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Influence of the Substrate Orientation on the Isothermal Solidification during TLP Bonding Single Crystal Superalloys
Available online at ScienceDirect
ScienceDirect J. Mater. Sci. Technol., 2014, 30(3), 213e216
Influence of the Substrate Orientation on the Isothermal Solidification during TLP Bonding Single Crystal Superalloys Naicheng Sheng, Bo Li, Jide Liu*, Tao Jin, Xiaofeng Sun, Zhuangqi Hu Superalloys Division, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China [Manuscript received June 5, 2013, in revised form June 24, 2013, Available online 3 December 2013]
Angle deviations between the two substrates during transient liquid phase (TLP) bonding single crystal superalloys cannot be avoided. In the present work, specimens have been prepared to investigate the influences of the various substrate orientations. It is found that the width of the non-isothermal solidification zone (NSZ) is linear with the square root of the isothermal solidification time. This suggests that the isothermal solidification process is B-diffusion controlled in different substrate orientation deviations. And also the width of the NSZ increases with increasing angle deviation, indicating that the isothermal solidification time needed in the TLP bonding increases with increasing orientation deviation between the two substrates. KEY WORDS: Transient liquid phase (TLP) bonding; Single crystal superalloys; Orientation deviation; Bonding time
1. Introduction Single crystal superalloys are developed to meet the requirements of the advanced aeroengines in recent decades. With increasing thermal efficiency requirements, the geometry and size of the single crystal turbine blades are getting more complex[1]. However, it is still difficult to cast these complex or large single crystals without any defects, such as freckles, stray grains or low angle grain boundaries[2]. These defects make the casting processes of the single crystals more expensive. Transient liquid phase (TLP) bonding is an effective method in repairing and preparing the single crystal blades[3]. During the production of blades using TLP bonding, small single crystal casting segments are prepared through directional solidification, and then the small segments are TLP bonded. The bonding zones are usually in the low stress region of the blade. After that, a series of heat treatments are applied to these specimens to acquire the most advanced mechanical properties. TLP bonding was developed by Duvall[4] and it has been used widely in bonding superalloys or other crack sensitive alloys now[5e7]. In principle, TLP bonding involves the system that is eutectic or peritectic[4]. B is usually used as the melting point depressant (MPD) in bonding superalloys. As has been investigated, the TLP bonding can be divided into several independent Corresponding author. Assoc. Prof., Ph.D.; Tel.: þ86 24 83971787; Fax: þ86 24 83971758; E-mail address: [email protected] (J. Liu). 1005-0302/$ e see front matter Copyright Ó 2013, The editorial office of Journal of Materials Science & Technology. Published by Elsevier Limited. All rights reserved. http://dx.doi.org/10.1016/j.jmst.2013.11.013 *
stages[4,8,9]. First, melting of the interlayer inserted between two substrate metals occurs gradually, and then reaction at the interface between the MPD and the substrate happens leading to the widening of the liquid until the liquid and the substrate metal reach equilibrium completely. This process takes only a short time of the whole bonding stage. Subsequently, the diffusion of B in the liquid into the substrate leads to the reversal solidification of the liquid zone, named isothermal solidification process which occupies most of the time of the bonding process. After the liquid zone is isothermally solidified completely, a controlled heat treatment is imposed to attain the required microstructures and mechanical properties. Many factors such as bonding temperature[10], compositions of the interlayer[11,12] and substrate grain sizes or grain boundaries[13e15] have important influences on the bonding process of polycrystal alloys. Isothermal solidification stage is the most important stage during TLP bonding and determines the final quality of the bonding zone, so the isothermal solidification zone is more concerned in the bonding process. But when it is used to bond single crystal, the bonding phenomenon becomes different as the microstructure of the single crystal superalloys and the polycrystalline superalloys varies from each other, for example, the anisotropy of the crystal structure, grain boundaries. Angle deviations introduced from casting segments or preparation processes between the two substrates cannot be avoided when bonding the single crystal superalloys. The influence of the relative angle deviations between the substrates ought to be investigated during TLP bonding single crystal superalloys. Isothermal solidification stage is the most important during TLP bonding, so we focused our attentions on the bonding behaviors of the isothermal solidification stage.
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2. Experimental [001] orientation single crystal bars were prepared using high rate solidification (HRS) method. The NieCreB powder prepared by the inert gas atomization method was used in our investigations. The composition of the base metal and the interlayer are illustrated in Table 1. Specimens were prepared by putting two substrates, with angle deviations between the two primary dendrites, together and were named fixed orientation assembly, as illustrated in Fig. 1. The test pieces were cut directly from the as-cast single crystal bars by electrical discharge machine (EDM) to pieces of 16 mm in diameter and 3.5 mm in thickness. Then the cutting test pieces were polished to 800 grits, cleaned in acetone and assembled with two Mo wires in the center to control the width of liquid zone during TLP bonding, and the liquid zone was about 120 mm after bonding. The assembly was then heated to 1200 C with a heating rate of 0.8 K/s in a vacuum furnace under a pressure of about 5.1 103 Pa for different isothermal time, and then they were furnace cooled after bonding. The angle deviations of the specimens are 0 , 10 , 20 , 30 and 40 . Each angle is isothermally solidified for 1, 2, 3 and 4 h. The optical microscopy (OM) and scanning electron microscopy (SEM) were both used to examine the microstructures of the assemblies after bonding. All specimens were cut into two identical pieces perpendicular to the bonding line from the center and then they were polished and etched using a solution of CuSO4/HCl/C2H4OH. Metallographic photos were taken by LEIKA optical microscope at 200 magnifications in different regions of the bonding zone. The mean width of the nonisothermal solidification zone (NSZ) produced through the area over the length of the fixed orientation assembly was calculated using the commercial software Image Tool. Each statistic was conducted three times by the same method to ensure that the results acknowledged were reliable enough. 3. Results and Discussion 3.1. Microstructures observations of the bonding zone Fig. 2 shows the typical SEM and OM microstructures of the bonding zone. As illustrated in Fig. 2(a), there are three zones separated by the morphologies of the microstructures, and they are non-isothermally solidified zone (NSZ for short) in the center, isothermally solidified zone (ISZ) and diffusion affected zone (DAZ). In the present work, after widening process, ISZ and DAZ were formed simultaneously as a result of the diffusion of the B into the substrates during bonding at 1200 C. The Brich phases in the DAZ were also reported in other systems[16,17], and these precipitates formed as a result of the enrichment of B in the substrate. There are fine precipitates, short-bar precipitates and long acicular precipitates in the DAZ, and they are CreMoe W rich borides[16]. To satisfy the service requirements, extra efforts are needed to get rid of these deleterious phases which
Table 1 Chemical compositions of base metal and powder interlayer (wt%)
Base metal Filler metal
Cr
Co
Mo
W
Al
Ti
B
Ni
6.0 15
7.5 e
1.2 e
5.8 e
5.9 e
1.1 e
e 3.5
Bal. Bal.
Fig. 1 Preparation of the specimen using the [001] direction single crystal bar.
may serve as the origin of cracks, and cracks along the precipitates have been observed during the preparation of TEM specimens. The ISZ formed as a Ni-base solid solution at the bonding temperature, and after sufficient time the interlayer liquid would transform into ISZ completely. NSZ was produced during the cooling from bonding temperature when the interlayer liquid still exists. There are usually eutectics in the NSZ and the transformation process has been rationalized[18]. During bonding at 1200 C, the composition of the interlayer liquid reached equilibrium to the substrate first, when it cooled, the Ni-base fcc solid solution would nucleate homogeneously from the liquid or heterogeneously from the substrate interface and the composition of the liquid would gradually switch to binary eutectic line along the tie line in the NieCreB ternary phase diagram[19]. After that, the transformation of the residual liquid to Ni-base fcc solid solution and Ni3B started, then the composition transformed along the binary eutectic line until it reached the ternary eutectic point. The residual liquid transformed to Ni-base fcc solid solution, Ni3B and CrB eutectic. Fig. 2(d) shows the bonding zone isothermally solidified completely. The interlayer liquid has transformed to ISZ completely and the bond can be divided into the ISZ and DAZ. 3.2. Kinetics of the different substrate orientations Angle deviations during bonding of single crystal superalloy segments cannot be avoided, as it is hard to cast segments along the desired orientation precisely and also deviation can be introduced during the preparation process before TLP bonding. The influence of the orientation on the isothermal solidification kinetics is concerned. Previous work on the factors affecting the TLP bonding process of single crystals or polycrystalline alloys has focused mainly on the interlayer composition, interlayer width, bonding temperature and their effects on the isothermal solidification[17]. Isothermal solidification stage is a process controlled by the diffusion of the melting point depressant elements into the base metal, leading to the final isothermal solidification after sufficient time. Fick’s second diffusion law has been applied to calculate the thickness of the isothermal solidification zone during the growth[20e22]. After the stage of dissolution and homogenization of the interlayer liquid the isothermal solidification zone will grow epitaxially from substrate/liquid interface into the residual liquid gradually, and the thickness of the growing isothermal
N. Sheng et al.: J. Mater. Sci. Technol., 2014, 30(3), 213e216
215
Fig. 2 Microstructures of bonding region: (a) microstructure of the bonding zone (2 h), (b) precipitates of the DAZ (2 h), (c) eutectic of the NSZ (2 h), (d) bonding zone after completely solidification (4 h).
solidification zone is assumed as y ¼ W0 =2 W =2, where W0 is the width of the interlayer liquid after the dissolution stage, W is the instantaneous width of the liquid zone during the isothermal solidification stage. As has been acknowledged, second diffusion law can be expressed as follows: vC v2 C ¼ D$ 2 vt vy
(1)
And the error function solution of the Fick’s second law has been well known as below: pffiffiffiffiffiffiffiffi C ¼ M þ N $erf y= 4Dt
the same relationship between the width of the residual eutectic with the isothermal solidification time. Fig. 3 shows the isothermal solidification behavior of the different relative angles. The linear relationship between the width of the non-isothermally solidification zone with the square root of the isothermal solidification time is fitted. It can be seen that the width of NSZ is linear with the square root of the isothermal solidification time, and this confirms that solidification process is mainly determined by the B diffusion into the substrate. The relationship of width of the NSZ with the relative angle between the two substrates is illustrated in Fig. 4, and the
(2)
After a series of specific boundary conditions the coefficient M, N of the above equation can be obtained, where the thickness of the diffusion satisfies the following relationship: pffiffiffiffiffiffiffiffi y ¼ k$ 4Dt
(3)
where k is a constant, and is determined by the concentration of MPD elements in the liquid and the solid substrate obtained from the phase diagram at a given temperature[23]. D is the diffusion coefficient of the melting point depressant element in the substrate which is mainly determined by the structure of the substrate if only B diffusion is considered in the present system[24]. As can be obtained from the equation (3), the thickness of the isothermal solidification zone is linear with the square root of the isothermal solidification time (t), indicating
Fig. 3 Isothermal solidification kinetics of the different substrate orientation deviations.
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(2) The width of the NSZ is linear with the square root of the isothermal solidification time, and also the time for the interlayer liquid to be completely solidified increases with increasing orientation deviations.
Acknowledgments This work was financially supported by the National Basic Research Program (973 Program) of China under Grant Nos. 2010CB631200 and 2010CB631206, the National Natural Science Foundation of China (NSFC) under Grant Nos. 50971124, 50904059, 51071165 and 51204156. Fig. 4 Relationship of the width of the NSZ with the substrate orientation deviations.
relationship that the residual eutectic width increases with increasing relative angle can be seen. As has been introduced above, the thickness of the isothermal solidification zone can be expressed as Eq. (3). The thickness of the ISZ is determined by the coefficient k, the diffusion coefficient D (B in substrate) and the bonding time. The coefficient k is mainly determined by the composition of the residual liquid during isothermal solidification[22]. The factors that affect the interlayer liquid composition may change the coefficient k which affects the bonding width of the isothermal solidification. When the substrate orientation exists, the equilibrium interlayer liquid composition changes. This is due to the composition heterogeneity of the substrate between the dendrite and the interdendrite regions, and the introduction of the orientation changes the area of the liquid and the substrate. Also, the diffusion coefficient will decrease as the orientation deviation changes the B diffusion direction in the substrate. Furthermore, there are lots of precipitates in the substrate and these substrates will serve as the consumption of B in the path of the B diffusion into the substrate. These factors interact in the whole bonding stage, and finally change the growth of the ISZ. As observed from Fig. 3, the isothermal solidification time increases with increasing substrate orientation deviations. So when single crystal segments are bonded using TLP in engineering areas, more time is required for the complete solidification if angle deviations are introduced during the process. Such instructions may avoid the incomplete solidification of the interlayer liquid which will be detrimental to the mechanical behavior of the bonds. 4. Conclusions (1) Three regions exist in the bonding zone separated by the morphology: the non-isothermal solidification zone (NSZ), isothermal solidification zone (ISZ) and the diffusion affected zone (DAZ). There are lots of eutectics in the NSZ, and fine, short-bar or acicular precipitates in the DAZ.
REFERENCES [1] R.C. Reed, The Superalloys: Fundamentals and Applications, Cambridge University Press, Cambridge, 2006. [2] C.M. Biondo, P.B. Gardens, W.J. Gostic, C.D. Parmley, Repaired Nickel Based Superalloys, US Patent, No. 5783318, 1998. [3] M.A. Burke, P.D. Freyer, M.A. Hebbar, B.B. Seth, G.W. Swartzbeck, T.W. Zagar, Turbine Blades Made from Multiple Single Crystal Cast Superalloy Segments, US Patent, No. 6331217, 2001. [4] D.S. Duvall, W.A. Owczarski, D.F. Paulonis, Weld. J. 51 (1972) 41e49. [5] W.F. Gale, Y. Guan, Metall. Mater. Trans. A 27 (1996) 3621e3629. [6] W.F. Gale, Y. Xu, Metall. Mater. Trans. A 30 (1999) 2723e2726. [7] O.A. Idowu, O.A. Ojo, M.C. C, Metall. Mater. Trans. A 37 (2006) 2787e2796. [8] T.W. Eagar, Annu. Rev. Mater. Sci. 22 (1992) 23e46. [9] W.F. Gale, D.A. Butts, Sci. Technol. Weld. Join 9 (2004) 283e300. [10] S.K. Tung, L.C. Lim, M.O. Lai, Scripta Mater. 34 (1996) 763e769. [11] M. Pouranvari, A. Ekrami, A.H. Kokabi, J. Alloy. Compd. 469 (2009) 270e275. [12] A. Ghoneim, O.A. Ojo, Intermetallics 18 (2010) 582e586. [13] H. Kokawa, C.H. Lee, T.H. North, Metall. Mater. Trans. A 32 (1991) 1627e1631. [14] K. Ikeuchi, Y. Zhou, H. Kokawa, T.H. North, Metall. Mater. Trans. A 23 (2005) 2905e2915. [15] K. Saida, Y. Zhou, T.H. North, J. Mater. Sci. 28 (1993) 6427e6432. [16] M. Mosallaee, A. Ekarami, K. Ohsasa, K. Matsuura, Metall. Mater. Trans. A 39 (2008) 2389e2402. [17] S. Steuer, R.F. Singer, Metall. Mater. Trans. A 44 (2013) 2226e 2232. [18] N.C. Sheng, J.D. Liu, T. Jin, X.F. Sun, Z.Q. Hu, Metall. Mater. Trans. A 44 (2013) 1793e1804. [19] S. Omori, Nippon Kinzoku Gakkaishi 49 (1985) 935e939. [20] H.N. Ikawa, T.Y. Isai, Trans. Jpn. Weld. Soc. 10 (1979) 25e29. [21] Y.K. Nakao, K. Nishimoto, K.J. Shinozaki, C.Y. Kang, in: Superalloy, The Metallurgical Society, 1988. [22] I.T. Poku, M. Dollar, T.B. Massalski, Metall. Mater. Trans. A 19 (1988) 675e686. [23] S.H. Omori, K. Koyama, Y. Hashimoto, K. Yamada, J. Jpn. Inst. Met. 49 (1985) 935e939. [24] Y.Y. Chu, P. Ji, J. Ke, Acta Metall. Sin. (Engl. Lett.) 5 (1992) 87e93.
ScienceDirect J. Mater. Sci. Technol., 2014, 30(3), 213e216
Influence of the Substrate Orientation on the Isothermal Solidification during TLP Bonding Single Crystal Superalloys Naicheng Sheng, Bo Li, Jide Liu*, Tao Jin, Xiaofeng Sun, Zhuangqi Hu Superalloys Division, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China [Manuscript received June 5, 2013, in revised form June 24, 2013, Available online 3 December 2013]
Angle deviations between the two substrates during transient liquid phase (TLP) bonding single crystal superalloys cannot be avoided. In the present work, specimens have been prepared to investigate the influences of the various substrate orientations. It is found that the width of the non-isothermal solidification zone (NSZ) is linear with the square root of the isothermal solidification time. This suggests that the isothermal solidification process is B-diffusion controlled in different substrate orientation deviations. And also the width of the NSZ increases with increasing angle deviation, indicating that the isothermal solidification time needed in the TLP bonding increases with increasing orientation deviation between the two substrates. KEY WORDS: Transient liquid phase (TLP) bonding; Single crystal superalloys; Orientation deviation; Bonding time
1. Introduction Single crystal superalloys are developed to meet the requirements of the advanced aeroengines in recent decades. With increasing thermal efficiency requirements, the geometry and size of the single crystal turbine blades are getting more complex[1]. However, it is still difficult to cast these complex or large single crystals without any defects, such as freckles, stray grains or low angle grain boundaries[2]. These defects make the casting processes of the single crystals more expensive. Transient liquid phase (TLP) bonding is an effective method in repairing and preparing the single crystal blades[3]. During the production of blades using TLP bonding, small single crystal casting segments are prepared through directional solidification, and then the small segments are TLP bonded. The bonding zones are usually in the low stress region of the blade. After that, a series of heat treatments are applied to these specimens to acquire the most advanced mechanical properties. TLP bonding was developed by Duvall[4] and it has been used widely in bonding superalloys or other crack sensitive alloys now[5e7]. In principle, TLP bonding involves the system that is eutectic or peritectic[4]. B is usually used as the melting point depressant (MPD) in bonding superalloys. As has been investigated, the TLP bonding can be divided into several independent Corresponding author. Assoc. Prof., Ph.D.; Tel.: þ86 24 83971787; Fax: þ86 24 83971758; E-mail address: [email protected] (J. Liu). 1005-0302/$ e see front matter Copyright Ó 2013, The editorial office of Journal of Materials Science & Technology. Published by Elsevier Limited. All rights reserved. http://dx.doi.org/10.1016/j.jmst.2013.11.013 *
stages[4,8,9]. First, melting of the interlayer inserted between two substrate metals occurs gradually, and then reaction at the interface between the MPD and the substrate happens leading to the widening of the liquid until the liquid and the substrate metal reach equilibrium completely. This process takes only a short time of the whole bonding stage. Subsequently, the diffusion of B in the liquid into the substrate leads to the reversal solidification of the liquid zone, named isothermal solidification process which occupies most of the time of the bonding process. After the liquid zone is isothermally solidified completely, a controlled heat treatment is imposed to attain the required microstructures and mechanical properties. Many factors such as bonding temperature[10], compositions of the interlayer[11,12] and substrate grain sizes or grain boundaries[13e15] have important influences on the bonding process of polycrystal alloys. Isothermal solidification stage is the most important stage during TLP bonding and determines the final quality of the bonding zone, so the isothermal solidification zone is more concerned in the bonding process. But when it is used to bond single crystal, the bonding phenomenon becomes different as the microstructure of the single crystal superalloys and the polycrystalline superalloys varies from each other, for example, the anisotropy of the crystal structure, grain boundaries. Angle deviations introduced from casting segments or preparation processes between the two substrates cannot be avoided when bonding the single crystal superalloys. The influence of the relative angle deviations between the substrates ought to be investigated during TLP bonding single crystal superalloys. Isothermal solidification stage is the most important during TLP bonding, so we focused our attentions on the bonding behaviors of the isothermal solidification stage.
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N. Sheng et al.: J. Mater. Sci. Technol., 2014, 30(3), 213e216
2. Experimental [001] orientation single crystal bars were prepared using high rate solidification (HRS) method. The NieCreB powder prepared by the inert gas atomization method was used in our investigations. The composition of the base metal and the interlayer are illustrated in Table 1. Specimens were prepared by putting two substrates, with angle deviations between the two primary dendrites, together and were named fixed orientation assembly, as illustrated in Fig. 1. The test pieces were cut directly from the as-cast single crystal bars by electrical discharge machine (EDM) to pieces of 16 mm in diameter and 3.5 mm in thickness. Then the cutting test pieces were polished to 800 grits, cleaned in acetone and assembled with two Mo wires in the center to control the width of liquid zone during TLP bonding, and the liquid zone was about 120 mm after bonding. The assembly was then heated to 1200 C with a heating rate of 0.8 K/s in a vacuum furnace under a pressure of about 5.1 103 Pa for different isothermal time, and then they were furnace cooled after bonding. The angle deviations of the specimens are 0 , 10 , 20 , 30 and 40 . Each angle is isothermally solidified for 1, 2, 3 and 4 h. The optical microscopy (OM) and scanning electron microscopy (SEM) were both used to examine the microstructures of the assemblies after bonding. All specimens were cut into two identical pieces perpendicular to the bonding line from the center and then they were polished and etched using a solution of CuSO4/HCl/C2H4OH. Metallographic photos were taken by LEIKA optical microscope at 200 magnifications in different regions of the bonding zone. The mean width of the nonisothermal solidification zone (NSZ) produced through the area over the length of the fixed orientation assembly was calculated using the commercial software Image Tool. Each statistic was conducted three times by the same method to ensure that the results acknowledged were reliable enough. 3. Results and Discussion 3.1. Microstructures observations of the bonding zone Fig. 2 shows the typical SEM and OM microstructures of the bonding zone. As illustrated in Fig. 2(a), there are three zones separated by the morphologies of the microstructures, and they are non-isothermally solidified zone (NSZ for short) in the center, isothermally solidified zone (ISZ) and diffusion affected zone (DAZ). In the present work, after widening process, ISZ and DAZ were formed simultaneously as a result of the diffusion of the B into the substrates during bonding at 1200 C. The Brich phases in the DAZ were also reported in other systems[16,17], and these precipitates formed as a result of the enrichment of B in the substrate. There are fine precipitates, short-bar precipitates and long acicular precipitates in the DAZ, and they are CreMoe W rich borides[16]. To satisfy the service requirements, extra efforts are needed to get rid of these deleterious phases which
Table 1 Chemical compositions of base metal and powder interlayer (wt%)
Base metal Filler metal
Cr
Co
Mo
W
Al
Ti
B
Ni
6.0 15
7.5 e
1.2 e
5.8 e
5.9 e
1.1 e
e 3.5
Bal. Bal.
Fig. 1 Preparation of the specimen using the [001] direction single crystal bar.
may serve as the origin of cracks, and cracks along the precipitates have been observed during the preparation of TEM specimens. The ISZ formed as a Ni-base solid solution at the bonding temperature, and after sufficient time the interlayer liquid would transform into ISZ completely. NSZ was produced during the cooling from bonding temperature when the interlayer liquid still exists. There are usually eutectics in the NSZ and the transformation process has been rationalized[18]. During bonding at 1200 C, the composition of the interlayer liquid reached equilibrium to the substrate first, when it cooled, the Ni-base fcc solid solution would nucleate homogeneously from the liquid or heterogeneously from the substrate interface and the composition of the liquid would gradually switch to binary eutectic line along the tie line in the NieCreB ternary phase diagram[19]. After that, the transformation of the residual liquid to Ni-base fcc solid solution and Ni3B started, then the composition transformed along the binary eutectic line until it reached the ternary eutectic point. The residual liquid transformed to Ni-base fcc solid solution, Ni3B and CrB eutectic. Fig. 2(d) shows the bonding zone isothermally solidified completely. The interlayer liquid has transformed to ISZ completely and the bond can be divided into the ISZ and DAZ. 3.2. Kinetics of the different substrate orientations Angle deviations during bonding of single crystal superalloy segments cannot be avoided, as it is hard to cast segments along the desired orientation precisely and also deviation can be introduced during the preparation process before TLP bonding. The influence of the orientation on the isothermal solidification kinetics is concerned. Previous work on the factors affecting the TLP bonding process of single crystals or polycrystalline alloys has focused mainly on the interlayer composition, interlayer width, bonding temperature and their effects on the isothermal solidification[17]. Isothermal solidification stage is a process controlled by the diffusion of the melting point depressant elements into the base metal, leading to the final isothermal solidification after sufficient time. Fick’s second diffusion law has been applied to calculate the thickness of the isothermal solidification zone during the growth[20e22]. After the stage of dissolution and homogenization of the interlayer liquid the isothermal solidification zone will grow epitaxially from substrate/liquid interface into the residual liquid gradually, and the thickness of the growing isothermal
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Fig. 2 Microstructures of bonding region: (a) microstructure of the bonding zone (2 h), (b) precipitates of the DAZ (2 h), (c) eutectic of the NSZ (2 h), (d) bonding zone after completely solidification (4 h).
solidification zone is assumed as y ¼ W0 =2 W =2, where W0 is the width of the interlayer liquid after the dissolution stage, W is the instantaneous width of the liquid zone during the isothermal solidification stage. As has been acknowledged, second diffusion law can be expressed as follows: vC v2 C ¼ D$ 2 vt vy
(1)
And the error function solution of the Fick’s second law has been well known as below: pffiffiffiffiffiffiffiffi C ¼ M þ N $erf y= 4Dt
the same relationship between the width of the residual eutectic with the isothermal solidification time. Fig. 3 shows the isothermal solidification behavior of the different relative angles. The linear relationship between the width of the non-isothermally solidification zone with the square root of the isothermal solidification time is fitted. It can be seen that the width of NSZ is linear with the square root of the isothermal solidification time, and this confirms that solidification process is mainly determined by the B diffusion into the substrate. The relationship of width of the NSZ with the relative angle between the two substrates is illustrated in Fig. 4, and the
(2)
After a series of specific boundary conditions the coefficient M, N of the above equation can be obtained, where the thickness of the diffusion satisfies the following relationship: pffiffiffiffiffiffiffiffi y ¼ k$ 4Dt
(3)
where k is a constant, and is determined by the concentration of MPD elements in the liquid and the solid substrate obtained from the phase diagram at a given temperature[23]. D is the diffusion coefficient of the melting point depressant element in the substrate which is mainly determined by the structure of the substrate if only B diffusion is considered in the present system[24]. As can be obtained from the equation (3), the thickness of the isothermal solidification zone is linear with the square root of the isothermal solidification time (t), indicating
Fig. 3 Isothermal solidification kinetics of the different substrate orientation deviations.
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(2) The width of the NSZ is linear with the square root of the isothermal solidification time, and also the time for the interlayer liquid to be completely solidified increases with increasing orientation deviations.
Acknowledgments This work was financially supported by the National Basic Research Program (973 Program) of China under Grant Nos. 2010CB631200 and 2010CB631206, the National Natural Science Foundation of China (NSFC) under Grant Nos. 50971124, 50904059, 51071165 and 51204156. Fig. 4 Relationship of the width of the NSZ with the substrate orientation deviations.
relationship that the residual eutectic width increases with increasing relative angle can be seen. As has been introduced above, the thickness of the isothermal solidification zone can be expressed as Eq. (3). The thickness of the ISZ is determined by the coefficient k, the diffusion coefficient D (B in substrate) and the bonding time. The coefficient k is mainly determined by the composition of the residual liquid during isothermal solidification[22]. The factors that affect the interlayer liquid composition may change the coefficient k which affects the bonding width of the isothermal solidification. When the substrate orientation exists, the equilibrium interlayer liquid composition changes. This is due to the composition heterogeneity of the substrate between the dendrite and the interdendrite regions, and the introduction of the orientation changes the area of the liquid and the substrate. Also, the diffusion coefficient will decrease as the orientation deviation changes the B diffusion direction in the substrate. Furthermore, there are lots of precipitates in the substrate and these substrates will serve as the consumption of B in the path of the B diffusion into the substrate. These factors interact in the whole bonding stage, and finally change the growth of the ISZ. As observed from Fig. 3, the isothermal solidification time increases with increasing substrate orientation deviations. So when single crystal segments are bonded using TLP in engineering areas, more time is required for the complete solidification if angle deviations are introduced during the process. Such instructions may avoid the incomplete solidification of the interlayer liquid which will be detrimental to the mechanical behavior of the bonds. 4. Conclusions (1) Three regions exist in the bonding zone separated by the morphology: the non-isothermal solidification zone (NSZ), isothermal solidification zone (ISZ) and the diffusion affected zone (DAZ). There are lots of eutectics in the NSZ, and fine, short-bar or acicular precipitates in the DAZ.
REFERENCES [1] R.C. Reed, The Superalloys: Fundamentals and Applications, Cambridge University Press, Cambridge, 2006. [2] C.M. Biondo, P.B. Gardens, W.J. Gostic, C.D. Parmley, Repaired Nickel Based Superalloys, US Patent, No. 5783318, 1998. [3] M.A. Burke, P.D. Freyer, M.A. Hebbar, B.B. Seth, G.W. Swartzbeck, T.W. Zagar, Turbine Blades Made from Multiple Single Crystal Cast Superalloy Segments, US Patent, No. 6331217, 2001. [4] D.S. Duvall, W.A. Owczarski, D.F. Paulonis, Weld. J. 51 (1972) 41e49. [5] W.F. Gale, Y. Guan, Metall. Mater. Trans. A 27 (1996) 3621e3629. [6] W.F. Gale, Y. Xu, Metall. Mater. Trans. A 30 (1999) 2723e2726. [7] O.A. Idowu, O.A. Ojo, M.C. C, Metall. Mater. Trans. A 37 (2006) 2787e2796. [8] T.W. Eagar, Annu. Rev. Mater. Sci. 22 (1992) 23e46. [9] W.F. Gale, D.A. Butts, Sci. Technol. Weld. Join 9 (2004) 283e300. [10] S.K. Tung, L.C. Lim, M.O. Lai, Scripta Mater. 34 (1996) 763e769. [11] M. Pouranvari, A. Ekrami, A.H. Kokabi, J. Alloy. Compd. 469 (2009) 270e275. [12] A. Ghoneim, O.A. Ojo, Intermetallics 18 (2010) 582e586. [13] H. Kokawa, C.H. Lee, T.H. North, Metall. Mater. Trans. A 32 (1991) 1627e1631. [14] K. Ikeuchi, Y. Zhou, H. Kokawa, T.H. North, Metall. Mater. Trans. A 23 (2005) 2905e2915. [15] K. Saida, Y. Zhou, T.H. North, J. Mater. Sci. 28 (1993) 6427e6432. [16] M. Mosallaee, A. Ekarami, K. Ohsasa, K. Matsuura, Metall. Mater. Trans. A 39 (2008) 2389e2402. [17] S. Steuer, R.F. Singer, Metall. Mater. Trans. A 44 (2013) 2226e 2232. [18] N.C. Sheng, J.D. Liu, T. Jin, X.F. Sun, Z.Q. Hu, Metall. Mater. Trans. A 44 (2013) 1793e1804. [19] S. Omori, Nippon Kinzoku Gakkaishi 49 (1985) 935e939. [20] H.N. Ikawa, T.Y. Isai, Trans. Jpn. Weld. Soc. 10 (1979) 25e29. [21] Y.K. Nakao, K. Nishimoto, K.J. Shinozaki, C.Y. Kang, in: Superalloy, The Metallurgical Society, 1988. [22] I.T. Poku, M. Dollar, T.B. Massalski, Metall. Mater. Trans. A 19 (1988) 675e686. [23] S.H. Omori, K. Koyama, Y. Hashimoto, K. Yamada, J. Jpn. Inst. Met. 49 (1985) 935e939. [24] Y.Y. Chu, P. Ji, J. Ke, Acta Metall. Sin. (Engl. Lett.) 5 (1992) 87e93.