- Email: [email protected]
Interface structure and formation mechanism of diffusion-bonded joints of SiC ceramic to TiAl-based alloy
Scripta mater. 43 (2000) 49 –53 www.elsevier.com/locate/scriptamat
INTERFACE STRUCTURE AND FORMATION MECHANISM OF DIFFUSION-BONDED JOINTS OF SiC CERAMIC TO TiAlBASED ALLOY H.J. Liu, J.C. Feng and Y.Y. Qian National Key Laboratory of Advanced Welding Production Technology; Harbin Institute of Technology; Harbin 150001, People’s Republic of China (Received October 4, 1999) (Accepted in revised form January 20, 2000) Keywords: Diffusion bonding; Interface; Structural ceramic; Intermetallic compound Introduction SiC ceramics are considered one of the most promising structural materials for high temperature applications. The development of bonding technology, especially diffusion bonding, is widening the application field of SiC ceramics. There have been some reports on diffusion bonding of SiC ceramics to refractory metals (1–5) and heat-resistant alloy (6). TiAl-based alloys have a great potential to become important candidates for advanced applications in aerospace and military industries. The researches on diffusion bonding of TiAl-based alloys to other different materials have progressed in recent years (7–9). The concept of utilizing ceramic, intermetallic and metallic materials to attain one complete armor system by the process of diffusion bonding is a recent approach for defeating armor projectiles (10).In order to achieve a functionally-graded armor system that has the necessary material and mechanical characteristics for ballistic protection, it is necessary to demonstrate the feasibility of diffusion bonding. Therefore, a previous study of diffusion bonding of SiC ceramic to pure TiAl was carried out (11). The present study focuses on the interface structure and formation mechanism of diffusion-bonded joints of SiC to TiAl-based alloy. Experimental Procedure The ceramic material used in experiments was pressureless-sintered ␣-SiC with a small amount of Al2O3 as sintering additive. The TiAl-based alloy was Ti-43Al-1.7Cr-1.7Nb (at %) cast alloy composed of lamellar (␥⫹␣2) duplex-phase microstructures, as shown in Fig. 1, and this alloy was named TAD in this study. The dimensions of cylindrical SiC samples were 6 mm in diameter and 4 mm in height, and the thickness of TAD foils was 0.2 mm. The surfaces to be bonded were ground and polished through diamond paste and cleaned in ethanol and acetone prior to bonding. The stacked SiC/TAD/SiC assemblies were diffusion bonded in a vacuum furnace (Centorr-3520) equipped with a hydraulic system. The bonding parameters are shown in Table 1. The diffusion-bonded SiC/TAD/SiC joints were cross-sectioned, perpendicular to the bonding interfaces, using a low-speed diamond saw, and the cross-sections of these joints were metallographically polished using 0.5-m diamond paste as final polish and cleaned in acetone. The polished 1359-6462/00/$–see front matter. © 2000 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved. PII: S1359-6462(00)00360-2
50
DIFFUSION-BONDED JOINTS
Vol. 43, No. 1
Figure 1. Microstructure of the TAD base material.
cross-sections and fracture surfaces of the joints were observed by scanning electron microscopy (SEM, S-570). The composition analysis of the reaction products were performed by electron probe X-ray microanalysis (EPMA, JXA-8600). The kinds and crystal structures of the reaction products were identified from the fracture surfaces of the joints by X-ray diffraction (XRD, JDX-3530M). For SEM and EPMA characterization, the cross-sections of each joint were deposited with carbon using a depositor. Results and Discussion The back-scattered electron image and element distribution of the SiC/TAD interface bonded at 1573 K for 240 min are shown in Fig. 2. It can be clearly seen from the figure that two kinds of reaction layers have occurred between SiC and TAD. For the sake of convenience, the reaction layer adjacent to SiC is called A layer, and the reaction layer, dotted with a small amount of grey phase, adjacent to TAD is called B layer. Obviously, A layer is a single-phase layer, and B layer is composed of duplex phases where the white phase is matrix. It’s also not difficult to observe from the element distribution in Fig. 2 that there is almost no Si in A layer and in the grey dotted-phase of B layer, and no Al in the white matrix of B layer. This implies that A layer and the grey dotted-phase of B layer are mainly composed of Ti, C and Al, and the white matrix of B layer is mainly composed of Ti, Si and C. The back-scattered electron images of the SiC/TAD interfaces bonded under different bonding conditions are shown in Fig. 3. It can be seen from the figure that the SiC/TAD joints are all similar in interface structure to one another, although bonding temperature and bonding time are not the same. In other words, the interface structure of the SiC/TAD joints can be all expressed by SiC/A/B/TAD, regardless of bonging temperature and bonding time, but the thickness of each reaction layer increases with bonding temperature and bonding time. Fig. 4 shows the XRD patterns from the fracture surfaces of the SiC/TAD joint bonded at 1573 K for 60 min. Obviously, the phases identified from the fracture surface on the SiC side are SiC, TiC and Ti5Si3Cx (xⱕ1) where SiC is an original phase in the SiC base material. The phases identified from the fracture surface on the TAD side are TiAl, Ti3Al, TiC and Ti5Si3Cx (xⱕ1) where TiAl and Ti3Al are TABLE 1 Diffusion Bonding Parameters for SiC/TAD Joints Temperature (K) 1473–1573
Time (min)
Pressure (MPa)
Vacuum (mPa)
Heating rate (K/min)
Cooling rate (K/min)
15–240
35
6.6
15
15
Vol. 43, No. 1
DIFFUSION-BONDED JOINTS
51
Figure 2. Back-scattered electron image and element distribution of the SiC/TAD interface bonded at 1573 K for 240 min.
the original phases in the TAD base material. Therefore, there are two kinds of reaction products or formed phases in the diffusion-bonded joints of SiC ceramic to TiAl-based alloy, namely face-centered cubic TiC and hexagonal Ti5Si3Cx . Table 2 shows the average contents of major elements, which were obtained by EPMA, in each reaction layer of the SiC/TAD joint bonded at 1573 K for 240 min. It can be seen from the table that the stoichiometric proportion of Ti to C is approximately 1:1 in A layer and in the dotted phase of B layer. Based on this and the XRD results mentioned above, it can be inferred that A layer is composed of TiC and the dotted phase of B layer is TiC, but it should be noted that there is a certain amount of Al in TiC. Similarly, the stoichiometric proportion of Ti, Si and C in the matrix of B layer is approximately 5:3:1, so the matrix of B layer must be Ti5Si3Cx identified by XRD. That is to say, B layer is composed of Ti5Si3Cx and TiC. Therefore, the interface structure of diffusion-bonded SiC/TAD joints is SiC/TiC/(Ti5Si3Cx ⫹TiC)/TAD. The interface structure of SiC/TAD joints is different from that of SiC/TiAl joints (11), so their formation mechanisms must be different to some extent. Fig. 5 is the interface reaction model for diffusion-bonded SiC/TAD joints. The whole reaction process can be divided into three stages. In the first stage (see Fig. 5b), Ti3Al in the TAD base material decomposes at the original SiC/TAD interface into TiAl and free Ti according to Reaction 1, and the free Ti reacts with SiC at the original SiC/TAD interface into TiC and free Si according to Reaction 2, thus a single-phase TiC layer occurs on the SiC side. Because the TiAl phase produced according Reaction 1 is the same as the TiAl phase in the TAD base material, a new TiAl layer is not observed on the TAD side.
Figure 3. Back-scattered electron images of the SiC/TAD interfaces bonded under different bonding conditions: (a) at 1573 K for 30 min and (b) at 1473 K for 120 min.
52
DIFFUSION-BONDED JOINTS
Vol. 43, No. 1
Figure 4. XRD patterns from the fracture surfaces of the SiC/TAD joint bonded at 1573 K for 60 min: (a) on the SiC side and (b) on the TAD side.
Ti3Al 3 TiAl ⫹ Ti
(1)
SiC ⫹ Ti 3 TiC ⫹ Si
(2)
In the second stage (see Fig. 5c), the continuance of Reaction 1 and Reaction 2 certainly raises the amount of free Si at the SiC/TiC interface and makes it diffuse through the TiC layer and gather at the TiC/TAD interface. When the concentration of free Si rises to a certain amount, TiC reacts with free Si and free Ti, which has formed according to Reaction 1, into Ti5Si3Cx according to Reaction 3, thus a duplex-phase (Ti5Si3Cx ⫹TiC) layer occurs at the TiC/TAD interface. The occurrence of the duplex-phase (Ti5Si3Cx ⫹TiC) layer instead of the single-phase Ti5Si3Cx layer is because of a small amount of TiC next to the TiC/TAD interface that does not react and resides in the Ti5Si3Cx phase. Ti ⫹ Si ⫹ TiC 3 Ti5Si3Cx
(3)
In the third stage (see Fig. 5d), TiC continues to grow at the SiC/TiC interface and the thickness of the TiC layer extends to the SiC side. Ti5Si3Cx continues to grow at the TiC/(Ti5Si3Cx ⫹TiC) interface and the thickness of the (Ti5Si3Cx ⫹TiC) layer extends to the TiC side. As a matter of fact, the thickness of each reaction layer increases with bonding time according to a parabolic law. In addition, a small amount of Al has diffused from the TAD base material and mainly dissolved in the TiC phase, thus there is a certain amount of Al in the TiC phase. TABLE 2 Average Contents of Major Elements in Each Reaction Layer of the SiC/TAD Joint Bonded at 1573 K for 240 min (at %) Layer
Ti
Al
Si
C
A B (matrix) B (dotted phase)
43.4 53.9 41.3
8.9 3.3 9.2
3.5 30.7 4.4
44.2 12.1 45.1
Vol. 43, No. 1
DIFFUSION-BONDED JOINTS
53
Figure 5. Interface reaction model for SiC/TAD joints: (a) close contact of SiC to TAD; (b) occurrence of a TiC layer; (c) occurrence of a (Ti5Si3Cx ⫹TiC) layer; and (d) growth of the TiC and (Ti5Si3Cx ⫹TiC) layers.
Summary and Conclusions Diffusion bonding of SiC ceramic to TiAl-based alloy was carried out at 1473–1573 K for 15–240 min under a pressure of 35 MPa. The kinds of the reaction products and the interface structures of the joints were investigated by SEM, EPMA and XRD. Based on this, a formation mechanism of the interface structure was elucidated. Two kinds of reaction products or new phases have formed during the diffusion bonding of SiC to TAD, namely face-centered cubic TiC and hexagonal Ti5Si3Cx. The interface structure of diffusionbonded SiC/TAD joints is SiC/TiC/(Ti5Si3Cx ⫹TiC)/TAD, and this structure will not change with bonding time once it forms. The interface structure of SiC/TAD joints forms according to the three-stage mechanism, namely the occurrence of a single-phase TiC layer; the occurrence of a duplex-phase (Ti5Si3Cx ⫹TiC) layer; and the growth of the TiC and (Ti5Si3Cx ⫹TiC) layers. In addition, the thickness of each reaction layer increases with bonding time according to a parabolic law. Acknowledgments The authors would like to express their gratitude to Professor M. Naka, with the Joining and Welding Research Institute, Osaka University, Japan, for his help with microanalysis. The research was funded by the three institutions of China, namely Harbin Institute of Technology, the Scientific and Technological Committee of National Defence, and the National Key Laboratory of Advanced Welding Production Technology. References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.
M. Naka, J. C. Feng, and J. C. Schuster, Metall. Mater. Trans. A. 28, 1385 (1997). J. C. Feng, M. Naka, and J. C. Schuster, J. Mater. Sci. Lett. 16, 1116 (1997). J. C. Feng, H. J. Liu, and Z. R. Li, Trans. Chin. Weld. Inst. 2, 20 (1997). T. Fukai, M. Naka, and J. C. Schuster, Trans. JWRI. 1, 59 (1996). A. E. Martinelli and R. A. L. Drew, Mater. Sci. Eng. A. 191, 239 (1995). M. Backhaus-Ricoult, Acta Metall. Mater. 40, S95 (1992). M. Holmquist, V. Recina, J. Ockborn, B. Pettersson, and E. Zumalde, Scripta Mater. 39, 1101 (1998). S. Seto, K. Matsumoto, T. Masuyama, M. Hiroshima, and H. Ishikawa, Q. J. Jap. Weld. Soc. 16, 59 (1998). T. Jungling, B. Kieback, W. Glatz, and H. Clemens, in Proceedings of the TMS ’95 Annual Meeting on Gamma Titanium Aluminides, ed. Y. W. Kim, R. Wagner, and M. Yamaguchi, Nevada, p. 627 (1995). H. A. Wickman and E. S. C. Chin, in Proceedings of the TMS ’95 Annual Meeting on Gamma Titanium Aluminides, ed. Y. W. Kim, R. Wagner, and M. Yamaguchi, Nevada, p. 499 (1995). H. J. Liu, J. C. Feng, Y. Y. Qian, and Z. R. Li, J. Mater. Sci. Lett. 18, 1011 (1999).
INTERFACE STRUCTURE AND FORMATION MECHANISM OF DIFFUSION-BONDED JOINTS OF SiC CERAMIC TO TiAlBASED ALLOY H.J. Liu, J.C. Feng and Y.Y. Qian National Key Laboratory of Advanced Welding Production Technology; Harbin Institute of Technology; Harbin 150001, People’s Republic of China (Received October 4, 1999) (Accepted in revised form January 20, 2000) Keywords: Diffusion bonding; Interface; Structural ceramic; Intermetallic compound Introduction SiC ceramics are considered one of the most promising structural materials for high temperature applications. The development of bonding technology, especially diffusion bonding, is widening the application field of SiC ceramics. There have been some reports on diffusion bonding of SiC ceramics to refractory metals (1–5) and heat-resistant alloy (6). TiAl-based alloys have a great potential to become important candidates for advanced applications in aerospace and military industries. The researches on diffusion bonding of TiAl-based alloys to other different materials have progressed in recent years (7–9). The concept of utilizing ceramic, intermetallic and metallic materials to attain one complete armor system by the process of diffusion bonding is a recent approach for defeating armor projectiles (10).In order to achieve a functionally-graded armor system that has the necessary material and mechanical characteristics for ballistic protection, it is necessary to demonstrate the feasibility of diffusion bonding. Therefore, a previous study of diffusion bonding of SiC ceramic to pure TiAl was carried out (11). The present study focuses on the interface structure and formation mechanism of diffusion-bonded joints of SiC to TiAl-based alloy. Experimental Procedure The ceramic material used in experiments was pressureless-sintered ␣-SiC with a small amount of Al2O3 as sintering additive. The TiAl-based alloy was Ti-43Al-1.7Cr-1.7Nb (at %) cast alloy composed of lamellar (␥⫹␣2) duplex-phase microstructures, as shown in Fig. 1, and this alloy was named TAD in this study. The dimensions of cylindrical SiC samples were 6 mm in diameter and 4 mm in height, and the thickness of TAD foils was 0.2 mm. The surfaces to be bonded were ground and polished through diamond paste and cleaned in ethanol and acetone prior to bonding. The stacked SiC/TAD/SiC assemblies were diffusion bonded in a vacuum furnace (Centorr-3520) equipped with a hydraulic system. The bonding parameters are shown in Table 1. The diffusion-bonded SiC/TAD/SiC joints were cross-sectioned, perpendicular to the bonding interfaces, using a low-speed diamond saw, and the cross-sections of these joints were metallographically polished using 0.5-m diamond paste as final polish and cleaned in acetone. The polished 1359-6462/00/$–see front matter. © 2000 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved. PII: S1359-6462(00)00360-2
50
DIFFUSION-BONDED JOINTS
Vol. 43, No. 1
Figure 1. Microstructure of the TAD base material.
cross-sections and fracture surfaces of the joints were observed by scanning electron microscopy (SEM, S-570). The composition analysis of the reaction products were performed by electron probe X-ray microanalysis (EPMA, JXA-8600). The kinds and crystal structures of the reaction products were identified from the fracture surfaces of the joints by X-ray diffraction (XRD, JDX-3530M). For SEM and EPMA characterization, the cross-sections of each joint were deposited with carbon using a depositor. Results and Discussion The back-scattered electron image and element distribution of the SiC/TAD interface bonded at 1573 K for 240 min are shown in Fig. 2. It can be clearly seen from the figure that two kinds of reaction layers have occurred between SiC and TAD. For the sake of convenience, the reaction layer adjacent to SiC is called A layer, and the reaction layer, dotted with a small amount of grey phase, adjacent to TAD is called B layer. Obviously, A layer is a single-phase layer, and B layer is composed of duplex phases where the white phase is matrix. It’s also not difficult to observe from the element distribution in Fig. 2 that there is almost no Si in A layer and in the grey dotted-phase of B layer, and no Al in the white matrix of B layer. This implies that A layer and the grey dotted-phase of B layer are mainly composed of Ti, C and Al, and the white matrix of B layer is mainly composed of Ti, Si and C. The back-scattered electron images of the SiC/TAD interfaces bonded under different bonding conditions are shown in Fig. 3. It can be seen from the figure that the SiC/TAD joints are all similar in interface structure to one another, although bonding temperature and bonding time are not the same. In other words, the interface structure of the SiC/TAD joints can be all expressed by SiC/A/B/TAD, regardless of bonging temperature and bonding time, but the thickness of each reaction layer increases with bonding temperature and bonding time. Fig. 4 shows the XRD patterns from the fracture surfaces of the SiC/TAD joint bonded at 1573 K for 60 min. Obviously, the phases identified from the fracture surface on the SiC side are SiC, TiC and Ti5Si3Cx (xⱕ1) where SiC is an original phase in the SiC base material. The phases identified from the fracture surface on the TAD side are TiAl, Ti3Al, TiC and Ti5Si3Cx (xⱕ1) where TiAl and Ti3Al are TABLE 1 Diffusion Bonding Parameters for SiC/TAD Joints Temperature (K) 1473–1573
Time (min)
Pressure (MPa)
Vacuum (mPa)
Heating rate (K/min)
Cooling rate (K/min)
15–240
35
6.6
15
15
Vol. 43, No. 1
DIFFUSION-BONDED JOINTS
51
Figure 2. Back-scattered electron image and element distribution of the SiC/TAD interface bonded at 1573 K for 240 min.
the original phases in the TAD base material. Therefore, there are two kinds of reaction products or formed phases in the diffusion-bonded joints of SiC ceramic to TiAl-based alloy, namely face-centered cubic TiC and hexagonal Ti5Si3Cx . Table 2 shows the average contents of major elements, which were obtained by EPMA, in each reaction layer of the SiC/TAD joint bonded at 1573 K for 240 min. It can be seen from the table that the stoichiometric proportion of Ti to C is approximately 1:1 in A layer and in the dotted phase of B layer. Based on this and the XRD results mentioned above, it can be inferred that A layer is composed of TiC and the dotted phase of B layer is TiC, but it should be noted that there is a certain amount of Al in TiC. Similarly, the stoichiometric proportion of Ti, Si and C in the matrix of B layer is approximately 5:3:1, so the matrix of B layer must be Ti5Si3Cx identified by XRD. That is to say, B layer is composed of Ti5Si3Cx and TiC. Therefore, the interface structure of diffusion-bonded SiC/TAD joints is SiC/TiC/(Ti5Si3Cx ⫹TiC)/TAD. The interface structure of SiC/TAD joints is different from that of SiC/TiAl joints (11), so their formation mechanisms must be different to some extent. Fig. 5 is the interface reaction model for diffusion-bonded SiC/TAD joints. The whole reaction process can be divided into three stages. In the first stage (see Fig. 5b), Ti3Al in the TAD base material decomposes at the original SiC/TAD interface into TiAl and free Ti according to Reaction 1, and the free Ti reacts with SiC at the original SiC/TAD interface into TiC and free Si according to Reaction 2, thus a single-phase TiC layer occurs on the SiC side. Because the TiAl phase produced according Reaction 1 is the same as the TiAl phase in the TAD base material, a new TiAl layer is not observed on the TAD side.
Figure 3. Back-scattered electron images of the SiC/TAD interfaces bonded under different bonding conditions: (a) at 1573 K for 30 min and (b) at 1473 K for 120 min.
52
DIFFUSION-BONDED JOINTS
Vol. 43, No. 1
Figure 4. XRD patterns from the fracture surfaces of the SiC/TAD joint bonded at 1573 K for 60 min: (a) on the SiC side and (b) on the TAD side.
Ti3Al 3 TiAl ⫹ Ti
(1)
SiC ⫹ Ti 3 TiC ⫹ Si
(2)
In the second stage (see Fig. 5c), the continuance of Reaction 1 and Reaction 2 certainly raises the amount of free Si at the SiC/TiC interface and makes it diffuse through the TiC layer and gather at the TiC/TAD interface. When the concentration of free Si rises to a certain amount, TiC reacts with free Si and free Ti, which has formed according to Reaction 1, into Ti5Si3Cx according to Reaction 3, thus a duplex-phase (Ti5Si3Cx ⫹TiC) layer occurs at the TiC/TAD interface. The occurrence of the duplex-phase (Ti5Si3Cx ⫹TiC) layer instead of the single-phase Ti5Si3Cx layer is because of a small amount of TiC next to the TiC/TAD interface that does not react and resides in the Ti5Si3Cx phase. Ti ⫹ Si ⫹ TiC 3 Ti5Si3Cx
(3)
In the third stage (see Fig. 5d), TiC continues to grow at the SiC/TiC interface and the thickness of the TiC layer extends to the SiC side. Ti5Si3Cx continues to grow at the TiC/(Ti5Si3Cx ⫹TiC) interface and the thickness of the (Ti5Si3Cx ⫹TiC) layer extends to the TiC side. As a matter of fact, the thickness of each reaction layer increases with bonding time according to a parabolic law. In addition, a small amount of Al has diffused from the TAD base material and mainly dissolved in the TiC phase, thus there is a certain amount of Al in the TiC phase. TABLE 2 Average Contents of Major Elements in Each Reaction Layer of the SiC/TAD Joint Bonded at 1573 K for 240 min (at %) Layer
Ti
Al
Si
C
A B (matrix) B (dotted phase)
43.4 53.9 41.3
8.9 3.3 9.2
3.5 30.7 4.4
44.2 12.1 45.1
Vol. 43, No. 1
DIFFUSION-BONDED JOINTS
53
Figure 5. Interface reaction model for SiC/TAD joints: (a) close contact of SiC to TAD; (b) occurrence of a TiC layer; (c) occurrence of a (Ti5Si3Cx ⫹TiC) layer; and (d) growth of the TiC and (Ti5Si3Cx ⫹TiC) layers.
Summary and Conclusions Diffusion bonding of SiC ceramic to TiAl-based alloy was carried out at 1473–1573 K for 15–240 min under a pressure of 35 MPa. The kinds of the reaction products and the interface structures of the joints were investigated by SEM, EPMA and XRD. Based on this, a formation mechanism of the interface structure was elucidated. Two kinds of reaction products or new phases have formed during the diffusion bonding of SiC to TAD, namely face-centered cubic TiC and hexagonal Ti5Si3Cx. The interface structure of diffusionbonded SiC/TAD joints is SiC/TiC/(Ti5Si3Cx ⫹TiC)/TAD, and this structure will not change with bonding time once it forms. The interface structure of SiC/TAD joints forms according to the three-stage mechanism, namely the occurrence of a single-phase TiC layer; the occurrence of a duplex-phase (Ti5Si3Cx ⫹TiC) layer; and the growth of the TiC and (Ti5Si3Cx ⫹TiC) layers. In addition, the thickness of each reaction layer increases with bonding time according to a parabolic law. Acknowledgments The authors would like to express their gratitude to Professor M. Naka, with the Joining and Welding Research Institute, Osaka University, Japan, for his help with microanalysis. The research was funded by the three institutions of China, namely Harbin Institute of Technology, the Scientific and Technological Committee of National Defence, and the National Key Laboratory of Advanced Welding Production Technology. References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.
M. Naka, J. C. Feng, and J. C. Schuster, Metall. Mater. Trans. A. 28, 1385 (1997). J. C. Feng, M. Naka, and J. C. Schuster, J. Mater. Sci. Lett. 16, 1116 (1997). J. C. Feng, H. J. Liu, and Z. R. Li, Trans. Chin. Weld. Inst. 2, 20 (1997). T. Fukai, M. Naka, and J. C. Schuster, Trans. JWRI. 1, 59 (1996). A. E. Martinelli and R. A. L. Drew, Mater. Sci. Eng. A. 191, 239 (1995). M. Backhaus-Ricoult, Acta Metall. Mater. 40, S95 (1992). M. Holmquist, V. Recina, J. Ockborn, B. Pettersson, and E. Zumalde, Scripta Mater. 39, 1101 (1998). S. Seto, K. Matsumoto, T. Masuyama, M. Hiroshima, and H. Ishikawa, Q. J. Jap. Weld. Soc. 16, 59 (1998). T. Jungling, B. Kieback, W. Glatz, and H. Clemens, in Proceedings of the TMS ’95 Annual Meeting on Gamma Titanium Aluminides, ed. Y. W. Kim, R. Wagner, and M. Yamaguchi, Nevada, p. 627 (1995). H. A. Wickman and E. S. C. Chin, in Proceedings of the TMS ’95 Annual Meeting on Gamma Titanium Aluminides, ed. Y. W. Kim, R. Wagner, and M. Yamaguchi, Nevada, p. 499 (1995). H. J. Liu, J. C. Feng, Y. Y. Qian, and Z. R. Li, J. Mater. Sci. Lett. 18, 1011 (1999).