Texture Control and High-Temperature Strength of Directionally Solidified Al 2 O 3 /YAG/ZrO 2 Eutectic Composite Rods

Transcription

1 Materials Transactions, Vol. 45, No. 8 (2004) pp to 2702 #2004 The Japan Institute of Metals Texture Control and High-Temperature Strength of Directionally Solidified Al 2 O 3 /YAG/ZrO 2 Eutectic Composite Rods Yonosuke Murayama 1, Shuji Hanada 2, Jong Ho Lee 3, Akira Yoshikawa 4 and Tsuguo Fukuda 4 1 Niigata Institute of Technology, Kashiwazaki , Japan 2 Institute for Materials Research, Tohoku University, Sendai , Japan 3 Korea Institute of Industrial Technology, Daechondong 958-3, Bukku, Gwang Ju, , Korea 4 Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai , Japan A 2 O 3 /YAG/ZrO 2 eutectic Melt-Growth-Composite (MGC) rods with two different microstructures were prepared by unidirectional solidification using the modified-pulling-down method (MPD) under different processing parameters. Microstructure and crystallographic texture were investigated by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM) and electron backscattered pattern (EBSP) method. High-temperature strength was evaluated by compression tests at 1773 K and 1873 K. Geometric pattern structure and Chinese script pattern structure are evolved by controlling processing parameters. MPD rods have strong preferred growing orientations in Al 2 O 3 of h001i for the geometric pattern structure and of h300i for Chinese script pattern structure. Constituent phases in the MPD rod hold the orientation relationship. The yield stress for the geometric pattern structure is over 1 GPa at 1773 K, which is extremely higher than that for Chinese script pattern structure. High-temperature strength at 1773 K and 1873 K depends on strain rate and temperature in both the MPD rods. (Received April 14, 2004; Accepted June 8, 2004) Keywords: Al 2 O 3 /YAG/ZrO 2 eutectic composite, melt-growth-composite (MGC), crystallographic texture, electron backscattered pattern (EBSP), high-temperature strength, strength anisotropy 1. Introduction Recently, the global problems such as an exhaustion of natural resources and an environmental issue are attracting a great deal of attention. The main source of CO 2 emission in Japan is electric power generation systems. Therefore, one indispensable technology to solve the problem is the development of high-temperature structural material to improve thermal efficiency in gas turbines. 1) High-temperature structural material, whose operating temperature exceeds 1800 K, is required especially in an electric power generation system. 1) Melt-Growth-Composites (MGC) consisting of oxide eutectics, which are directionally solidified by Bridgman method, are candidate materials to meet the above requirements because of their high strength, excellent oxidation resistance and microstructural stability up to near their melting temperatures. 2 9) Especially, it has been reported that the eutectic MGC, which has the characteristic complex microstructure 2 11) consisting of interpenetrating single crystal oxides, 3 7) shows excellent high-temperature performance. Al 2 O 3 /YAG/ZrO 2 eutectic MGC rod, which was unidirectionally solidified by the modified-pulling-down method (MPD) showed the characteristic complex microstructure similar to the eutectic MGC grown by Bridgman method, which is called Chinese script pattern structure. The growth direction of Al 2 O 3 in the MPD rod is h300i, 11) which is the same with Al 2 O 3 /YAG or Al 2 O 3 /YAG/ZrO 2 eutectic MGC grown by Bridgman type method. 3 7) However, the MPD rod is anticipated to show finer microstructure and higher strength than the MGC because of a higher solidification rate and a larger temperature gradient in the MPD. It has been revealed that the high-temperature strength of an Al 2 O 3 single crystal is the highest in h001i direction, 12,13) and the strength of Al 2 O 3 -based eutectic MGC is dominated by orientation of the constituent Al 2 O 3 phase. These results suggest that an MPD rod with the Al 2 O 3 growing direction of h001i exhibits the highest strength. Recently, we succeeded in growing MPD rods with Al 2 O 3 growing directions of h001i and h300i by controlling processing parameters in crystal growth. So far the strength in h001i direction of constituent Al 2 O 3 phase in the MPD rod with Chinese script pattern structure has not been measured, because the MPD rod consists of eutectic grains with different growth orientation. In this paper microstructure, texture and high-temperature strength are investigated using the MPD rods with the two kinds of growing directions in Al 2 O Experimental Procedure An Al 2 O 3 /YAG/ZrO 2 eutectic composite in the shape of a round rod was grown by the MPD method. The starting materials were high-purity commercial powders of -Al 2 O 3, Y 2 O 3 and ZrO 2. The mixture of Al 2 O 3,Y 2 O 3 and ZrO 2 powders in the mole ratio of Al 2 O 3 :Y 2 O 3 :ZrO 2 = 65:16:19 for the eutectic composition as based on the Lakiza s work 14) was melted in an iridium crucible using a 30 kw radio frequency. The crucible for rod-shaped crystal growth has a circular basal plane whose diameter decides the diameter of the rod. A detailed explanation of MPD is mentioned elswhere. 15) The different processing parameters for the MPD rods are summarized in Table 1. The microstructure was observed by a field emission scanning electron microscope (FE-SEM; FILIPS XL-30) equipped with an electron backscattered pattern (EBSP) apparatus (TexSEM Laboratory). Samples for microstructural observation were polished using a vibratory polisher (BUEHLER VIBROMET 2) with a solution containing alumina powder of 0.05 mm. Sample surface was coated with carbon by vacuum evaporation to reduce charging in SEM observation. Since the quality of EBSPs was very sensitive to

2 2698 Y. Murayama, S. Hanada, J. H. Lee, A. Yoshikawa and T. Fukuda surface preparation, samples were ion milled (GATAN 600N). The crystallographic texture was examined by X- ray diffraction and EBSP analysis. To evaluate the high-temperature strength, compression tests were carried out using rectangular samples with the dimension of mm 3. The compression tests were conducted in an argon atmosphere at a constant cross head speed at 1773 K and 1873 K, using an Instron testing machine (Instron 8562). A strain rate was changed during compression testing between s 1 and s Results Table 1 Processing parameter. Sample name Geometric pattern Chinese script pattern Diameter of basal plane of crucible 3mm 5mm Seed Al 2 O 3 Al 2 O 3 /YAG/ZrO 2 Orientation of Al 2 O 3 in seed h001i h300i Pulling rate 60 mm/h 30 mm/h Length of after heater 30 mm 20 mm 3.1 Microstructure Depending on the processing parameters in Table 1, two different types of microstructures were observed on the whole section perpendicular to the growth direction of an Al 2 O 3 /YAG/ZrO 2 eutectic MGC grown by the MPD method. One typical example is shown in Figs. 1(a)(c)(e) and the other is in Figs. 1(b)(d)(f). The former and the latter will be called hereafter the geometric pattern and Chinese script pattern structures. In the micrograph of MPD rod with the geometric pattern structure, cell boundaries, where interlamellar spacing is irregular, are observed at a low magnification (Fig. 1(a)). In addition, rectangular maze-like eutectic structure of Al 2 O 3 and ZrO 2 is evolved at a high magnification (Fig. 1(c)). Both the phases are well elongated along the growth direction, as seen in Fig. 1(e). SEM observation of a MPD rod with Chinese script pattern structure indicated that the cross-section was composed of several eutectic grains with different shading contrasts, as shown previously. 11) Each eutectic grain consists of Al 2 O 3, YAG and ZrO 2, as seen in Figs. 1(b), (d) and (f). Although the constituent phases have a tendency to be elongated in the growth direction as shown in Fig. 1(f), their growth directions are considerably scattered, compared to those in the geometric pattern structure in Fig. 1(e). 3.2 Crystallographic texture The crystallographic texture at a cross section perpendicular to the growth direction was evaluated by X-ray diffraction and EBSP analysis. The X-ray diffraction pattern on a cross-section perpendicular to the growth direction is shown in Fig. 2. There is a distinct difference between the geometric pattern and Chinese script pattern structures concerning the presence of peaks of Al 2 O 3 h300i. No apparent peak of Al 2 O 3 h300i can be seen in the geometric pattern structure, while Al 2 O 3 h006i peak is clearly observed as seen in an inserted profile. Waku et al. have reported that the preferred growth orientations of an Al 2 O 3 /YAG/ZrO 2 eutectic MGC with Chinese script pattern structure grown by the Bridgman method are h300i in Al 2 O 3, h100i in ZrO 2, and h100i in YAG from an X-ray diffraction pattern similar to Fig. 2. 6,7) Lee et al. indicated that Al 2 O 3 has the preferred growth direction of h300i, but ZrO 2 and YAG have no distinct preferred growth direction in the Al 2 O 3 /YAG/ZrO 2 eutectic MGC grown by the modified-pulling-down method. 9,15) The pole figures of the constituent phases at a central area of the MPD rods, which are obtained by EBSP analysis at a cross section perpendicular to the growth direction, are shown in Fig. 3. The pole figures indicate clearly that the preferred growing orientations are h001i for Al 2 O 3 and h220i for ZrO 2 in the geometric pattern structure and h300i for Al 2 O 3 and h100i for ZrO 2 in Chinese script pattern structure. No definite orientation relation between Al 2 O 3 and ZrO 2 is observed in the MPD rods with the geometric and Chinese script pattern structures, but there seems to be =4 rotational relation around h110i of Al 2 O 3 or h100i of ZrO 2 in both the structures. The {800} pole figure of YAG shows that the h100i orientation deviates from the growth direction in the geometric and Chinese script pattern structures, although h400i and h800i peaks are observed in X-ray diffraction profiles in Fig. 2. The analyzed orientations of YAG seem to scatter probably because of less reliability of EBSP analysis arising from insufficient sample surface preparation as well as intrinsic scattering in orientation. Some accumulated peaks of the growth direction are shown in the inverse pole figures in Fig. 4. The inverse pole figure of Al 2 O 3 in Chinese script pattern structure indicates clearly that the preferred growth directions are near h100i and h010i. The formation of dominant h100i and h010i directions can be explained by twinning. 10,11,13) These twins may form to minimize compatibility strain at the interface between the two variants during the growing process. ZrO 2 indicates preferred direction of h001i. On the other hand, in the geometric pattern structure Al 2 O 3 has no preferred growing direction, but ZrO 2 has weak preferred direction deviated from h101i. YAG in both the structures seems to show some weak accumulated peaks in the growth direction. 3.3 High-temperature deformation behavior The strength of samples compressed to the growth direction at 1773 K and 1873 K is plotted in Fig. 5 in the form of the true stress-true strain curves. The peak stress of MPD rod with the geometric pattern structure is extremely higher than that with Chinese script pattern structure. The peak stress exceeds 1 GPa at 1773 K. Deformation softening can be seen after the peak stress in both MPD rods, especially in the geometric pattern structure. The strength of both MPD rods shows a strong temperature dependence. Furthermore, the strain rate change test in the growth direction indicates that the flow stress of both MPD rods changes remarkably depending on strain rate at 1773 K and 1873 K. This suggests that deformation is controlled by a thermally activated deformation process.

3 Texture Control and High-Temperature Strength of Directionally Solidified Al 2 O 3 /YAG/ZrO 2 Eutectic Composite Rods 2699 Fig. 1 FE-SEM micrographs of MPD rods with Geometric pattern structure, (a), (c) and (e) and Chinese script pattern structure, (b), (d) and (f). (a) (d) are cross-sections perpendicular to the growth direction. (e) and (f) are cross-sections parallel to the growth direction. 4. Discussion Fig. 2 X-ray diffraction pattern at a cross-section perpendicular to the growth direction, using a molybdenum X-ray tube. 4.1 Microstructure and texture Al 2 O 3 /YAG/ZrO 2 eutectic MGC consists of single crystal-like Al 2 O 3, YAG and ZrO 2. Mechanical properties of the MGC at elevated temperatures would be controlled by crystallographic orientation of Al 2 O 3, since Al 2 O 3 with hexagonal crystal structure exhibits significant orientation dependence of strength, 16 18) while YAG and ZrO 2 with cubic crystal structure shows almost no anisotropy in strength ) Therefore, in addition to microstructures the crystallographic texture of the constituent phases should be considered to understand the strength of the MGC rods. The preferred growing direction of Al 2 O 3 in Al 2 O 3 /YAG/ ZrO 2 eutectic MGC has been reported to be h300i in the MPD rod with Chinese script pattern structure. Though the different processing parameter in Table 1 is found to produce either geometric pattern structure or Chinese script pattern structure, it is not clear at present which parameter determines the preferred growing orientation. A preferred growing orientation other than h300i has never been reported

4 2700 Y. Murayama, S. Hanada, J. H. Lee, A. Yoshikawa and T. Fukuda Fig. 3 Pole figures at cross-section perpendicular to the growth direction of MPD rod with Geometric pattern structure and Chinese script pattern structure. Fig. 4 Inverse pole figures of growth direction obtained from EBSP analysis. for Al 2 O 3 in eutectic MGC grown by Bridgman type method. This difference will be associated with a seeding process. Bridgman type method not using a seed crystal needs a lower solidifying rate and a smaller temperature gradient for the crystal growth, compared with the MPD method. Al 2 O 3 / YAG binary eutectic MGC grown by the MPD method showed h300i preferred growing orientation of Al 2 O 3, even though an Al 2 O 3 seed crystal with h001i was used. However, Al 2 O 3 /YAG/ZrO 2 ternary eutectic MGC grown by the MPD method showed occasionally mixed preferred orientations of h300i and h001i of Al 2 O 3 in a cross-section. Al 2 O 3 phase grown from a h001i seed crystal in Al 2 O 3 /YAG binary Fig. 5 Strain rate change test at 1773 K and 1873 K. eutectic MGC rotates to h300i orientation obeying the twin relation and forms the Chinese script pattern structure with other phases. The interphase orientation relation between Al 2 O 3 and ZrO 2 may give an important influence on the preferred growing orientation. The formation of fine eutectic

5 Texture Control and High-Temperature Strength of Directionally Solidified Al 2 O 3 /YAG/ZrO 2 Eutectic Composite Rods 2701 Fig. 7 Appearance of specimens after compression test at 1873 K. (a) MPD rod with geometric pattern structure. (b) MPD rod with Chinese script pattern structure. Fig. 6 Pole figures of Al 2 O 3 at same cross-section. Orientation of each phase in other eutectic grain rotates about growth direction. microstructure of Al 2 O 3 /ZrO 2 with interphase orientation of h001i Al2 O 3 kh110i ZrO2 may make the rotation of Al 2 O 3 impossible. Figure 6 shows pole figures by EBSP analysis at another cross section of the same MPD rod as in Fig. 3. By comparing the pole figures in Figs. 3 and 6, it is found that the crystallographic orientation distributions are different in the same rod. The MPD rod with Chinese script pattern structure has distinct eutectic grain boundaries. 11) Each eutectic grain seems to be a single crystal and has almost the same preferred growing direction. However, the crystallographic orientations of eutectic grains are changed each other by rotation of the crystallographic orientation around the growth direction. 11) The MPD rod with the geometric pattern structure has no distinct eutectic grain boundaries, but has cell boundaries. The adjacent cells and their cell boundaries showed a very small difference in crystallographic orientation, which was confirmed by the EBSP analysis around interface boundaries. There is a definite difference between distant cells as shown in Fig. 6. However, they have almost same preferred growing direction, i.e. h001i of Al 2 O 3 in the MPD rod with geometric pattern structure and h300i in the MPD rod with Chinese script pattern structure. 4.2 Texture and high-temperature strength The flow stress of MPD rod with geometric pattern structure decreases drastically after showing a peak stress as shown in Fig. 5. Figure 7 shows the compressed MPD rods with geometric and Chinese script pattern structures at 1873 K. The rod with Chinese script pattern structure deforms throughout the rectangular specimen, while the rod with the geometric pattern structure deforms severely only at sample edges. Moreover, in the geometric pattern structure brittle fracture occurs along the growth direction like bundled firewood. This may be caused by preferential crack propagation along weak cell boundaries. However, since the severe deformation at the sample edges and the firewood-like fracture take place at a strain after the peak stress, the 0.2% proof stress measured from the stress-strain curves may be controlled by the strength of the constituent single crystals. It has been pointed out that the strength anisotropy in the Al 2 O 3 /YAG eutectic MGC is associated with the orientation dependence of high-temperature strength of single crystal Al 2 O 3. 13) The high-temperature strength obtained in this experiment is compared with the strength of single crystals of Al 2 O 3, 16 18) YAG 19) and ZrO 20,21) 2 as shown in Fig. 8. It is considered that the single crystals YAG and ZrO 2 with cubic crystal structure show a very small anisotropy in hightemperature strength. On the other hand, Al 2 O 3 with hexagonal structure shows the strong anisotropy. Since the primary slip system of -Al 2 O 3 is (001)1/3h110i basal slip above 1000 K, 22) the strength of Al 2 O 3 depends on the angle between testing direction and c-axis. As seen in Fig. 8, the high temperature strength of Al 2 O 3 decreases remarkably by only 6 deviation of testing direction from c-axis of Al 2 O 3, though the strength at c-axis is comparable to that of YAG. There are no available data on the strength of cubic ZrO 2 at Fig. 8 Relationship between strain rate and compressive strength for Al 2 O 3 /YAG/ZrO 2 eutectic composite, single crystal Al 2 O 3, YAG and ZrO 2.

6 2702 Y. Murayama, S. Hanada, J. H. Lee, A. Yoshikawa and T. Fukuda and above 1773 K but the strength of cubic ZrO 2 at 1673 K is comparable to that of Al 2 O 3 deformed to a-axis. The compression directions in this experiment are close to c-axis of Al 2 O 3 in geometric pattern structure and to a-axis in Chinese script pattern structure. It has been reported that the flow stress of Al 2 O 3 /YAG binary eutectic MGC obeys the mixture rule. 13) The strength of MPD rod with geometric pattern structure is comparable to the strength of single crystal YAG and Al 2 O 3 deformed to c-axis. On the other hand, the strength of MPD rod with Chinese script pattern structure is on the average of the strength values of single crystals YAG and Al 2 O 3 deformed to a-axis. The microstructural morphology of MPD rod seems to consist of two laminates with even volume fraction, in which one laminate is eutectic layer of Al 2 O 3 and ZrO 2 and the other is YAG. Though the influence of ZrO 2 on strength is vague, the strength of MPD rod seems to obey the mixture rule of the constituents composed of oriented single crystals. 5. Conclusions Al 2 O 3 /YAG/ZrO 2 eutectic MGC rods with two different microstructures and preferred growth orientations were grown by the modified-pulling-down method and hightemperature strength was investigated. The main conclusions about texture and high-temperature strength of MPD rod with are as follows. (1) Two different microstructures, geometric and Chinese script pattern structures, are obtained by controlling processing conditions. (2) Preferred growth orientations of MPD rod are h300i in Al 2 O 3 and h100i in ZrO 2 for Chinese script pattern structure and h001i in Al 2 O 3 and h220i in ZrO 2 for geometric pattern structure. YAG has no distinct preferred growth orientation, though several peaks are observed in the inverse pole figure. (3) Compressive strength exceeding 1 GPa at 1773 K in the growth direction of MPD rod with geometric pattern structure is much higher than that with Chinese script pattern structure. (4) Flow stress of MPD rod with both geometric and Chinese script pattern structures shows strain rate and deformation temperature dependence in compression tests at 1773 K and 1873 K. (5) Strength of MPD rod is comparable to the strength of single crystals YAG and Al 2 O 3 deformed to c-axis for geometric pattern structure, while it is the average of the strength of single crystals YAG and Al 2 O 3 deformed to a-axis for Chinese script pattern structure. Acknowledgments This work was performed through Special Coordination Funds of the Ministry of Education, Culture, Sports, Science and Technology of the Japanese Government. REFERENCES 1) A. Nitta: J. JSME 99 (1996) ) T. A. Parthasarathy, T. Y. Mah and L. E. Matson: J. Am. Ceram. Soc. 76 (1993) ) Y. Waku, N. Nakagawa, H. Ohtsubo, Y. Ohsora and Y. Koutoku: J. Japan Inst. Metals 59 (1995) ) Y. Waku, H. Ohtsubo, N. Nakagawa and Y. Koutoku: J. Mater. Sci. 31 (1996) ) Y. Waku, N. Nakagawa, T. Wakamoto, H. Ohtsubo, K. Shimizu and Y. Kohtoku: Nature 389 (1997) ) Y. Waku, S. Sakata, A. Mitani and K. Shimizu: Mater. Res. Innov. 15 (2001) ) Y. Waku, S. Sakata, A. Mitani, K. Shimizu and M. Hasebe: J. Mater. Sci. 37 (2002) ) B. M. Epelbaum, A. Yoshikawa, K. Shimamura, T. Fukuda, K. Suzuki and Y. Waku: J. Crystal Growth 198/199 (1999) ) J. H. Lee, A. Yoshikawa, T. Fukuda and Y. Waku: J. Cryst. Growth 231 (2001) ) C. S. Frazer, E. C. Dickey and A. Sayir: J. Cryst. Growth 233 (2001) ) Y. Murayama, J. H. Lee, A. Yoshikawa, S. Hanada and T. Fukuda: Mater. Trans. 45 (2004) ) H. Yoshida, A. Nakamura, T. Sakuma, N. Nakagawa and Y. Waku: Scr. Mater. 45 (2001) ) Y. Murayama, S. Hanada and Y. Waku: Mater. Trans. 44 (2003) ) S. M. Lakiza and L. M. Lopato: J. Am. Cerami. Soc. 80 (1997) ) J. H. Lee: Doctoral Thesis, Tohoku University (2001). 16) D. M. Kotchick and R. E. Tressler: J. Am. Ceram. Soc. 63 (1980) ) G. S. Corman: Ceram. Eng. Sci. Proc. 12 (1991) ) D. J. Gooch and G. W. Groves: J. Mater. Sci. 8 (1973) ) G. S. Corman: J. Mater. Sci. 12 (1993) ) T. A. Parthasarathy and R. S. Hay: Acta Mater. 44 (1996) ) U. Messerschmidt, D. Baither, B. Baufeld and M. Bartsch: Mater. Sci. Eng. A233 (1997) ) K. P. D. Lagerlof, A. H. Heuer, J. Castaing, J. P. Riviere and T. E. Mitchell: J. Am. Ceram. Soc. 77 (1994)