japan) and Gd₂O₃ (99.95%, Kanto Chemical, Japan). The powders in the eutectic composition 23% (mole fraction) of Gd₂O₃ and 77% (mole fraction) Al₂O₃ were processed using a mortar and pestle in a 3% (mass fraction) polyvinyl alcohol (Waco Pure Chemical Industries, Japan; degree of polymerization: ~500) solution. The mixture was pressed into a rod-shaped compact and was calcined at 1300 °C for 1 h.

The top of the sintered rod was melted by arc discharge; the slow-cooled samples of solid eutectic composite were produced after turning off the discharge. The sample was rapidly cooled by quenching the droplet between rotating aluminum rollers to form an amorphous film. The amorphous films were obtained from heating between 1000 °C and 1600 °C for 30 min to form a eutectic phase of GdAlO₃ and Al₂O₃. The films were heated at 1500 °C for various periods ranging from 30 s to 30 min for the investigation of the crystallization process of the ultra-fine microstructures in the eutectic-like composite of GdAlO₃ and Al₂O₃.

2.2 Specimen preparation and characterization by FIB-SEM and HR-TEM

For the FIB-SEM analysis, a SEIKO FIB/SEM Hybrid System SMI 3050SE was operated with the acceleration voltage of 30 kV, the beam current of 90 pA, and the beam size of 22 nm. An H-bar and lift-out technique was employed to prepare the TEM specimens by FIB milling. For all the specimens, the acceleration voltage of 30 kV and the beam current of 90 pA were applied. The TEM was a JEOL JEM-2100F and was operated at 200 kV at a point resolution 0.19 nm for the high resolution characterization.

3 Results and discussion

Figure 1 shows the HR-TEM analysis of a rapidly quenched specimen and heat-treated specimens of the Gd₂O₃-Al₂O₃ eutectic composition, sintered at 1000 °C for 30 min. The rapidly quenched specimen was found to be mostly amorphous, as shown in Fig. 1, establishing that an amorphous film can be successfully prepared by rapidly quenching the Gd₂O₃-Al₂O₃ eutectic melt.

The HR-TEM analysis of the lattice image of the eutectic 23%Gd₂O₃-77%Al₂O₃ after being sintered at 1000 °C for 30 min reveals a mixed phase of a crystalline region (dark) and a noncrystalline region (bright), as shown in Fig. 1. The dark, crystalline regions exhibit lattices, as indicated by A. The bright, noncrystalline regions have no order in atomic arrangement, as indicated by B.

Fig. 1 HR-TEM analysis of lattice image of eutectic 23%Gd₂O₃-77%Al₂O₃ sintered at 1000 °C for 30 min

A low magnification image of the region analyzed by EDS is shown in Fig. 2(a) and the qualitative data for the region are shown in Fig. 2(b) with the peaks for GdAlO₃ identified.

For the specimen sintered at 1300 °C for 30 s, dark and bright regions coexist in the TEM image at a low magnification, as shown in Fig. 3(a), in which the dark region is the crystalline Al₂O₃ and the bright region is the crystalline GdAlO₃. A high magnification image of GdAlO₃ from a bright area is shown in Fig. 3(b), and a high magnification image of Al₂O₃ from a dark area is shown in Fig. 3(c).

The dark areas (Al₂O₃) and the bright areas (GdAlO₃) appear to be in an initial or intermediate stage of crystallization, which is confirmed by the TEM analysis of the sample sintered at 1300 °C for 30 s, as shown Fig. 4.

In Figs. 5-7, the eutectic composites sintered at 1500 °C for 2 min, 8 min, or 30 min are compared with the crystalline Al₂O₃ appearing dark and the crystalline GdAlO₃ appearing bright, analyzed earlier by FIB-SEM and HR-TEM.

In Fig. 5, the bright area is GdAlO₃ and the dark area is Al₂O₃. Most of the Al₂O₃ component is crystallized from the amorphous phase along with GdAlO₃ after a heat treatment at 1500 °C for 2 min, as shown in Fig. 5, which can be explained well, based on the previous results by KAKEGAWA et al [13]. The completely crystallized components appeared after a heat treatment of 8 min, as shown in Fig. 6. A similar crystallized microstructure is present in the sample sintered for 30 min according to the FIB-SEM micrographs that show apparently fine, eutectic-like microstructures. These microstructures having an appearance of a cantaloupe skin are intertwined GdAlO₃ and Al₂O₃ crystals, as confirmed by the TEM analysis presented in Fig. 7. It is confirmed that the crystallization process of Gd₂O₃-Al₂O₃ completes after being sintered at 1500 °C for 8 min.

Fig. 2 Region analyzed by EDS (a) and qualitative data with peaks for GdAlO₃ identified (b)

Fig. 3 TEM images of sample sintered at 1300 °C for 30 s, showing dark areas of crystalline Al₂O₃ and bright areas of crystalline GdAlO₃ (a), Al₂O₃ in dark area in process of crystallization (b) and GdAlO₃ in bright area in process of crystallization (c)

The formation process of eutectic-like microstructures from the amorphous phase is presumably different from that of an ordinary eutectic microstructure, as mentioned in Refs. [7,8]. In general, an ordinary eutectic microstructure is formed by simultaneously crystallizing individual components at a eutectic temperature. In contrast, the crystallization and the growth of the GdAlO₃ and Al₂O₃ components from the amorphous phase begin separately at different heating temperatures. In the case of low temperatures, in which the Al₂O₃ component cannot crystallize, the microstructure consists of round-shaped GdAlO₃ crystals and the amorphous Al₂O₃ phase, as proved by KAKEGAWA et al [13].

Fig. 4 TEM analysis of sample sintered at 1300 °C for 30 s

When one component nucleates and grows, the component is consumed during the crystallization and its concentration is lowered. As a result, the crystal growth slows down. However, there is a directional fluctuation in the crystal growth [14,15]. If the growth occurs on a convex part of the crystal, the concentration of its component in the surrounding region increases and the crystal can grow further. In addition, the crystal grows while releasing the latent heat and enhancing the growth. In this mechanism, the crystal will not grow into a sphere but into a long and thin shape. There is also a directional fluctuation of the component in the amorphous phase that makes the long thin crystals to intertwine when they meet. GAP crystallizes in this way first and then Al₂O₃ crystallizes later.

Fig. 5 FIB-SEM and HR-TEM microstructures of eutectic composites sintered at 1500 °C for 2 min

Fig. 6 FIB-SEM and HR-TEM microstructures of eutectic composites sintered at 1500 °C for 8 min

Fig. 7 FIB-SEM and HR-TEM microstructures of eutectic composites sintered at 1500 °C for 30 min

The above explains why the observed microstructure is different from an ordinary eutectic microstructure. The fine, eutectic-like microstructure cannot materialize from the crystallization of a single component. As the heating temperature reaches an appropriate temperature, the Al₂O₃ component can crystallize along with the remaining amorphous Al₂O₃ phase. When heat treated at 1500 °C, the Al₂O₃ component begins to crystallize immediately along with the GdAlO₃ component and forms a fine eutectic-like microstructure with a cantaloupe skin appearance, as shown in Fig. 5.

4 Conclusions

An ultra-fine microstructure of GdAlO₃ and Al₂O₃ is formed from the amorphous phase following an eutectic-like stage by simultaneously crystallizing both the components at a sufficiently high temperature of 1500 °C for 8 min. The eutectic-like microstructure formed from the amorphous phase is very fine in texture compared with an ordinary eutectic microstructure.

Acknowledgements

This work has been part of the study under the “Human Resource Development Center for Economic Region Leading Industry” Project, supported by the Ministry of Education, Science & Technology (MEST) and by the National Research Foundation of Korea (NRF).

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(Edited by LI Xiang-qun)

Corresponding author: Y. H. HAN; Tel: +82-53-810-2539; Fax: +82-53-810-4628; E-mail: yhhanyu@yu.ac.kr

DOI: 10.1016/S1003-6326(12)61769-5