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Directional Solidification of TiAl-Based Alloys: Determination of the Primary Phase in Ti-50Al-5Nb Alloy
Rare Metal Materials and Engineering Volume 40, Issue 1, January 2011 Online English edition of the Chinese language journal ARTICLE
Cite this article as: Rare Metal Materials and Engineering, 2011, 40(1): 0001−0003.
Directional Solidification of TiAl-Based Alloys: Determination of the Primary Phase in Ti-50Al-5Nb Alloy Yang Huimin,
Su Yanqing,
Luo Liangshun,
Chen Hui,
Guo Jingjie,
Fu Hengzhi
National Key Lab. for Hot Precision Processing of Metals, Harbin Institute of Technology, Harbin 150001, China
Abstract: In order to obtain excellent comprehensive mechanical properties, it is important to control the lamellar orientation of TiAl-based alloys by directional solidification technology. The control of the lamellar orientation is related closely with the primary phase during the directional solidification of TiAl-based alloys. The identification of the primary phase in directionally solidified TiAl-based alloys is insufficient only according to an isolated two-dimensional micrograph of the lamellar orientation. In this paper, we identified accurately the primary phase according to the quasi three-dimensional micrograph of the lamellar microstructure. The results indicate that the primary phase with various lamellar orientations is β phase during the directional solidification of Ti-50Al-5Nb alloy. Key words: Ti-50Al-5Nb alloy; directional solidification; primary phase
TiAl-based alloys with fully lamellar microstructure are the most potential high temperature structural materials due to their high specific strength, specific modulus, light mass and good high temperature properties[1-3]. Unfortunately, the as-cast TiAl-based alloys have poor room-temperature ductility and fracture toughness property[4-6].The alloying method and microstructure control were usually used to optimize the mechanical properties of TiAl-based alloys. It has been found that Nb addition can improve the room- and elevated-temperature mechanical properties[7,8]. On the other hand, the properties of TiAl-based alloys for given compositions are predominantly affected by the microstructures. The mechanical properties of TiAl-based alloys with the structure of polysynthetically twinned (PST) crystal are of remarkable anisotropism[9-11]. The different primary phases influence the final lamellar orientations and thus affect the mechanical properties of TiAl-based alloys. When the β-Ti phase is the primary phase, the lamellar microstructure with the lamellar orientation is aligned parallelly (A direction) or inclined at an angle of 45° (B direction) to the growth direction. Whereas, when the α-Ti phase is the primary phase, the lamellar orientation is perpendicular to the growth direction (C direction). For TiAl-based alloys, good compre-
hensive mechanical properties can be obtained by directional solidification (DS) with the lamellar orientation aligned parallelly or inclined at an angle of 45° to the growth direction[12,13]. Therefore, in order to obtain good comprehensive mechanical properties, we expect the full transformation of the β phase from the liquid during the solidification of TiAl-based alloys. In general, the primary phase is controlled by the composition and the solidification condition during directional solidification. For TiAl-based alloys, the primary phase is usually identified according to the two-dimensional metallographical micrograph of the final lamellar orientation[14-16]. In this paper we presented an idea that the identification of the primary phase in directionally solidified TiAl-based alloys is insufficient only according to the two-dimensional metallographical micrograph. Moreover we investigated the directionally solidified microstructure of Ti-50Al-5Nb alloy and identified the primary phase according to quasi three-dimensional metallographical observation.
1 Experimental The alloy with chemical composition of Ti-50Al-5Nb (atomic fraction) was used in the DS processing. The master ingot of the alloy was prepared from 99.9% pure Ti, 99.9% pure Al and
Received date: March 31, 2010 Foundation item: Supported by the National Science Foundation of China (50901025, 50771041, 50975060), State Key Lab of Advanced Metals Materials (2009ZD-06) and Program of Excellent Team in Harbin Institute of Technology Corresponding author: Yang Huimin, Ph. D., School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, P. R. China, Tel: 0086-451-86418415, E-mail: [email protected] Copyright © 2011, Northwest Institute for Nonferrous Metal Research. Published by Elsevier BV. All rights reserved.
1
Yang Huimin et al. / Rare Metal Materials and Engineering, 2011, 40(1): 0001-0003
29Al-71Nb alloy (mass fraction, %). The sample of 3 mm in diameter was placed in an alumina crucible of 4 mm in inner diameter. The DS experiment was carried out in a Bridgman type furnace[17] with two heating-zones. After 3-4 h heating and 10 min temperature stabilization, the sample was pulled at selected velocity. At the end of the experiment, the crucible was dropped into Ga-In-Sn liquid alloy to quench the solid/liquid interface. The temperature gradient was measured to be approximately 11.4 K/mm. The DS sample was sectioned longitudinally by electro-discharge machining. The phases and microstructures were observed by Olympus-GX71 optical microscopy (OM). The sample for the observation of OM was mechanically polished and then etched using a solution of HF: HNO3: H2O=1:1:8 (volume ratio).
2
Results and Discussion
Experimental research shows that the lamellar microstructure with C direction is not always formed in Ti-50Al-5Nb alloy. The lamellar microstructure with C direction can be obtained only at the growth rate of 5 μm/s. Fig.1 shows the longitudinal sections of the directionally solidified Ti-50Al-5Nb alloy at V=5 μm/s. It is found that the lamellar orientation is inclined at angles of 0° and 90° to the growth direction. The growth of the grains with lamellar orientation aligned parallelly to the growth direction shows a greater advantage (Fig.1c). The grains with the lamellar orientation aligned perpendicularly to the growth direction nucleate from the liquid and grow after the sample is pulled for 17 mm (Fig.1b). For the simultaneous growth of the lamellar orientation inclined at angle of 0°and 90° to the growth direction, the growth distance is about 11 mm. During the DS processing of TiAl-based alloys, the final lamellar orientation is related closely with the primary phase which is the intrinsic factor of the final lamellar orientation formation. If we identify the primary phase only according to the two-dimensional metallographical micrograph of the lamellar orientation, the primary phase with A direction is β phase, and the phase with C direction is α phase. Whereas, the lamellar orientation is affected by many factors in addition to the primary phase during the DS process of TiAl-based alloys. However, the determination of the primary phase is very important for better controlling the lamellar orientation of Ti-50Al-5Nb alloy. We need to identify the primary phase during the DS processing of Ti-50Al-5Nb alloy at the growth rate of 5 μm/s. In TiAl-based alloys, the lamellar orientations are influenced by a combination of the liquid solidification and the solid-state phase transformation. However, during the DS processing by modifying solidification procedure of TiAl-based alloys, the primary phase can be probably identified according to the relationship between the final lamellar orientation and the growth directions[14]. If the β phase, as the primary phase, nucleates and grows from the liquid during the DS processing, the final lamellar orientation is inclined at angles of 0°or 45° to the growth
a
b
c
500 µm
500 µm 200 mm
Fig.1 Longitudinal sections of the directionally solidified Ti-50Al5Nb alloys at V=5 μm/s: (a) the macrostructure and (b,c) the parts of the microstructure
Direction; whereas, if the α phase is the primary phase, the lamellar orientation is inclined at an angle of 90° to the growth direction. Thereby, it can be easily identified that the primary phase with A and B direction is the β phase, and the phase with C direction is the α phase according to the conventional two-dimensional metallographical micrograph of the lamellar microstructure. But, we need to identify further the phase containing the lamellar microstructure with C direction because the different section planes made for the observation of OM remarkably have an influence on the lamellar orientation. As shown in Fig.2, when the section plane is sectioned along IJKL plane, the lamellar orientation is inclined at an angle of 54.7° to the growth direction in Fig.2b. When the section plane is sectioned along MNOP plane, the lamellar orientation is inclined at an angle of 75.5° to the growth direction in Fig.2c. Whereas, the lamellar orientation is inclined at an angle of 90° to the growth direction along BCGF plane (Fig.2a). For the phase containing the lamellar microstructure with C direction, it can precipitate from both the β phase and α phase. Therefore, the identification of the primary phase in the directionally solidified TiAl-based alloys is insufficient only according to the two-dimensional metallographical micrograph of the lamellar orientation. The phase with various lamellar orientations can be accurately identified according to the quasi three-dimensional metallographical method. Fig.3 shows the three-dimensional optical micrograph of Ti-50Al-5Nb alloy at the growth rate of 5 μm/s. QRWV section is the part of Fig.1b which shows the grains with A and C directions. RSTW section is the longitudinal section which is perpendicular to QRWV section. For the grain with C direction, QRWV section shows the grain with the lamellar orientation inclined at an angle of 90° to the growth direction; whereas, RSTW section shows the grain with the lamellar orientation inclined at an angle which is not perpendicular to the growth direction. Consequently, according to Fig.2 and Fig.3, we can easily identify that the primary phase with C direction is not the α phase because the lamellar orientation in the transverse section is not perpendicular to the growth direction for the grain with C direction in the longitudinal
2
Yang Huimin et al. / Rare Metal Materials and Engineering, 2011, 40(1): 0001-0003
b
c
Growth Direction
a
1) The primary phase with various lamellar orientations is β phase at the growth rate of 5 μm/s. 2) The identification of the primary phase in the directionally solidified TiAl-based alloys is insufficient only according to the two-dimensional metallographical micrograph of the lamellar orientation because the lamellar orientation is influenced by the section plane made for observation. 3) The primary phase with the lamellar orientation inclined at an angle of 90° to the growth direction is the β phase.
References Fig.2 Schematics of the various lamellar orientations observed from
1 Malinov, S.; Sha, W. Mater. Sci. Eng. A., 2004, 365: 202
different section planes when the β phase is the primary phase:
2 Lu, W.; Chen, C. L.; Xi, Y. J. Intermetallics, 2007, 15: 989
(a) the theoretical lamellar orientation; (b) the lamellar orien-
3 Weaver, M. L.; Calhoun, C. W.; Garmestani, H. J. Mater. Sci.,
tation along the IJKL section plane; (c) the lamellar orientation along the CGNM section plane which inclined at an angle of 60° to ACGE plane
2002, 37: 2483 4 Liu, C. T.; Wright, J. L.; Deevi, S. C. Mater. Sci. Eng. A., 2002, (329-331): 416 5 Jin, Y. G.; Wang, J. N.; Yang, J. Scripta Mater, 2004, 51: 113 6 Hénaff, G.; Gloanec, A. L. Intermetallics, 2005, 13: 543 7 Liu, Z. C.; Lin, J. P.; Li, S. J. Intermetallics, 2002, 10: 653 8 Lu, W.; Chen, C. L.; Xi, Y. J. Intermetallics, 2007, 15: 989 9 Yokoshima, S.; Yamaguchi, M. Acta Mater, 1996, 44: 873 10 Heatherly, L.; Pgeorge, E.; Liu, C. T. Intermetallics, 1997, 5: 281 11 Kishida, K.; Inui, H.; Yamaguchi, M. Intermetallics, 1999, 7: 1131 12 Inui, H.; Oh, M. H.; Nakamura, A. Acta Metall Mater, 1992, 40: 3095
Fig.3 Three-dimensional optical micrograph of Ti-50Al-5Nb alloy at the growth rate of 5 μm/s
13 Yamanaka, T.; Johnson, D. R.; Inui, H. Intermetallics, 1999, 7: 779 14 Kim, M. C.; Oh, M. H.; Lee, J. H. Mater. Sci. Eng. A., 1997,
section. Therefore, the phase with C direction is the β phase. For the DS using the method of modifying solidification procedure of TiAl-based alloys in processing, when the α phase is the primary phase, the final lamellar orientation is only perpendicular to the growth direction according to the quasi three-dimensional metallographical observation.
3 Conclusions
(239-240): 570 15 Luo, W. Z.; Shen, J.; Min, Z. X. Materials Letters, 2009, 63: 1419 16 Liu, Y. C.; Yang, G. C.; Guo, X. F. J. Crystal Growth, 2001, 222: 645 17 Luo, L. S.; Su, Y. Q.; Guo, J. J. J Alloys and Compounds, 2008, 461: 121
3
Cite this article as: Rare Metal Materials and Engineering, 2011, 40(1): 0001−0003.
Directional Solidification of TiAl-Based Alloys: Determination of the Primary Phase in Ti-50Al-5Nb Alloy Yang Huimin,
Su Yanqing,
Luo Liangshun,
Chen Hui,
Guo Jingjie,
Fu Hengzhi
National Key Lab. for Hot Precision Processing of Metals, Harbin Institute of Technology, Harbin 150001, China
Abstract: In order to obtain excellent comprehensive mechanical properties, it is important to control the lamellar orientation of TiAl-based alloys by directional solidification technology. The control of the lamellar orientation is related closely with the primary phase during the directional solidification of TiAl-based alloys. The identification of the primary phase in directionally solidified TiAl-based alloys is insufficient only according to an isolated two-dimensional micrograph of the lamellar orientation. In this paper, we identified accurately the primary phase according to the quasi three-dimensional micrograph of the lamellar microstructure. The results indicate that the primary phase with various lamellar orientations is β phase during the directional solidification of Ti-50Al-5Nb alloy. Key words: Ti-50Al-5Nb alloy; directional solidification; primary phase
TiAl-based alloys with fully lamellar microstructure are the most potential high temperature structural materials due to their high specific strength, specific modulus, light mass and good high temperature properties[1-3]. Unfortunately, the as-cast TiAl-based alloys have poor room-temperature ductility and fracture toughness property[4-6].The alloying method and microstructure control were usually used to optimize the mechanical properties of TiAl-based alloys. It has been found that Nb addition can improve the room- and elevated-temperature mechanical properties[7,8]. On the other hand, the properties of TiAl-based alloys for given compositions are predominantly affected by the microstructures. The mechanical properties of TiAl-based alloys with the structure of polysynthetically twinned (PST) crystal are of remarkable anisotropism[9-11]. The different primary phases influence the final lamellar orientations and thus affect the mechanical properties of TiAl-based alloys. When the β-Ti phase is the primary phase, the lamellar microstructure with the lamellar orientation is aligned parallelly (A direction) or inclined at an angle of 45° (B direction) to the growth direction. Whereas, when the α-Ti phase is the primary phase, the lamellar orientation is perpendicular to the growth direction (C direction). For TiAl-based alloys, good compre-
hensive mechanical properties can be obtained by directional solidification (DS) with the lamellar orientation aligned parallelly or inclined at an angle of 45° to the growth direction[12,13]. Therefore, in order to obtain good comprehensive mechanical properties, we expect the full transformation of the β phase from the liquid during the solidification of TiAl-based alloys. In general, the primary phase is controlled by the composition and the solidification condition during directional solidification. For TiAl-based alloys, the primary phase is usually identified according to the two-dimensional metallographical micrograph of the final lamellar orientation[14-16]. In this paper we presented an idea that the identification of the primary phase in directionally solidified TiAl-based alloys is insufficient only according to the two-dimensional metallographical micrograph. Moreover we investigated the directionally solidified microstructure of Ti-50Al-5Nb alloy and identified the primary phase according to quasi three-dimensional metallographical observation.
1 Experimental The alloy with chemical composition of Ti-50Al-5Nb (atomic fraction) was used in the DS processing. The master ingot of the alloy was prepared from 99.9% pure Ti, 99.9% pure Al and
Received date: March 31, 2010 Foundation item: Supported by the National Science Foundation of China (50901025, 50771041, 50975060), State Key Lab of Advanced Metals Materials (2009ZD-06) and Program of Excellent Team in Harbin Institute of Technology Corresponding author: Yang Huimin, Ph. D., School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, P. R. China, Tel: 0086-451-86418415, E-mail: [email protected] Copyright © 2011, Northwest Institute for Nonferrous Metal Research. Published by Elsevier BV. All rights reserved.
1
Yang Huimin et al. / Rare Metal Materials and Engineering, 2011, 40(1): 0001-0003
29Al-71Nb alloy (mass fraction, %). The sample of 3 mm in diameter was placed in an alumina crucible of 4 mm in inner diameter. The DS experiment was carried out in a Bridgman type furnace[17] with two heating-zones. After 3-4 h heating and 10 min temperature stabilization, the sample was pulled at selected velocity. At the end of the experiment, the crucible was dropped into Ga-In-Sn liquid alloy to quench the solid/liquid interface. The temperature gradient was measured to be approximately 11.4 K/mm. The DS sample was sectioned longitudinally by electro-discharge machining. The phases and microstructures were observed by Olympus-GX71 optical microscopy (OM). The sample for the observation of OM was mechanically polished and then etched using a solution of HF: HNO3: H2O=1:1:8 (volume ratio).
2
Results and Discussion
Experimental research shows that the lamellar microstructure with C direction is not always formed in Ti-50Al-5Nb alloy. The lamellar microstructure with C direction can be obtained only at the growth rate of 5 μm/s. Fig.1 shows the longitudinal sections of the directionally solidified Ti-50Al-5Nb alloy at V=5 μm/s. It is found that the lamellar orientation is inclined at angles of 0° and 90° to the growth direction. The growth of the grains with lamellar orientation aligned parallelly to the growth direction shows a greater advantage (Fig.1c). The grains with the lamellar orientation aligned perpendicularly to the growth direction nucleate from the liquid and grow after the sample is pulled for 17 mm (Fig.1b). For the simultaneous growth of the lamellar orientation inclined at angle of 0°and 90° to the growth direction, the growth distance is about 11 mm. During the DS processing of TiAl-based alloys, the final lamellar orientation is related closely with the primary phase which is the intrinsic factor of the final lamellar orientation formation. If we identify the primary phase only according to the two-dimensional metallographical micrograph of the lamellar orientation, the primary phase with A direction is β phase, and the phase with C direction is α phase. Whereas, the lamellar orientation is affected by many factors in addition to the primary phase during the DS process of TiAl-based alloys. However, the determination of the primary phase is very important for better controlling the lamellar orientation of Ti-50Al-5Nb alloy. We need to identify the primary phase during the DS processing of Ti-50Al-5Nb alloy at the growth rate of 5 μm/s. In TiAl-based alloys, the lamellar orientations are influenced by a combination of the liquid solidification and the solid-state phase transformation. However, during the DS processing by modifying solidification procedure of TiAl-based alloys, the primary phase can be probably identified according to the relationship between the final lamellar orientation and the growth directions[14]. If the β phase, as the primary phase, nucleates and grows from the liquid during the DS processing, the final lamellar orientation is inclined at angles of 0°or 45° to the growth
a
b
c
500 µm
500 µm 200 mm
Fig.1 Longitudinal sections of the directionally solidified Ti-50Al5Nb alloys at V=5 μm/s: (a) the macrostructure and (b,c) the parts of the microstructure
Direction; whereas, if the α phase is the primary phase, the lamellar orientation is inclined at an angle of 90° to the growth direction. Thereby, it can be easily identified that the primary phase with A and B direction is the β phase, and the phase with C direction is the α phase according to the conventional two-dimensional metallographical micrograph of the lamellar microstructure. But, we need to identify further the phase containing the lamellar microstructure with C direction because the different section planes made for the observation of OM remarkably have an influence on the lamellar orientation. As shown in Fig.2, when the section plane is sectioned along IJKL plane, the lamellar orientation is inclined at an angle of 54.7° to the growth direction in Fig.2b. When the section plane is sectioned along MNOP plane, the lamellar orientation is inclined at an angle of 75.5° to the growth direction in Fig.2c. Whereas, the lamellar orientation is inclined at an angle of 90° to the growth direction along BCGF plane (Fig.2a). For the phase containing the lamellar microstructure with C direction, it can precipitate from both the β phase and α phase. Therefore, the identification of the primary phase in the directionally solidified TiAl-based alloys is insufficient only according to the two-dimensional metallographical micrograph of the lamellar orientation. The phase with various lamellar orientations can be accurately identified according to the quasi three-dimensional metallographical method. Fig.3 shows the three-dimensional optical micrograph of Ti-50Al-5Nb alloy at the growth rate of 5 μm/s. QRWV section is the part of Fig.1b which shows the grains with A and C directions. RSTW section is the longitudinal section which is perpendicular to QRWV section. For the grain with C direction, QRWV section shows the grain with the lamellar orientation inclined at an angle of 90° to the growth direction; whereas, RSTW section shows the grain with the lamellar orientation inclined at an angle which is not perpendicular to the growth direction. Consequently, according to Fig.2 and Fig.3, we can easily identify that the primary phase with C direction is not the α phase because the lamellar orientation in the transverse section is not perpendicular to the growth direction for the grain with C direction in the longitudinal
2
Yang Huimin et al. / Rare Metal Materials and Engineering, 2011, 40(1): 0001-0003
b
c
Growth Direction
a
1) The primary phase with various lamellar orientations is β phase at the growth rate of 5 μm/s. 2) The identification of the primary phase in the directionally solidified TiAl-based alloys is insufficient only according to the two-dimensional metallographical micrograph of the lamellar orientation because the lamellar orientation is influenced by the section plane made for observation. 3) The primary phase with the lamellar orientation inclined at an angle of 90° to the growth direction is the β phase.
References Fig.2 Schematics of the various lamellar orientations observed from
1 Malinov, S.; Sha, W. Mater. Sci. Eng. A., 2004, 365: 202
different section planes when the β phase is the primary phase:
2 Lu, W.; Chen, C. L.; Xi, Y. J. Intermetallics, 2007, 15: 989
(a) the theoretical lamellar orientation; (b) the lamellar orien-
3 Weaver, M. L.; Calhoun, C. W.; Garmestani, H. J. Mater. Sci.,
tation along the IJKL section plane; (c) the lamellar orientation along the CGNM section plane which inclined at an angle of 60° to ACGE plane
2002, 37: 2483 4 Liu, C. T.; Wright, J. L.; Deevi, S. C. Mater. Sci. Eng. A., 2002, (329-331): 416 5 Jin, Y. G.; Wang, J. N.; Yang, J. Scripta Mater, 2004, 51: 113 6 Hénaff, G.; Gloanec, A. L. Intermetallics, 2005, 13: 543 7 Liu, Z. C.; Lin, J. P.; Li, S. J. Intermetallics, 2002, 10: 653 8 Lu, W.; Chen, C. L.; Xi, Y. J. Intermetallics, 2007, 15: 989 9 Yokoshima, S.; Yamaguchi, M. Acta Mater, 1996, 44: 873 10 Heatherly, L.; Pgeorge, E.; Liu, C. T. Intermetallics, 1997, 5: 281 11 Kishida, K.; Inui, H.; Yamaguchi, M. Intermetallics, 1999, 7: 1131 12 Inui, H.; Oh, M. H.; Nakamura, A. Acta Metall Mater, 1992, 40: 3095
Fig.3 Three-dimensional optical micrograph of Ti-50Al-5Nb alloy at the growth rate of 5 μm/s
13 Yamanaka, T.; Johnson, D. R.; Inui, H. Intermetallics, 1999, 7: 779 14 Kim, M. C.; Oh, M. H.; Lee, J. H. Mater. Sci. Eng. A., 1997,
section. Therefore, the phase with C direction is the β phase. For the DS using the method of modifying solidification procedure of TiAl-based alloys in processing, when the α phase is the primary phase, the final lamellar orientation is only perpendicular to the growth direction according to the quasi three-dimensional metallographical observation.
3 Conclusions
(239-240): 570 15 Luo, W. Z.; Shen, J.; Min, Z. X. Materials Letters, 2009, 63: 1419 16 Liu, Y. C.; Yang, G. C.; Guo, X. F. J. Crystal Growth, 2001, 222: 645 17 Luo, L. S.; Su, Y. Q.; Guo, J. J. J Alloys and Compounds, 2008, 461: 121
3