Processing, microstructure, and properties of

laser remelted Al₂O₃/Y₃Al₅O₁₂(YAG) eutectic in situ composite

SU Hai-jun(苏海军), ZHANG Jun(张 军), LIU Lin(刘 林), FU Heng-zhi(傅恒志)

State Key Laboratory of Solidification Processing, Northwestern Polytechnical University,

Xi’an 710072, China

Received 15 July 2007; accepted 10 September 2007

Abstract: Laser remelting and rapid solidification were performed in preparing the high-performance Al₂O₃/Y₃Al₅O₁₂(YAG) eutectic in situ composite. The microstructure characteristic and solidification behavior were studied using scanning electron microscopy(SEM), energy dispersive spectroscopy(EDS), X-ray diffractometry(XRD) and simultaneous thermal analysis(STA). The hardness and fracture toughness were obtained using an indentation technique. The results show that the laser remelted Al₂O₃/YAG composite has a homogeneous eutectic microstructure without microcrack and pore. The component phases of Al₂O₃ and YAG are three-dimensionally and continuously reticular connected, and finely coupled without grain boundaries, colonies and amorphous phases between interfaces. The eutectic interspacing is greatly refined with increasing the scanning rate and average is only 1 mm. The synthetically thermal analysis indicates that the eutectic temperature of Al₂O₃-YAG is 1 824 ℃, well matching the phase diagram of Al₂O₃-Y₂O₃ system. The maximum hardness reaches 19.5 GPa and the room fracture toughness is 3.6 MPa?m^1/2.

Key words: Al₂O₃/Y₃Al₅O₁₂(YAG); eutectic; in situ composite; laser remelting; microstructure; properties

1 Introduction

To solve increasingly serious energy resources problems and keep sound, sustainable economic growth, it is highly required to rapidly improve the thermal efficiency and decrease the energy consumption in advanced power generator field. The second law of thermodynamics determines that the efficiency of a thermal engine increases with the maximum temperature in the cycle, and consequently, leading to vigorous searching for a new structural material with excellent mechanical properties at elevated temperatures (1 400-  1 700 ℃) over the world[1].

Currently, oxide-based ceramic materials are well recognized as ones that can survive in oxidizing environments at temperatures approaching or exceeding 1 400 ℃[2]. However, most monophasic polycrystalline oxide ceramics prepared by powder sintering method can not be used at elevated temperatures due to the glassy phase at the grain boundaries, which results in a significant reduction in strength above 1 100 ℃[3]. As the eutectic phases are all grown simultaneously from melt, eutectic in situ composite fabricated by directional solidification shows extremely fine microstructure in the submicron range, high-density interfacial area and strong bonded interface, and consequently, resulting in high microstructural and chemical stability up to temperatures even close to the melting point. Additionally, it combines the properties of distinct constitutes into one material, eliminates the manmade interface incompatibility and effectively avoids grain boundaries. Therefore, applying directional solidification technique to prepare eutectic ceramic in situ composite is very hopeful to provide a novel, potential method to improve properties of oxide ceramics. WAKU et al[4] reported that directionally solidified Al₂O₃/YAG and Al₂O₃/GdAlO₃ eutectic ceramics exhibited excellent mechanical properties at high temperatures. For example, Al₂O₃/YAG can maintain flexural strength of 360-500 MPa from room temperature to 1 800 ℃ (melting temperature 1 826 ℃) in air[3]. The compression creep strength at 1 600 ℃ is about 13 times higher than that of sintered composites with the same chemical composition[5]. The eutectic also has outstanding corrosion resistance in humid environments at high temperatures[6]. So, directionally solidified Al₂O₃/YAG eutectic in situ composite has become the most promising candidate as high- temperature structural materials [7].

However, most previous studies about the Al₂O₃/YAG eutectic mainly focus on investigations of the mechanical properties under slow growth rate by directional solidification[3-5]. The microstructure, solidification behavior and mechanical properties of the eutectic under rapid growth rate are rarely studied up to now. Recently, laser zone melting technique has been successfully used to directionally grow oxide eutectic ceramics with advantages of the absence of any container or die (avoiding possible sources of contamination), high melting temperature, steep temperature gradient, fast growing rate and low cost. The present study aims to present the preparation, rapid solidification charac- teristics and growth mechanism of Al₂O₃/YAG eutectic ceramic in situ composite. Then the hardness and room-temperature fracture toughness of the as-solidified composites are investigated by the micro-indentation method.

2 Experimental

2.1 Sample preparation

The starting materials were prepared by mixing the high purity nano-powders of Al₂O₃ and Y₂O₃ corresponding to eutectic composition using ball-milling. The mole fraction ratio of Al₂O₃ to Y₂O₃ was 82?18[8]. The precursor rods of d 7 mm×60 mm samples were prepared after mixing, pressing and sintering at 1 500 ℃ for 2 h. With 5 kW ROFIN-SINAR850 CO₂ laser, the laser zone remelting was carried out in a vacuum chamber. The sample was moved by the numerically-controlled worktable with 5-axis and 4-direction coupled motion to realize the laser beam scanning along the sample axis. The sample was solidified just behind the melting zone, as illustrated in Fig.1. In order to avoid volatilization and fire waste of the composition, Ar gas is filled into the vacuum chamber at a flow rate of 10 L/min. Considerable experiments show that under Ar atmosphere with the laser power of 190-220 W, the scanning rate of 10-1000 mm/s and the beam diameter of 4 mm, the crackless samples with smooth surface and high density can be obtained.

2.2 Characterizations

The microstructure and component of the composite were determined by the scanning electron microscopy

Fig.1 Experimental setup for laser zone remelting method applied to growth of eutectic rods (v is scanning rate, and R is solidification rate)

(SEM, JSM-5800), the energy disperse spectroscopy (EDS, Link-Isis) and the X-ray diffractometry(XRD, Rigakumsg-158). Quantitative calculation of the phase volume fraction was performed by the digital image analysis software of SISC IAS V8.0. The thermal behavior and phase transformation were analyzed by the simultaneous thermal analyzer (STA, Netzsch-409CD). Approximately 37 mg of the as-solidified sample was heated in a Mo crucible at a heating rate 10 ℃/s up to   1 950 ℃ in a flowing stream of Ar, and the corresponding cooling rate was 10 ℃/s.

The hardness and fracture toughness were determined by a Vickers indentation technique on the polished surface of the composite, using applied load of 9.8 N for 15 s. The indentation size and the crack length were measured from optical microscopy and SEM. The indentation morphology and crack propagation were observed through the laser scanning confocal microscope (LSCM, Lext 3000). The hardness and fracture toughness are calculated from the equation proposed by ANTIS et al[9] for the median radial cracks. The equations are expressed as follows:

H_V＝0.464p/a²                                                (1)

K_IC＝0.016(H_V/E)^-1/2p/c^3/2                        (2)

E=E_fφ_f+E_mφ_m                                                (3)

where  H_V is the Vickers hardness, K_IC is the fracture toughness, E, E_f and E_m are elastic moduli of the composite, YAG and Al₂O₃, respectively, φ_f and φ_m are the volume factions of YAG and Al₂O₃ phases, respectively, p is the indentation load, c is the distance from the center of the indentation to the crack tip and a is half of the indentation diagonal.

3 Results and discussion

3.1 Thermal behavior

The DTA-DDTA curves in the heating and cooling processes are shown in Figs.2 (a) and (b), respectively.

Fig.2 DTA and DDTA curves of laser remelted Al₂O₃/YAG eutectic composite: (a) Heating process; (b) Cooling process

From Fig.2(a), it can be seen that a sharp endothermic peak occurs at the heating state in the temperature range of 1 815-1 824 ℃, indicating the melting of the eutectic structure, which agrees well with the eutectic reaction point (1 826 ℃) of the phase diagram of the Al₂O₃-Y₂O₃ system[8] and verifies that the composition of the composite lies in the eutectic point or very near the eutectic point. However, in the cooling state, there are two exothermic peaks appearing at 1 767 ℃ and 1 684 ℃, respectively, and the latter is much sharper than the former, as shown in Fig.2(b). In the Al₂O₃-Y₂O₃ system, there is a narrow coupled growth zone for eutectic solidification. In common, the Y₂O₃ is relatively easy to volatilize as compared with the Al₂O₃ at elevated temperatures. The volatilization of Y₂O₃ may induce the composite to deviate the eutectic composition, consequently, resulting in a hypereutectic composition. Therefore, during the solidification, the first exothermic peak may be due to the nucleation of the primary YAG phase, and then the composite enters into the Al₂O₃/YAG equilibrium eutectic solidification process, accordingly, occurring the second exothermic peak. Moreover, in the system of Al₂O₃/YAG, YAG also commonly acts as the first phase to nucleate at the eutectic, leading to two exothermic heats. Similar phenomenon during the solidification process has also been observed by YASUDA et al[10] in the Al₂O₃-23.5%Y₂O₃ hypereutectic composite.

On the other hand, in the Al₂O₃-Y₂O₃ system, the phase selection is determined by the maximum melt temperature, the cooling rate, and the nucleation of YAG [8, 11-12]. CASLAVSK et al[11] systemically investigated the Al₂O₃-Y₂O₃ system by using optical DTA. The study indicates that there are eutectic reactions: one is the Al₂O₃/YAG equilibrium eutectic at 1 826 ℃ and the other is Al₂O₃/YAlO₃(YAP) metastable eutectic at 1 702 ℃. When the melt is cooled from above 2 000 ℃, the nucleation of YAG is completely inhibited, and consequently the Al₂O₃/YAP metastable eutectic system is selected. Contrariwise, when the eutectic melt is cooled from below 2 000 ℃, the solidification obeys the Al₂O₃/YAG equilibrium eutectic path, and is easy to form the Al₂O₃/YAG eutectic. Moreover, the cooling rate also significantly affects the phase selection. The YAP phase has been found by MIZUTANI et al[12] in the Al₂O₃/YAG eutectic during the solidification when the melt was heated up to 1 900 ℃ and then cooled at a rate higher than 5 ℃/s. So, the first weak exothermic peak during the solidification may also be attributed to the formation of the metastable phase YAP, and then the YAP transforms into YAG, afterwards the melt enters into eutectic solidification. However, because the oxide eutectic system is extremely complex and there is few experimental data, the accurate nucleation mechanism and thermal behavior should be further investigated.

3.2 X-ray diffraction(XRD) analysis

The XRD spectrum of the laser remelted Al₂O₃/YAG eutectic composite is shown in Fig.3. Identified crystalline phases are corundum (α-Al₂O₃) and yttrium aluminum garnet (Y₃Al₅O₁₂, YAG). The alumina shows very little solubility of either yttria or YAG. No

Fig.3 XRD pattern of laser remelted Al₂O₃/YAG eutectic composite

other phase is found. However, in the primary sintered composite with the same composite, there are the traces of YAP and Y₂A₄O₉ phases[13], which indicates that the laser remelting and rapid solidification of the Al₂O₃/YAG binary eutectic composite evidently change the phase composition of the metastable phases YAP or Y₂A₄O₉, which deteriorates the mechanical properties.

3.3 Eutectic microstructure

Fig.4 shows the typical microstructures of the laser remelted Al₂O₃/YAG eutectic grown at different scanning rates. The black phase is alumina and the gray phase corresponds to YAG. The YAG domains of different sizes and shapes are embedded in the matrix of Al₂O₃ phase. The eutectic microstructure exhibits an interpenetrating network, like “Chinese script”, in which Al₂O₃ phase with a hexagonal structure and YAG phase with a garnet structure are three-dimensionally and continuously connected and finely coupled without grain boundaries between interfaces (Figs.4(a), (b) and (c)). The sintered composite with the same composition shows typical polycrystalline ceramic structure with random grain orientations and evident boundaries (Fig.4(d)). The volume fraction of Al₂O₃/YAG eutectic phases, as measured from the analysis of SEM images, is 44%Al₂O₃/56%YAG, which is consistent with that expected for eutectic composition. The similar microstructure was also found by PASTOR et al[14] in Al₂O₃/YAG eutectics grown by the laser- heated floating zone method.

The eutectic presents a fine structure with typical phase spacing from the micron to sub-micron range. The size of the domains of each phase decreases with increasing the scanning rate (Figs.4(a)-(c)), according to the HUNT and JACK theory[15]. The average eutectic spacing is only 1 mm, which is almost 1/10 of that can be obtained availably using Bridgman method[3-5] and 1/2 of that obtained by laser-heated floating zone method[14]. The fine microstructure is primarily due to the rapid solidification of laser zone remelting.

During the laser rapid solidification, the temperature gradient(G) in the solid/liquid interface is high up to 10⁶ K/m, which means that higher solidification rate(v) can be achieved at the eutectic solidification, namely, G/v＞DT/D (DT is the localized constitutional supercooling; D is the solute diffusion coefficient) can be satisfied. Corresponding to the high G/R ratio, oriented eutectic macrostructures in directional solidification are either lamellar or fibrous. When the minor-phase volume fraction is higher than 0.28, a lamellar structure is commonly favored. For the Al₂O₃/YAG binary eutectic, the minor-phase volume fraction is high up to 44% on one hand, which leads to the complexly irregular lamellar eutectic structures. On the other hand, most oxides exhibit a relatively high entropy of fusion, in this case, Al₂O₃ and YAG have very high entropies of fusion

Fig.4 Typical microstructures of laser remelted Al₂O₃/YAG binary eutectics grown at different laser scanning rates: (a) 40 mm/s;   (b) 200 mm/s; (c) 600 mm/s; (d) Sintered Al₂O₃/YAG composite with same composition

of 5.74R and 14.72R (R is the gas constant), which results in a strong faceted eutectic growth of Al₂O₃ and YAG phases at solid/liquid interface according to the entropy of fusion criterion of HUNT and JACKSON[15]. So, the second phase YAG with relatively high viscosity compared to the Al₂O₃ phase grows and interweaves cooperatively with the Al₂O₃ phase, which eventually determines the formation of complex irregular eutectic microstructures. In addition, laser zone remelting process is a procedure far from an equilibrium state and the growth direction of eutectic is not coincident with heat-flux direction but exists an angle, both of which contribute to the complexity of entangled microstructures.

Moreover, from Fig.4 it can be seen that the eutectic structure is not completely uniform in local zone, as the Al-Si or Fe-C alloy has irregular eutectic structures. It is primarily due to the fact that the evolution of eutectic spacing is also strongly affected by the branching. In common, for regular eutectic, the average eutectic spacing is determined by solute diffusion and interfacial energy[15], and the branching of eutectics favors the optimal eutectic spacing determined by the inherent physical properties to adapt to the local growth condition. However, Al₂O₃/YAG binary eutectic belongs to the irregular eutectic as illustrated above, so the difficulty of branching because of the anisotropy of faceted growth of the two continuous phases makes it hard to yield favorite interspacing, and consequently, leading to the coexistence of the coarsened and fine eutectic structures. In addition, the local structural inhomogeneity of the eutectic is also relevant to the nonuniformity of heat distribution of the melted zone.

3.4 Mechanical properties

The hardness and fracture toughness are measured by means of the Vickers indentation technique. Fig.5

Fig.5 Three dimension indentation patterns and crack propagations generated by Vickers indenter

shows the three dimension indenter impression and cracks generated in the laser remelted Al₂O₃/YAG eutectic composite. For calculating the fracture toughness according to the Eqns.(1)-(3), at least 6 cracks are measured in each sample. The elastic modulus of eutectic phases, E_m, E_f and the volume fraction, φ_m, φ_f, are listed in Table 1. The calculated elastic modulus E for the eutectic is 345 GPa.

Table 1 Elastic modulus and volume fraction of eutectic phases of composite

The calculation results obtained indicate that the maximum hardness of the composite is 19.5 GPa, which is intervenient between those of Al₂O₃/YAG (16.3 GPa) and Al₂O₃/ZrO₂ (20 GPa)[16] eutectic composites. The room temperature fracture toughness of the composite reaches 3.6 MPa^.m^1/2, which is higher than that of monocrystal Al₂O₃ (2.4 MPa^.m^1/2), monocrystal YAG (3.0 MPa^.m^1/2)[17] and Al₂O₃/YAG eutectic composite (2.0-2.4 MPa^.m^1/2)[14]. This indicates that the laser zone remelting and rapid solidification can effectively improve the mechanical properties of the Al₂O₃/YAG binary eutectic composite. The improved   hardness and fracture toughness are primarily attributed to the fine eutectic microstructure and the crack branching, deflection, bending and arrest related to the laser rapid solidification. The crack branching, deflection, bending and arrest are interestedly found in the as-solidified eutectic, as shown in Fig.5. The indentation cracks were all straight in the no-doped Al₂O₃/YAG eutectics in previous reports[7, 14], which were thought as the reason for the poor toughness. The deflected cracks are only found in the CeO₂-doped Al₂O₃/YAG eutectic with improved toughness in Ref.[18]. As a result, the deflected, branched and bended cracks effectively restrain the extend of dominant cracks, and toughen the solidified eutectics.

Fig.6 shows the dependence of the hardness and fracture toughness with respect to the laser scanning rate. It is indicated that the hardness gradually increases with increasing the scanning rate, but the fracture toughness exhibits a noticeable reduction when the scanning rate is elevated to a high value. High scanning rate leads to the reduction of the thickness of solidified eutectic and more defects; moreover, the crack deflection and crack arrest are more effective in eutectics with lager domain sizes, both of which eventually deteriorate the toughness with higher scanning rate. So, the best mechanical properties can be obtained by selecting the optimal laser power density and scanning rate. Moreover, the fracture toughness at high temperatures may be higher than that of room temperature because of the thermal residual stress and motion-induced dislocation. The fracture toughness up to 4 MPa?m^1/2 at 1 723 ℃ in the Al₂O₃/ YAG eutectic has been reported by OCHIAI et al[17].

Fig.6 Dependence of fracture toughness and hardness of laser remelted Al₂O₃/YAG eutectic composite with respect to laser scanning rates

4 Conclusions

1) Laser remelted Al₂O₃/YAG eutectic in situ composites exhibit a “Chinese script” microstructure with an interpenetrating network of Al₂O₃ (44%) and YAG (56%) interweaved. The microstructures vary from laser processing parameters, and the domain sizes decrease with increasing the laser scanning rate. The average eutectic spacing is only 1 mm.

2) Synthetically thermal analysis indicates that the eutectic temperature of Al₂O₃/YAG is 1 824 ℃, well matching the phase diagram of Al₂O₃-Y₂O₃ system.

3) The Al₂O₃/YAG eutectic belongs to a typical irregular one. Both of the Al₂O₃ and YAG grow in a faceted growth behavior.

4) The maximum hardness and the room temperature fracture toughness of the composite measured by the micro-indentation test are 19.5 GPa and 3.6 MPa?m^1/2, respectively. The hardness increases with increasing the scanning rate and the fracture toughness decreases when the scanning rate is high. The crack branching, deflection, bending and arrest are found in the as-solidified eutectic, which greatly contributes to the improved toughness.

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(Edited by YANG Bing)

Foundation item: Project(50102004) supported by the National Natural Science Foundation of China; Project(04G53048) supported by the Aeronautical Science Foundation of China; Project(20040699035) supported by the Specialized Research Fund for the Doctoral Program of Higher Education of China; Project(W018101) supported by the Research Fund of Northwestern Polytechnical University, China; Project supported by the Developing Program for Outstanding Persons in Northwestern Polytechnical University, China

Corresponding author: SU Hai-jun; Tel: +86-13772141015; E-mail: shjnpu@yahoo.com.cn