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IN718 system
Journal of Alloys and Compounds 685 (2016) 896e904
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
TLP bonding of IN738/MBF20/IN718 system A. Yarmou Shamsabadi a, R. Bakhtiari b, *, G. Eisaabadi B. c a
Department of Metallurgical Engineering, College of Engineering, Kermanshah Branch, Islamic Azad University, Kermanshah, Iran Department of Materials Engineering, Faculty of Engineering, Razi University, Kermanshah, Iran c Department of Materials Science and Engineering, Faculty of Engineering, Arak University, Arak, 38156-8-8349, Iran b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 December 2015 Received in revised form 16 June 2016 Accepted 19 June 2016 Available online 21 June 2016
The effects of temperature (1050, 1100 and 1150 C) and time (1e20 min) on the microstructure of the transient liquid phase (TLP) joints of IN-738/MBF-20/IN-718 system were investigated using OM, SEM and SEM/EDS analysis. During bonding, no pressure was applied and just the weight of the upper specimen was effective. A continuous eutectic structure was observed at the joint centerline of the samples bonded at 1050 C for 30 min. The same was seen at the joints made at 1100 C for 1 min which were detected as Cr- and Ni-borides. The morphology and composition of phases at diffusion affected zone (DAZ) were somewhat different in IN-718 and IN-738 halves. Increasing the bonding time at 1050 and 1100 C bonding temperatures increased the degree of isothermal solidification. In case of the samples, TLP-bonded at 1150 C, the diffusion of the liquid phase into the grain boundaries of the base metals occurred and a wavy bonding interface and no DAZ phases were observed. A critical bonding temperature (between 1100 and 1150 C) was seen above which the rate of isothermal solidification was reduced. Larson-Miller equation (P ¼ TB [C þ ln(tB)]) was used to estimate the required time for completion of the isothermal solidification. The calculated value of 33 for system-dependent constant C, resulted in the best estimate at different bonding temperatures. © 2016 Elsevier B.V. All rights reserved.
Keywords: TLP bonding Dissimilar joint IN-738 IN-718 Microstructure
1. Introduction Joining and assembling of the components used in gas turbines, such as those made of IN-718 and IN-738 superalloys, is necessary due to their complex shape and configuration. Due to exposure of these components at high temperatures, formation of thermal fatigue and corrosion cracks is common which needs repair processes involving joining methods [1]. Also, dissimilar joining is used to manufacture the parts with higher efficiency. Although, fusion welding, brazing and diffusion bonding are the most common joining processes for Ni-based superalloys [2], each of these have some limitations such as cracking at heat affected zone (HAZ), formation of brittle intermetallics at the joints and the need for using high pressure during diffusion bonding [3]. Therefore, transient liquid phase (TLP) bonding that is a combination of diffusion bonding and brazing is introduced as an alternative method for joining the superalloys. It is reported [4] that the bonding temperature and time significantly influence the microstructure and mechanical properties of the joints made using TLP. For example,
* Corresponding author. E-mail address: [email protected] (R. Bakhtiari). http://dx.doi.org/10.1016/j.jallcom.2016.06.185 0925-8388/© 2016 Elsevier B.V. All rights reserved.
the maximum shear strength of TLP joints of IN-738/MBF-80/FSX414 system was reported to be corresponded to the samples homogenized at 1200 C [5,6]. The present study aimed to investigate the influence of time and temperature on the microstructure of the dissimilar TLP joints of IN-738/MBF-20/IN-718 system. 2. Materials and methods The chemical composition of the as-received IN-718 and IN-738 superalloys and MBF-20 amorphous Ni-base interlayer used in this study are given in Table 1. The thickness of interlayer was 0.05 mm. Specimens from the base metals with dimensions of 10 5 5 mm were prepared using electro-discharge machining. The contact surface of the specimens was grounded using SiC paper (600 grit) and then ultrasonically cleaned in an acetone bath. High temperature steel fixture was used to fix the coupons and the interlayer during the TLP bonding (see Fig. 1). TLP bonding was conducted in an electrical furnace under a vacuum of 2 105 torr at 1050, 1100 and 1150 C for 1e120 min. During bonding, no pressure was applied and just the weight of the upper specimen was effective. The microstructure of the joints was studied using optical
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Table 1 Chemical composition of the base metals and interlayer (wt%).
IN-718 IN-738 MBF-20 [7]
Ni
Cr
Co
W
Fe
C
B
Mo
Ta
Nb
Al
Ti
Si
Bal Bal Bal
18 15.84 7.00
0.06 8.5 e
e 2.48 e
17.86 0.07 3
0.03 0.11 0.06
0.007 0.12 3.20
3.30 1.88 e
0.05 1.69 e
4.41 0.92 e
0.4 3.46 e
0.92 3.47
e e 4.50
microscopy, scanning electron microscopy (SEM) and SEM/EDS analysis. The IN-738 half of the joints were etched using Murakami etchant (10 g K3Fe(CN)6 þ 10 g KOH þ 100 ml H2O) and a different etchant (10 ml HNO3 þ 10 ml C2H4O2 þ 15 ml HCl) was used for etching the IN-718 half of the joints. 3. Results and discussion 3.1. Bonding at 1100 C 3.1.1. Optical microscope examination Fig. 2 shows the optical micrograph of the joint made at 1100 C for 1 min. As can be seen, the bonding region includes two distinct zones as isothermal solidification zone (ISZ) and athermal solidification zone. At bonding temperature, which is higher than the interlayer liquidus temperature, the interlayer melts. Diffusion of melting point depressant (MPD) elements such as B and Si from the molten interlayer into the base metals increases the melting point of the interlayer which causes isothermal solidification at the bonding temperature and forms ISZ. During isothermal solidification, lack of solute atoms segregation at solid/liquid interface prevents the formation of new phases and results in a unique microstructure (austenitic solid solution, g) in ISZ. During cooling from the bonding temperature, the retained liquid of the interlayer solidifies athermally which forms ASZ including various phases [2,8]. During athermal solidification, B (as a MPD element) segregates into the liquid phase and shifts the chemical composition of the remaining liquid toward the eutectic point (CE). Eventually, the remaining liquid solidifies during eutectic transformation at which compounds such as brittle borides could form. The MPD element of the interlayer (B) diffuses into the base metal either during solution of the base metal or during isothermal solidification. Once the concentration of the B in the base metal becomes larger than its solid solubility limit (CS), the precipitation of secondary phase occurs. Therefore, the microstructure of diffusion affected zone (DAZ) is affected by diffusion of MPD element
Fig. 1. Schematic arrangement of the IN-738/MBF-80/IN-718 system for TLP bonding.
from the interlayer into the base metal. Therefore, the width of DAZ is proportional with a zone in which B content is larger than the solid solubility limit [2,8,9]. Fig. 3 is another micrograph from the samples bonded at 1100 C/1 min condition. According to this figure, boride precipitated at grain boundaries of base metals that are 50 mm from the interface of ISZ/base metal. This suggests that the formation of the borides and therefore the width of DAZ are controlled by diffusion of B within the base metals. It must be noted that the width of DAZ is not affected by diffusion of Si [2] due to (a) considerably higher solubility limit of the Si in Ni compared to that of B [13] and (b) the significantly smaller diffusion coefficient of Si in Ni (3.09 1014 m2s1) compared to that of B in Ni (6.22 1011 m2s1) that prevents the precipitation of silicide in the base metals. Gale and Wallach [14] examined the TLP bonding of pure Ni using NieSieB interlayer at 1065 C and reported the formation of Ni23B6 in the DAZ. Also, they stated that borides form during the soaking of the samples at bonding temperature and not during the subsequent cooling. 3.1.2. SEM examination SEM micrograph of the joint made at 1100 C/1min is shown in Fig. 4 and the EDS analysis of the selected points is given in Table 2. During short time bonding (1100 C/1min), limited diffusion of B and Si into the base metal forms a layer of gamma solid solution phase adjacent to the base metal toward the center of bonding zone. The microstructural evolution at ASZ is controlled by formation of dendrites of gamma phase and segregation of solute atoms during athermal solidification. Table 2 shows that the eutectic compounds of points B and C formed during segregation of boron with other elements of interlayer such as Ni and Cr. Also, observation of base metal alloying elements such as Co, Nb and Mo at points B and C shows the diffusion of these elements into the molten interlayer which affected the phase formation. Continuous enrichment of interdendritic liquid in the interlayer with solute elements of the base metal increases the concentration of these elements in the
Fig. 2. Optical micrograph of the joint made at 1100 C for 1 min.
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Fig. 3. Optical micrograph of the joint made at 1100 C for 1 min showing boride grain boundaries.
interlayer to values higher than their solid solubility limit in ISZ. Therefore, eutectic compounds (points B and C in Fig. 4) can form during athermal solidification of the retained liquid [2]. The accurate determination of compounds of points B and C needs TEM studies. According to Table 2, these phases could be Cr- and Niborides, respectively. Idowu et al. [10] indicated the presence of Ni23B6 and Cr5B3 eutectic compounds at the joint centerline of TLP bonded IN738LC superalloy. Ohsasa et al. [11] reported similar eutectic microstructure in TLP bonding zone of Ni using Ni-15.2 wt% Cr-4.0 wt% B ternary interlayer. Also, during the TLP bonding, the dendrites of gamma formed as the primary phase and with cooling of bonding temperature, L/g þ Ni3 B eutectic reaction occurs at 1042 C. Finally, the solidification completed through a ternary eutectic reaction, L/g þ Ni3 B þ CrB, at 997 C. The EDS analysis in Table 2, indicates that ISZ zones in Fig. 2b (i.e. zone A) including austenitic solid solution formed during TLP bonding of IN718/MBF20/IN738 system due to depletion of the interlayer from Si and B. The driving force for isothermal solidification and formation of Ni-base austenitic solid solution is depletion of interlayer from MPD elements that subsequently increases the melting point of interlayer and results in isothermal solidification during holding at bonding temperature [12]. Isothermal solidification occurred alongside with depletion of base metals
Fig. 4. (a) SE and (b) BSE images of the joint made at 1100 C for 1 min.
Table 2 SEM/EDS analysis of selected points in Figs. 4a, 5 and 6 and 9. Figure
Point (zone)
Ni
Al
Ti
Cr
Fe
Co
Ta
Nb
Mo
W
Si
Fig. 4a
Matrix (A) Cr-boride (B) Ni- boride (C) Needle-like particles (A) Black particles (B) White particles (C) Needle-like particles (A) Blocky particles (B) A B C D E
69.36 8.49 83.92 60.78 64.20 43.97 40.74 54.55 08.45 81.69 55.32 17.75 66.47
0.83 e 0.64 2.13 1.79 3.42 0.30 0.54 0.11 0.75 e 0.08 0.89
0.67 0.23 3.14 2.03 2.47 2.46 0.91 0.23 0.36 2.75 3.51 1.12 1.13
14.01 85.2 6.84 17.56 14.61 25.53 26.04 18.23 83.20 8.22 2.54 46.95 14.78
7.46 1.13 0.31 0.39 0.01 0.02 14.04 16.97 1.42 0.37 1.27 5.23 8.01
2.01 0.53 1.56 10.54 9.84 5.26 e e 0.76 1.61 0.95 0.67 2.42
e e e e e e e 1.08 e e e e e
0.78 e 2.51 1.50 0.66 4.67 10.41 4.82 0.09 1.76 22.63 12.58 1.23
0.54 3.09 0.38 1.61 0.01 4.72 6.72 3.55 4.21 0.63 e 14.33 0.66
1.53 0.43 e 3.15 5.19 9.94 e e 0.56 1.23 3.20 0.05 1.60
2.81 0.9 0.70 0.31 1.22 0.01 0.84 0.03 0.84 0.99 10.58 1.24 2.81
Fig. 5
Fig. 6 Fig. 9
Fig. 5. (a) BSE and (b) SE image of the DAZ in IN-738 half of the joint made at 1100 C for 1 min.
Fig. 6. (a) BSE and (b) SE image of the DAZ in IN-718 half of the joint made at 1100 C for one minute.
Fig. 7. Optical micrograph of the joint made at 1100 C for: (a) 15, (b) 30 and (c) 60 min.
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borides and the needle-like ones could be Al, Co, Ni, Cr and Ti borides. Comparison of Figs. 5 and 6 indicates that the thickness of needle-like precipitates and size of the blocky precipitates in IN718 half of the joint is significantly larger than those of IN-738 half. 3.2. Effect of bonding time at 1100 C 3.2.1. Optical microscope examination The effect of bonding time on the microstructure of the joint made at 1100 C is depicted in Fig. 7. According to Fig. 7a, while a considerable amount of eutectic is present in the joint made for 15 min, the amount of eutectic phases decreased significantly with increasing the bonding time to 30 min. No eutectic was observed in the joint made for 60 min that indicates completion of the isothermal solidification. Also, comparison of series of images in Fig. 7 shows that ISZ became wider with increasing the bonding time that is due to the more depletion of interlayer from MPD elements. 3.2.2. SEM examination Fig. 8a and b shows the microstructure of a sample that was cooled after bonding for 15 min at 1100 C (before complete isothermal solidification). It can be seen that athermal solidification of the retained liquid resulted in formation of intermetallic compounds surrounded by gamma phase at the joint centerline. The EDS/line scan analysis across the bonding zone in Fig. 8b shows that the concentration of Cr and Ni within the interlayer is very different from those of the DAZ indicating insufficient interdiffusion between the interlayer and base metals during 15 min bonding. SEM micrograph of the sample bonded at 1100 C for 30 min is shown in Fig. 9 and the EDS analysis of the selected points are presented in Table 2. The eutectic structure that indicate the incomplete isothermal solidification can be seen in the joint centerline and adjacent to the IN-718 base metal. It is very well known that isothermal solidification is controlled by the diffusion of the MPD elements in base metals. Therefore, the presence of eutectic structure adjacent to the IN-718 half suggests that the diffusion coefficient of B in IN-718 is smaller than that in IN-738. From the results of EDS analysis in Table 2, points A and B are Ni and Cr-borides and points C and D are Cr, Mo and Nb-borides (similar to those observed for the joints made for 15 min). Fig. 8. (a) SE images from the joint made at 1100 C for 15 min and (b) the EDS line scan analysis of the dotted line in (a).
from alloying elements such as Al, Mo, Cr, Ti and Co and their subsequent diffusion into the interlayer. The influence of each element on equilibrium melting point of the interlayer depends on the distribution coefficient (K) [12]. Therefore, the diffusion of Cr and Fe with K > 1 into the interlayer and depletion of the interlayer from Si and B with K < 1 increases the equilibrium melting point of the interlayer and results in formation of ISZ. The SEM images of DAZ of IN-738 and IN-718 halves of the samples bonded at 1100 C for one minute are shown in Figs. 5 and 6, respectively. The EDS analysis of the indicated points in Figs. 5 and 6 are shown in Table 2. Two types of phases (blocky and needle-like) precipitated in IN-738 half of the joint (Fig. 4). According to the EDS analysis in Table 2, each of the phases is mainly Ni, Cr, Co and partly Mo, Nb-enriched. Nb, Mo and Cr are strong boride formers. Idowu et al. [10] for TLP joints of IN738LC superalloy reported that the blocky precipitates could be W, Mo and Cr
3.3. Bonding at 1050 C 3.3.1. Optical microscope examination The effect of bonding time on microstructure of the joint made at 1050 C is shown in Fig. 10. From Fig. 10a, the presence of eutectic structure in the centerline of the sample bonded for 30 min indicates incomplete isothermal solidification. Also, no eutectic was observed in the microstructure of the joints made for 75 and 120 min. Therefore, isothermal solidification has been completed for these joints due to the longer time that was available for diffusion of MPD elements into the base metals. Fick’s second law for concentration of the MPD element in the base metal is written as [8]:
( " # " #) 1 xþw xw Cðx; tÞ ¼ Cm þ ðC0 Cm Þ erf erf 1 1 2 ð4DtÞ2 ð4DtÞ2
(1)
where Cm and C0 are the initial concentration of MPD element in the base metal and interlayer, respectively; C(x,t) is the MPD element concentration at time (t) and distance (x) from the joint
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901
Fig. 9. SEM micrograph of the joint made at 1100 C for 30 min showing eutectic structure: (a) at the joint centerline and (b) adjacent to IN-718.
centerline and D is diffusion coefficient of the MPD element in the base metal. According to Equation (1), at a constant bonding time, the increase in the bonding temperature from 1050 to 1100 C (i.e. increasing the diffusion coefficient, D) reduces the concentration of the MPD element, C(x,t) and subsequently reduces the fraction of eutectic in the joint centerline. Comparison of Figs. 7b and 10a confirms that width of the eutectic zone was decreased (i. e. the rate of isothermal solidification increased) with increasing the bonding temperature from 1050 C to 1100 C. 3.3.2. SEM examination Fig. 11 shows EDS/line scan analysis across the joint made at 1050 C for 75 min. The figure shows that the concentrations of alloying elements, especially that of Cr and Ni, are uniform across the bonding zone that indicates the presence of no eutectic or intermetallic. This is due to the equilibrium nature of isothermal solidification that prevents segregation of the alloying elements at solid/liquid interface and subsequent formation of undesired intermetallics across the bonding zone. 3.4. Bonding at 1150 C 3.4.1. Optical microscope examination Fig. 12 shows the microstructure of the TLP joint made at 1150 C
for 30 min. Comparison of Fig. 12 with Figs. 7b and 10b reveals a considerable difference between the microstructure of the samples bonded at this temperature and those bonded at 1050 C and 1100 C. The differences can be summarized as the followings: - Liquefaction phases (Fig. 12a) formed during diffusion of molten interlayer into the partial liquated grain boundaries in the base metals. Therefore, 1150 C could be higher than the base metals liquefaction temperature. - Formation of a continuous eutectic structure at the interface of base metals/bonding zone (Fig. 12b) at 1150 C, whereas, these phases were seen as isolated or continuous ones in the joint centerline at other bonding temperatures. - Needle-like or blocky phases at DAZ of samples bonded at 1050 C and 1100 C were replaced by grain boundary phases at 1150 C (Fig. 12a) - The interface of base metals/bonding zone was wavy at 1150 C (Fig. 12c) in comparison with nearly straight one at other bonding temperatures. The micrograph of the joint made at 1150 C for 60 min in Fig. 13 reveals incomplete isothermal solidification and formation of eutectic compounds. However, it can be seen that the diffusion of B into the base metal resulted in complete isothermal solidification in
Fig. 10. Optical micrograph of the joint made at 1050 C for (a) 30, (b) 75 and (c) 120 min.
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Fig. 11. EDS/line scan analysis across the joint made at 1050 C for 75 min.
joint made at 1150 C for 30 min are presented in Fig. 14. The presence of Si indicates the penetration of the molten interlayer to the zones A, B and C. Furthermore, the concentration of Ta and Ti in zone A, W, Mo, Cr and Nb in zone B and Cr, W and Mo in zone C are considerably higher than those in the base metals. Therefore, it can be concluded that the unusual chemical composition of these zones is due to the mixing of the molten interlayer and melts from the partial liquation of the base metals. The high content of Ti, W and Ta at zones A, B and C could not be due to solid-state diffusion and shows the possibility of liquefaction or partial re-melting at the grain boundaries of the base metals. Zone D (i.e. isothermal solidification zone) shows the Ni-base solid solution formed during the isothermal solidification of the molten interlayer. The higher concentration of the Ni and Si in this zone compared to those of the joints made at 1050 and 1100 C is probably due to the higher dissolution of the base metal and higher diffusion of alloying elements at 1150 C compared to those at the lower bonding temperatures. It has been reported that formation of g g0 eutectic occurs due to Ti segregation during athermal solidification. Cr, Mo and W have low solubility in g0 as the main constituent of the g g0 eutectic. Therefore, during the eutectic reaction, these elements segregate into the retained liquid. Enrichment of the retained liquid from Cr that is a strong boride-former explains the formation of Cr-borides [15,16].
the marked area [6,17].
3.5. Effect of bonding temperature and time on isothermal solidification
3.4.2. SEM examination The SEM image and the EDS analysis of the selected zones of the
Comparison of the microstructure of the joints made for 60 min indicates formation of eutectic compounds at 1150 C
Fig. 12. Optical micrograph of the joint made at 1150 C for 30 min.
Fig. 13. Optical micrographs of the joint made at 1150 C for 60 min (arrows show ISZ in base metals).
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Fig. 14. SEM micrograph of the joint made at 1150 C for 30 min: (a) BSE and (b) SE.
(Fig. 13) and eutectic-free joint at 1100 C (Fig. 7). Therefore, increasing the bonding temperature from 1100 to 1150 C increased the time needed to complete isothermal solidification and therefore decreased rate of the isothermal solidification. Therefore, it can be concluded that a critical bonding temperature (between 1100 and 1150 C) exists for IN718/MBF20/IN738 system above which increasing the bonding temperature will decrease the rate of isothermal solidification. This can be explained with increased dissolution of base metal with increasing temperature, altering the elements that control the rate of isothermal solidification [16,18] and reduced diffusion rate of the MPD element from the molten interlayer into the base metals [18]. It must be noted that according to the Fick’s second law of diffusion, vC=vt ¼ Dv2 C=vX 2 , the change in the concentration with time ðvC=vtÞ depends on both the concentration gradient ðv2 C=vX 2 Þ and diffusion coefficient of MPD element in the base metals (D).
Increasing the bonding temperature increases the diffusion coefficient and decreases the v2 C=vX 2 . When the temperature is lower than the critical bonding temperature (1050 and 1100 C), the effect of increasing the temperature on increase of the diffusion coefficient is more significant than the decrease of v2 C=vX 2 and therefore, increasing the temperature increases the rate of isothermal solidification. On the other hand, at temperatures higher than the critical temperature (1150 C), the decrease in v2 C=vX 2 is more significant than the increase in diffusion coefficient and therefore, further increase of the temperature will decrease the rate of isothermal solidification [19,20]. The minimum required time for isothermal (and therefore, eutectic free) solidification is of great importance. Fig. 15 shows the variation of size of the ASZ with bonding time at 1050 and 1100 C. It can be seen that the size of ASZ decreased with increasing the bonding time at the both temperatures. According to this figure,
Fig. 15. Variation of ASZ size with bonding temperature.
Fig. 16. Variation of ISZ size with Larson-Miller parameter (P).
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rate of reduction in size of the ASZ is more significant at 1100 C compared to that of 1050 C.
- The best estimate of the time required for the complete isothermal solidification of the examined system corresponded to the constant (C) value of 33 in Larson-Miller equation.
3.6. Parametric study of the isothermal solidification Acknowledgment Since the isothermal solidification rate is controlled by diffusion of MPD elements into the base metal that subsequently depends on the bonding temperature and time, a parametric approach was used to analyze the effect of bonding time and temperature on the width of ISZ using the Larsson-Miller parameter P. This parameter is given by P ¼ T B ½C þ lnðt B Þ, Where T B is the bonding temperature in Kelvin, t is the bonding time in minutes and C is a systemdependent constant. The parameter P is generally used for analyzing creep rupture data [21]. The values of ISZ width for the samples bonded at 1050, 1100 and 1150 C were plotted as a function of the parameter P using different values for C, and the quality of fitting was judged from the extent of the data points scatter. It was found that the best fitting and the minimum scatter for the ISZ data was obtained with a C value of 33 (Fig. 16). The linear relationship between the width of ISZ and LMP in Fig. 16 indicates that Larson-Miller parameter can be used as a criterion for prediction of the ISZ width. The required time for complete isothermal solidification of the joints made at different bonding temperatures can be estimated using LarssonMiller parameter. 4. Conclusions Dissimilar TLP joints of IN-738/IN-718 system were made using MBF-20 interlayer at different bonding temperatures and times. The results of microstructure studies showed that: - For the joints made at 1100 C/1min, Cr- and Ni-borides were observed as the eutectic structure. DAZ in IN718 half was enriched from Nb, Mo and Cr and DAZ in IN738 half was enriched from W, Mo and Cr borides. With increasing bonding time at 1100 C, volume fraction of eutectic phases at the joint centerline decreased and ISZ became wider. - For the joints made at 1050 C, continuous centerline eutectics were formed during 30 min bonding. In comparison with the joints made at 1100 C for the same time, the eutectic structure of the sample bonded at 1050 C was thinner. Further increase of the bonding time to 75 and 120 min at 1050 C, resulted in more complete isothermal solidification. - For the joints made at 1150 C for 30 min, the diffusion of liquid interlayer into the grain boundaries of base metals occurred, no DAZ was observed and the bonding interface became wavy. Even after bonding for 60 min, the isothermal solidification was not completed. - A critical bonding temperature (between 1100 and 1150 C) was observed below which increasing the bonding temperature increased the rate of isothermal solidification and above which, this rate decreased. - The Larson-Miller parametric approach can be used for studying the effect of bonding time and temperature on the ISZ width of the IN738/MBF-20/IN718 system.
The authors would like to acknowledge Islamic Azad University, Kermanshah Branch for financial supports of this research. References [1] O.A. Ojo, On Liquation Cracking of Cast Inconel 738LC Superalloy Welds, Ph.D. Dissertation, University of Manitoba, Canada, 2004. [2] M. Pouranvari, A. Ekrami, A. Kokabi, Transient liquid phase bonding of wrought IN718 nickel based superalloy using standard heat treatment cycles: microstructure and mechanical properties, Mater. Des. 50 (2013) 694e701. [3] A. Rabinkin, Brazing with (NiCoCr)eBeSi amorphous brazing filler metals: alloys, processing, joint structure, properties, applications, Sci. Technol. Weld. Join. 9 (3) (2004) 181e199. [4] R. Bakhtiari, A. Ekrami, T. Khan, Microstructure-mechanical properties relation of TLP-bonded FSX-414 superalloy: effect of homogenization design, J. Mater. Eng. Perform. 24 (4) (2015) 1687e1696. [5] B.A. Khazaei, G. Asghari, R. Bakhtiari, TLP bonding of dissimilar FSX-414/IN738 system with MBF80 interlayer: prediction of solid/liquid interface location, Trans. Nonferrous Metals Soc. China 24 (4) (2014) 996e1003. [6] B. Abbasi-Khazaei, A. Jahanbakhsh, R. Bakhtiari, TLP bonding of dissimilar FSX414/IN-738 system with MBF-80 interlayer: the effect of homogenizing treatment on microstructure and mechanical properties, Mater. Sci. Eng. A 651 (2016) 93e101. [7] High performance brazing filler metal. http://www.metglas.com. [8] M. Arafin, et al., Transient liquid phase bonding of Inconel 718 and Inconel 625 with BNi-2: modeling and experimental investigations, Mater. Sci. Eng. A 447 (1) (2007) 125e133. [9] A. Ghoneim, O. Ojo, Understanding reversed temperature dependence of diffusional solidification time in single crystal superalloy brazement, Philos. Mag. 91 (28) (2011) 3649e3666. [10] O. Idowu, O. Ojo, M. Chaturvedi, Microstructural study of transient liquid phase bonded cast INCONEL 738LC superalloy, Metallurgical Mater. Trans. A 37 (9) (2006) 2787e2796. [11] K. Ohsasa, T. Narita, T. Shinmura, Numerical modeling of the transient liquid phase bonding process of Ni using Ni-B-Cr ternary filler metal, J. Phase Equilibria 20 (3) (1999) 199e206. [12] M. Pouranvari, A. Ekrami, A. Kokabi, TLP bonding of cast IN718 nickel based superalloy: processemicrostructureestrength characteristics, Mater. Sci. Eng. A 568 (2013) 76e82. [13] S.D. Henry, J.M. Davidson, M.A. Fleming, Binary alloy phase diagrams, in: H.O.H. Baker (Ed.), Alloy phase Diagrams, ASM International, Ohio, 1992, pp. 118e156. [14] W. Gale, E. Wallach, Microstructural development in transient liquid-phase bonding, Metall. Trans. A 22 (10) (1991) 2451e2457. [15] V. Jalilvand, et al., Microstructural evolution during transient liquid phase bonding of Inconel 738LC using AMS 4777 filler alloy, Mater. Charact. 75 (2013) 20e28. [16] O. Idowu, N. Richards, M. Chaturvedi, Effect of bonding temperature on isothermal solidification rate during transient liquid phase bonding of Inconel 738LC superalloy, Mater. Sci. Eng. A 397 (1) (2005) 98e112. [17] R. Bakhtiari, A. Ekrami, T. Khan, The effect of TLP bonding temperature on microstructural and mechanical property of joints made using FSX-414 superalloy, Mater. Sci. Eng. A 546 (2012) 291e300. [18] A. Shulga, Boron and carbon behavior in the cast Ni-base superalloy EP962, J. Alloys Compd. 436 (1) (2007) 155e160. [19] N. Wikstrom, A. Egbewande, O. Ojo, High temperature diffusion induced liquid phase joining of a heat resistant alloy, J. Alloys Compd. 460 (1) (2008) 379e385. [20] M.M. Atabaki, Microstructural evolution in the partial transient liquid phase diffusion bonding of Zircaloy-4 to stainless steel 321 using active titanium filler metal, J. Nucl. Mater. 406 (3) (2010) 330e344. [21] S. Venkadesan, et al., Effect of ageing on the microstructural stability of coldworked titanium-modified 15Cr-15Nid2.5 Mo austenitic stainless steel, J. Nucl. Mater. 186 (2) (1992) 177e184.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
TLP bonding of IN738/MBF20/IN718 system A. Yarmou Shamsabadi a, R. Bakhtiari b, *, G. Eisaabadi B. c a
Department of Metallurgical Engineering, College of Engineering, Kermanshah Branch, Islamic Azad University, Kermanshah, Iran Department of Materials Engineering, Faculty of Engineering, Razi University, Kermanshah, Iran c Department of Materials Science and Engineering, Faculty of Engineering, Arak University, Arak, 38156-8-8349, Iran b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 December 2015 Received in revised form 16 June 2016 Accepted 19 June 2016 Available online 21 June 2016
The effects of temperature (1050, 1100 and 1150 C) and time (1e20 min) on the microstructure of the transient liquid phase (TLP) joints of IN-738/MBF-20/IN-718 system were investigated using OM, SEM and SEM/EDS analysis. During bonding, no pressure was applied and just the weight of the upper specimen was effective. A continuous eutectic structure was observed at the joint centerline of the samples bonded at 1050 C for 30 min. The same was seen at the joints made at 1100 C for 1 min which were detected as Cr- and Ni-borides. The morphology and composition of phases at diffusion affected zone (DAZ) were somewhat different in IN-718 and IN-738 halves. Increasing the bonding time at 1050 and 1100 C bonding temperatures increased the degree of isothermal solidification. In case of the samples, TLP-bonded at 1150 C, the diffusion of the liquid phase into the grain boundaries of the base metals occurred and a wavy bonding interface and no DAZ phases were observed. A critical bonding temperature (between 1100 and 1150 C) was seen above which the rate of isothermal solidification was reduced. Larson-Miller equation (P ¼ TB [C þ ln(tB)]) was used to estimate the required time for completion of the isothermal solidification. The calculated value of 33 for system-dependent constant C, resulted in the best estimate at different bonding temperatures. © 2016 Elsevier B.V. All rights reserved.
Keywords: TLP bonding Dissimilar joint IN-738 IN-718 Microstructure
1. Introduction Joining and assembling of the components used in gas turbines, such as those made of IN-718 and IN-738 superalloys, is necessary due to their complex shape and configuration. Due to exposure of these components at high temperatures, formation of thermal fatigue and corrosion cracks is common which needs repair processes involving joining methods [1]. Also, dissimilar joining is used to manufacture the parts with higher efficiency. Although, fusion welding, brazing and diffusion bonding are the most common joining processes for Ni-based superalloys [2], each of these have some limitations such as cracking at heat affected zone (HAZ), formation of brittle intermetallics at the joints and the need for using high pressure during diffusion bonding [3]. Therefore, transient liquid phase (TLP) bonding that is a combination of diffusion bonding and brazing is introduced as an alternative method for joining the superalloys. It is reported [4] that the bonding temperature and time significantly influence the microstructure and mechanical properties of the joints made using TLP. For example,
* Corresponding author. E-mail address: [email protected] (R. Bakhtiari). http://dx.doi.org/10.1016/j.jallcom.2016.06.185 0925-8388/© 2016 Elsevier B.V. All rights reserved.
the maximum shear strength of TLP joints of IN-738/MBF-80/FSX414 system was reported to be corresponded to the samples homogenized at 1200 C [5,6]. The present study aimed to investigate the influence of time and temperature on the microstructure of the dissimilar TLP joints of IN-738/MBF-20/IN-718 system. 2. Materials and methods The chemical composition of the as-received IN-718 and IN-738 superalloys and MBF-20 amorphous Ni-base interlayer used in this study are given in Table 1. The thickness of interlayer was 0.05 mm. Specimens from the base metals with dimensions of 10 5 5 mm were prepared using electro-discharge machining. The contact surface of the specimens was grounded using SiC paper (600 grit) and then ultrasonically cleaned in an acetone bath. High temperature steel fixture was used to fix the coupons and the interlayer during the TLP bonding (see Fig. 1). TLP bonding was conducted in an electrical furnace under a vacuum of 2 105 torr at 1050, 1100 and 1150 C for 1e120 min. During bonding, no pressure was applied and just the weight of the upper specimen was effective. The microstructure of the joints was studied using optical
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Table 1 Chemical composition of the base metals and interlayer (wt%).
IN-718 IN-738 MBF-20 [7]
Ni
Cr
Co
W
Fe
C
B
Mo
Ta
Nb
Al
Ti
Si
Bal Bal Bal
18 15.84 7.00
0.06 8.5 e
e 2.48 e
17.86 0.07 3
0.03 0.11 0.06
0.007 0.12 3.20
3.30 1.88 e
0.05 1.69 e
4.41 0.92 e
0.4 3.46 e
0.92 3.47
e e 4.50
microscopy, scanning electron microscopy (SEM) and SEM/EDS analysis. The IN-738 half of the joints were etched using Murakami etchant (10 g K3Fe(CN)6 þ 10 g KOH þ 100 ml H2O) and a different etchant (10 ml HNO3 þ 10 ml C2H4O2 þ 15 ml HCl) was used for etching the IN-718 half of the joints. 3. Results and discussion 3.1. Bonding at 1100 C 3.1.1. Optical microscope examination Fig. 2 shows the optical micrograph of the joint made at 1100 C for 1 min. As can be seen, the bonding region includes two distinct zones as isothermal solidification zone (ISZ) and athermal solidification zone. At bonding temperature, which is higher than the interlayer liquidus temperature, the interlayer melts. Diffusion of melting point depressant (MPD) elements such as B and Si from the molten interlayer into the base metals increases the melting point of the interlayer which causes isothermal solidification at the bonding temperature and forms ISZ. During isothermal solidification, lack of solute atoms segregation at solid/liquid interface prevents the formation of new phases and results in a unique microstructure (austenitic solid solution, g) in ISZ. During cooling from the bonding temperature, the retained liquid of the interlayer solidifies athermally which forms ASZ including various phases [2,8]. During athermal solidification, B (as a MPD element) segregates into the liquid phase and shifts the chemical composition of the remaining liquid toward the eutectic point (CE). Eventually, the remaining liquid solidifies during eutectic transformation at which compounds such as brittle borides could form. The MPD element of the interlayer (B) diffuses into the base metal either during solution of the base metal or during isothermal solidification. Once the concentration of the B in the base metal becomes larger than its solid solubility limit (CS), the precipitation of secondary phase occurs. Therefore, the microstructure of diffusion affected zone (DAZ) is affected by diffusion of MPD element
Fig. 1. Schematic arrangement of the IN-738/MBF-80/IN-718 system for TLP bonding.
from the interlayer into the base metal. Therefore, the width of DAZ is proportional with a zone in which B content is larger than the solid solubility limit [2,8,9]. Fig. 3 is another micrograph from the samples bonded at 1100 C/1 min condition. According to this figure, boride precipitated at grain boundaries of base metals that are 50 mm from the interface of ISZ/base metal. This suggests that the formation of the borides and therefore the width of DAZ are controlled by diffusion of B within the base metals. It must be noted that the width of DAZ is not affected by diffusion of Si [2] due to (a) considerably higher solubility limit of the Si in Ni compared to that of B [13] and (b) the significantly smaller diffusion coefficient of Si in Ni (3.09 1014 m2s1) compared to that of B in Ni (6.22 1011 m2s1) that prevents the precipitation of silicide in the base metals. Gale and Wallach [14] examined the TLP bonding of pure Ni using NieSieB interlayer at 1065 C and reported the formation of Ni23B6 in the DAZ. Also, they stated that borides form during the soaking of the samples at bonding temperature and not during the subsequent cooling. 3.1.2. SEM examination SEM micrograph of the joint made at 1100 C/1min is shown in Fig. 4 and the EDS analysis of the selected points is given in Table 2. During short time bonding (1100 C/1min), limited diffusion of B and Si into the base metal forms a layer of gamma solid solution phase adjacent to the base metal toward the center of bonding zone. The microstructural evolution at ASZ is controlled by formation of dendrites of gamma phase and segregation of solute atoms during athermal solidification. Table 2 shows that the eutectic compounds of points B and C formed during segregation of boron with other elements of interlayer such as Ni and Cr. Also, observation of base metal alloying elements such as Co, Nb and Mo at points B and C shows the diffusion of these elements into the molten interlayer which affected the phase formation. Continuous enrichment of interdendritic liquid in the interlayer with solute elements of the base metal increases the concentration of these elements in the
Fig. 2. Optical micrograph of the joint made at 1100 C for 1 min.
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Fig. 3. Optical micrograph of the joint made at 1100 C for 1 min showing boride grain boundaries.
interlayer to values higher than their solid solubility limit in ISZ. Therefore, eutectic compounds (points B and C in Fig. 4) can form during athermal solidification of the retained liquid [2]. The accurate determination of compounds of points B and C needs TEM studies. According to Table 2, these phases could be Cr- and Niborides, respectively. Idowu et al. [10] indicated the presence of Ni23B6 and Cr5B3 eutectic compounds at the joint centerline of TLP bonded IN738LC superalloy. Ohsasa et al. [11] reported similar eutectic microstructure in TLP bonding zone of Ni using Ni-15.2 wt% Cr-4.0 wt% B ternary interlayer. Also, during the TLP bonding, the dendrites of gamma formed as the primary phase and with cooling of bonding temperature, L/g þ Ni3 B eutectic reaction occurs at 1042 C. Finally, the solidification completed through a ternary eutectic reaction, L/g þ Ni3 B þ CrB, at 997 C. The EDS analysis in Table 2, indicates that ISZ zones in Fig. 2b (i.e. zone A) including austenitic solid solution formed during TLP bonding of IN718/MBF20/IN738 system due to depletion of the interlayer from Si and B. The driving force for isothermal solidification and formation of Ni-base austenitic solid solution is depletion of interlayer from MPD elements that subsequently increases the melting point of interlayer and results in isothermal solidification during holding at bonding temperature [12]. Isothermal solidification occurred alongside with depletion of base metals
Fig. 4. (a) SE and (b) BSE images of the joint made at 1100 C for 1 min.
Table 2 SEM/EDS analysis of selected points in Figs. 4a, 5 and 6 and 9. Figure
Point (zone)
Ni
Al
Ti
Cr
Fe
Co
Ta
Nb
Mo
W
Si
Fig. 4a
Matrix (A) Cr-boride (B) Ni- boride (C) Needle-like particles (A) Black particles (B) White particles (C) Needle-like particles (A) Blocky particles (B) A B C D E
69.36 8.49 83.92 60.78 64.20 43.97 40.74 54.55 08.45 81.69 55.32 17.75 66.47
0.83 e 0.64 2.13 1.79 3.42 0.30 0.54 0.11 0.75 e 0.08 0.89
0.67 0.23 3.14 2.03 2.47 2.46 0.91 0.23 0.36 2.75 3.51 1.12 1.13
14.01 85.2 6.84 17.56 14.61 25.53 26.04 18.23 83.20 8.22 2.54 46.95 14.78
7.46 1.13 0.31 0.39 0.01 0.02 14.04 16.97 1.42 0.37 1.27 5.23 8.01
2.01 0.53 1.56 10.54 9.84 5.26 e e 0.76 1.61 0.95 0.67 2.42
e e e e e e e 1.08 e e e e e
0.78 e 2.51 1.50 0.66 4.67 10.41 4.82 0.09 1.76 22.63 12.58 1.23
0.54 3.09 0.38 1.61 0.01 4.72 6.72 3.55 4.21 0.63 e 14.33 0.66
1.53 0.43 e 3.15 5.19 9.94 e e 0.56 1.23 3.20 0.05 1.60
2.81 0.9 0.70 0.31 1.22 0.01 0.84 0.03 0.84 0.99 10.58 1.24 2.81
Fig. 5
Fig. 6 Fig. 9
Fig. 5. (a) BSE and (b) SE image of the DAZ in IN-738 half of the joint made at 1100 C for 1 min.
Fig. 6. (a) BSE and (b) SE image of the DAZ in IN-718 half of the joint made at 1100 C for one minute.
Fig. 7. Optical micrograph of the joint made at 1100 C for: (a) 15, (b) 30 and (c) 60 min.
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borides and the needle-like ones could be Al, Co, Ni, Cr and Ti borides. Comparison of Figs. 5 and 6 indicates that the thickness of needle-like precipitates and size of the blocky precipitates in IN718 half of the joint is significantly larger than those of IN-738 half. 3.2. Effect of bonding time at 1100 C 3.2.1. Optical microscope examination The effect of bonding time on the microstructure of the joint made at 1100 C is depicted in Fig. 7. According to Fig. 7a, while a considerable amount of eutectic is present in the joint made for 15 min, the amount of eutectic phases decreased significantly with increasing the bonding time to 30 min. No eutectic was observed in the joint made for 60 min that indicates completion of the isothermal solidification. Also, comparison of series of images in Fig. 7 shows that ISZ became wider with increasing the bonding time that is due to the more depletion of interlayer from MPD elements. 3.2.2. SEM examination Fig. 8a and b shows the microstructure of a sample that was cooled after bonding for 15 min at 1100 C (before complete isothermal solidification). It can be seen that athermal solidification of the retained liquid resulted in formation of intermetallic compounds surrounded by gamma phase at the joint centerline. The EDS/line scan analysis across the bonding zone in Fig. 8b shows that the concentration of Cr and Ni within the interlayer is very different from those of the DAZ indicating insufficient interdiffusion between the interlayer and base metals during 15 min bonding. SEM micrograph of the sample bonded at 1100 C for 30 min is shown in Fig. 9 and the EDS analysis of the selected points are presented in Table 2. The eutectic structure that indicate the incomplete isothermal solidification can be seen in the joint centerline and adjacent to the IN-718 base metal. It is very well known that isothermal solidification is controlled by the diffusion of the MPD elements in base metals. Therefore, the presence of eutectic structure adjacent to the IN-718 half suggests that the diffusion coefficient of B in IN-718 is smaller than that in IN-738. From the results of EDS analysis in Table 2, points A and B are Ni and Cr-borides and points C and D are Cr, Mo and Nb-borides (similar to those observed for the joints made for 15 min). Fig. 8. (a) SE images from the joint made at 1100 C for 15 min and (b) the EDS line scan analysis of the dotted line in (a).
from alloying elements such as Al, Mo, Cr, Ti and Co and their subsequent diffusion into the interlayer. The influence of each element on equilibrium melting point of the interlayer depends on the distribution coefficient (K) [12]. Therefore, the diffusion of Cr and Fe with K > 1 into the interlayer and depletion of the interlayer from Si and B with K < 1 increases the equilibrium melting point of the interlayer and results in formation of ISZ. The SEM images of DAZ of IN-738 and IN-718 halves of the samples bonded at 1100 C for one minute are shown in Figs. 5 and 6, respectively. The EDS analysis of the indicated points in Figs. 5 and 6 are shown in Table 2. Two types of phases (blocky and needle-like) precipitated in IN-738 half of the joint (Fig. 4). According to the EDS analysis in Table 2, each of the phases is mainly Ni, Cr, Co and partly Mo, Nb-enriched. Nb, Mo and Cr are strong boride formers. Idowu et al. [10] for TLP joints of IN738LC superalloy reported that the blocky precipitates could be W, Mo and Cr
3.3. Bonding at 1050 C 3.3.1. Optical microscope examination The effect of bonding time on microstructure of the joint made at 1050 C is shown in Fig. 10. From Fig. 10a, the presence of eutectic structure in the centerline of the sample bonded for 30 min indicates incomplete isothermal solidification. Also, no eutectic was observed in the microstructure of the joints made for 75 and 120 min. Therefore, isothermal solidification has been completed for these joints due to the longer time that was available for diffusion of MPD elements into the base metals. Fick’s second law for concentration of the MPD element in the base metal is written as [8]:
( " # " #) 1 xþw xw Cðx; tÞ ¼ Cm þ ðC0 Cm Þ erf erf 1 1 2 ð4DtÞ2 ð4DtÞ2
(1)
where Cm and C0 are the initial concentration of MPD element in the base metal and interlayer, respectively; C(x,t) is the MPD element concentration at time (t) and distance (x) from the joint
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901
Fig. 9. SEM micrograph of the joint made at 1100 C for 30 min showing eutectic structure: (a) at the joint centerline and (b) adjacent to IN-718.
centerline and D is diffusion coefficient of the MPD element in the base metal. According to Equation (1), at a constant bonding time, the increase in the bonding temperature from 1050 to 1100 C (i.e. increasing the diffusion coefficient, D) reduces the concentration of the MPD element, C(x,t) and subsequently reduces the fraction of eutectic in the joint centerline. Comparison of Figs. 7b and 10a confirms that width of the eutectic zone was decreased (i. e. the rate of isothermal solidification increased) with increasing the bonding temperature from 1050 C to 1100 C. 3.3.2. SEM examination Fig. 11 shows EDS/line scan analysis across the joint made at 1050 C for 75 min. The figure shows that the concentrations of alloying elements, especially that of Cr and Ni, are uniform across the bonding zone that indicates the presence of no eutectic or intermetallic. This is due to the equilibrium nature of isothermal solidification that prevents segregation of the alloying elements at solid/liquid interface and subsequent formation of undesired intermetallics across the bonding zone. 3.4. Bonding at 1150 C 3.4.1. Optical microscope examination Fig. 12 shows the microstructure of the TLP joint made at 1150 C
for 30 min. Comparison of Fig. 12 with Figs. 7b and 10b reveals a considerable difference between the microstructure of the samples bonded at this temperature and those bonded at 1050 C and 1100 C. The differences can be summarized as the followings: - Liquefaction phases (Fig. 12a) formed during diffusion of molten interlayer into the partial liquated grain boundaries in the base metals. Therefore, 1150 C could be higher than the base metals liquefaction temperature. - Formation of a continuous eutectic structure at the interface of base metals/bonding zone (Fig. 12b) at 1150 C, whereas, these phases were seen as isolated or continuous ones in the joint centerline at other bonding temperatures. - Needle-like or blocky phases at DAZ of samples bonded at 1050 C and 1100 C were replaced by grain boundary phases at 1150 C (Fig. 12a) - The interface of base metals/bonding zone was wavy at 1150 C (Fig. 12c) in comparison with nearly straight one at other bonding temperatures. The micrograph of the joint made at 1150 C for 60 min in Fig. 13 reveals incomplete isothermal solidification and formation of eutectic compounds. However, it can be seen that the diffusion of B into the base metal resulted in complete isothermal solidification in
Fig. 10. Optical micrograph of the joint made at 1050 C for (a) 30, (b) 75 and (c) 120 min.
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Fig. 11. EDS/line scan analysis across the joint made at 1050 C for 75 min.
joint made at 1150 C for 30 min are presented in Fig. 14. The presence of Si indicates the penetration of the molten interlayer to the zones A, B and C. Furthermore, the concentration of Ta and Ti in zone A, W, Mo, Cr and Nb in zone B and Cr, W and Mo in zone C are considerably higher than those in the base metals. Therefore, it can be concluded that the unusual chemical composition of these zones is due to the mixing of the molten interlayer and melts from the partial liquation of the base metals. The high content of Ti, W and Ta at zones A, B and C could not be due to solid-state diffusion and shows the possibility of liquefaction or partial re-melting at the grain boundaries of the base metals. Zone D (i.e. isothermal solidification zone) shows the Ni-base solid solution formed during the isothermal solidification of the molten interlayer. The higher concentration of the Ni and Si in this zone compared to those of the joints made at 1050 and 1100 C is probably due to the higher dissolution of the base metal and higher diffusion of alloying elements at 1150 C compared to those at the lower bonding temperatures. It has been reported that formation of g g0 eutectic occurs due to Ti segregation during athermal solidification. Cr, Mo and W have low solubility in g0 as the main constituent of the g g0 eutectic. Therefore, during the eutectic reaction, these elements segregate into the retained liquid. Enrichment of the retained liquid from Cr that is a strong boride-former explains the formation of Cr-borides [15,16].
the marked area [6,17].
3.5. Effect of bonding temperature and time on isothermal solidification
3.4.2. SEM examination The SEM image and the EDS analysis of the selected zones of the
Comparison of the microstructure of the joints made for 60 min indicates formation of eutectic compounds at 1150 C
Fig. 12. Optical micrograph of the joint made at 1150 C for 30 min.
Fig. 13. Optical micrographs of the joint made at 1150 C for 60 min (arrows show ISZ in base metals).
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Fig. 14. SEM micrograph of the joint made at 1150 C for 30 min: (a) BSE and (b) SE.
(Fig. 13) and eutectic-free joint at 1100 C (Fig. 7). Therefore, increasing the bonding temperature from 1100 to 1150 C increased the time needed to complete isothermal solidification and therefore decreased rate of the isothermal solidification. Therefore, it can be concluded that a critical bonding temperature (between 1100 and 1150 C) exists for IN718/MBF20/IN738 system above which increasing the bonding temperature will decrease the rate of isothermal solidification. This can be explained with increased dissolution of base metal with increasing temperature, altering the elements that control the rate of isothermal solidification [16,18] and reduced diffusion rate of the MPD element from the molten interlayer into the base metals [18]. It must be noted that according to the Fick’s second law of diffusion, vC=vt ¼ Dv2 C=vX 2 , the change in the concentration with time ðvC=vtÞ depends on both the concentration gradient ðv2 C=vX 2 Þ and diffusion coefficient of MPD element in the base metals (D).
Increasing the bonding temperature increases the diffusion coefficient and decreases the v2 C=vX 2 . When the temperature is lower than the critical bonding temperature (1050 and 1100 C), the effect of increasing the temperature on increase of the diffusion coefficient is more significant than the decrease of v2 C=vX 2 and therefore, increasing the temperature increases the rate of isothermal solidification. On the other hand, at temperatures higher than the critical temperature (1150 C), the decrease in v2 C=vX 2 is more significant than the increase in diffusion coefficient and therefore, further increase of the temperature will decrease the rate of isothermal solidification [19,20]. The minimum required time for isothermal (and therefore, eutectic free) solidification is of great importance. Fig. 15 shows the variation of size of the ASZ with bonding time at 1050 and 1100 C. It can be seen that the size of ASZ decreased with increasing the bonding time at the both temperatures. According to this figure,
Fig. 15. Variation of ASZ size with bonding temperature.
Fig. 16. Variation of ISZ size with Larson-Miller parameter (P).
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rate of reduction in size of the ASZ is more significant at 1100 C compared to that of 1050 C.
- The best estimate of the time required for the complete isothermal solidification of the examined system corresponded to the constant (C) value of 33 in Larson-Miller equation.
3.6. Parametric study of the isothermal solidification Acknowledgment Since the isothermal solidification rate is controlled by diffusion of MPD elements into the base metal that subsequently depends on the bonding temperature and time, a parametric approach was used to analyze the effect of bonding time and temperature on the width of ISZ using the Larsson-Miller parameter P. This parameter is given by P ¼ T B ½C þ lnðt B Þ, Where T B is the bonding temperature in Kelvin, t is the bonding time in minutes and C is a systemdependent constant. The parameter P is generally used for analyzing creep rupture data [21]. The values of ISZ width for the samples bonded at 1050, 1100 and 1150 C were plotted as a function of the parameter P using different values for C, and the quality of fitting was judged from the extent of the data points scatter. It was found that the best fitting and the minimum scatter for the ISZ data was obtained with a C value of 33 (Fig. 16). The linear relationship between the width of ISZ and LMP in Fig. 16 indicates that Larson-Miller parameter can be used as a criterion for prediction of the ISZ width. The required time for complete isothermal solidification of the joints made at different bonding temperatures can be estimated using LarssonMiller parameter. 4. Conclusions Dissimilar TLP joints of IN-738/IN-718 system were made using MBF-20 interlayer at different bonding temperatures and times. The results of microstructure studies showed that: - For the joints made at 1100 C/1min, Cr- and Ni-borides were observed as the eutectic structure. DAZ in IN718 half was enriched from Nb, Mo and Cr and DAZ in IN738 half was enriched from W, Mo and Cr borides. With increasing bonding time at 1100 C, volume fraction of eutectic phases at the joint centerline decreased and ISZ became wider. - For the joints made at 1050 C, continuous centerline eutectics were formed during 30 min bonding. In comparison with the joints made at 1100 C for the same time, the eutectic structure of the sample bonded at 1050 C was thinner. Further increase of the bonding time to 75 and 120 min at 1050 C, resulted in more complete isothermal solidification. - For the joints made at 1150 C for 30 min, the diffusion of liquid interlayer into the grain boundaries of base metals occurred, no DAZ was observed and the bonding interface became wavy. Even after bonding for 60 min, the isothermal solidification was not completed. - A critical bonding temperature (between 1100 and 1150 C) was observed below which increasing the bonding temperature increased the rate of isothermal solidification and above which, this rate decreased. - The Larson-Miller parametric approach can be used for studying the effect of bonding time and temperature on the ISZ width of the IN738/MBF-20/IN718 system.
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