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Diffusion bonding of Ni3Al-based alloy using a Ni interlayer
Journal Pre-proof Diffusion bonding of Ni3Al-based alloy using a Ni interlayer Z.W. Yang, J. Lian, J. Wang, X.Q. Cai, Y. Wang, D.P. Wang, Z.M. Wang, Y.C. Liu PII:
S0925-8388(19)34570-0
DOI:
https://doi.org/10.1016/j.jallcom.2019.153324
Reference:
JALCOM 153324
To appear in:
Journal of Alloys and Compounds
Received Date: 6 June 2019 Revised Date:
2 December 2019
Accepted Date: 6 December 2019
Please cite this article as: Z.W. Yang, J. Lian, J. Wang, X.Q. Cai, Y. Wang, D.P. Wang, Z.M. Wang, Y.C. Liu, Diffusion bonding of Ni3Al-based alloy using a Ni interlayer, Journal of Alloys and Compounds (2020), doi: https://doi.org/10.1016/j.jallcom.2019.153324. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Yang Zhenwen: Conceptualization, Writing - Review & Editing Lian Jie: Investigation, Writing - Original Draft Wang Jing: Validation Cai Xiaoqiang: Formal analysis, Methodology Wang Ying: Project administration, Funding acquisition Wang Dongpo: Supervision Wang Zumin: Resources Liu Yongchang: Resources
Diffusion bonding of Ni3Al-based alloy using a Ni interlayer Z.W. Yang, J. Lian, J. Wang, X.Q. Cai, Y. Wang*, D.P. Wang, Z.M. Wang, Y.C. Liu Tianjin Key Lab of Advanced Joining Technology, School of Materials Science and Engineering, Tianjin University, Tianjin 300350, China
Abstract Ni3Al-based alloys were direct diffusion bonded to themselves at 950-1100°C for 10-60min under a pressure of 20MPa. The effects of the joining parameters on shear strength and interface bonding ratio of the direct diffusion bonded joints were studied in detail. The maximum joint strength achieved was 689MPa, when the joint was bonded at 1100°C for 60min under 20MPa, corresponding to an interface bonding ratio of 95%. However, the fracture surface of the bonded joint was characterized mainly by cleavage fracture, indicating that the fracture mode was brittle rupture. The formation mechanism of the direct diffusion bonded joint and the coarsening phenomenon of γ´ precipitates in the Ni3Al-based alloy were revealed. To inhibit the coarsening of γ´ caused by high bonding temperature, a Ni interlayer was introduced. When bonded at 1050°C with a 30µm thick Ni foil, the pure Ni interlayer completely vanished because of interdiffusion with the Ni3Al-based alloy; however, the performance of the joint was still inferior to the base metal because of the heterogeneity in the microstructure of the diffusion zone. Therefore, the fracture occurred mainly at the diffusion zone. When bonded at 1050°C with a 3µm thick electroplated Ni coating, the microstructure of the diffusion zone was consistent with that of the base metal because of sufficient interdiffusion. The shear strength basically reached the level of the direct diffusion bonded joint at 1100°C. The fracture morphology changed from cleavage fracture to alternating dimples and facets after introducing the electroplated Ni coating. Keywords: Intermetallics; Ni interlayer; Diffusion bonding; Mechanical properties *Corresponding author. Tel./Fax: +86 022 85356744 Email: [email protected] (Y. Wang)
1. Introduction In the past few decades, a large number of researchers have concentrated on intermetallic-based alloys and compounds as alternative materials in turbine engines [1–3]. Compared with the traditional high temperature structural materials, intermetallic-based alloys have the advantages of lighter weight, higher strength and good service performance at high temperature [4–6]. In particular, Ni3Al-based alloys have received much attention, and are considered as primary substitutes for Ni-based alloys [7–10]. Moreover, Ni-based alloys are used frequently as parts of complex components. Therefore, the opportunities for Ni3Al-based alloys as high temperature structural materials used in engineering fields may be significantly increased by the development of a suitable bonding method. Ojo et al. [11] reported that there was a serious tendency for cracks to be generated in the fusion and heat-affected zones when laser welding was used for joining Ni3Al-based alloys. Montazeri et al. [12] investigated the phenomenon of liquation cracking in laser-welded IN738LC superalloy joints, and the results suggested that the existence of Cr-Mo rich borides in the substrate increases the possibility of liquation cracking. In addition, the high temperature performance of the brazed joints was weakened because of formation of low re-melting temperature products, which limits the application of this method to bond Ni-based superalloy parts with high service performance requirements [13]. For diffusion bonding, melting of the base metal is not involved in the joining process. Moreover, the performance and microstructure of the obtained joints can approach that of to the base metal [14]. However, only a few papers [15,16] have concentrated on joining Ni-based alloys by diffusion bonding. The critical problem associated with this process is that the triggering of atomic self-diffusion at the interface needs higher bonding temperatures, pressures, and times because of the absence of a concentration
gradient in comparison with dissimilar materials joining [17–20]; and this may degrade the performance of Ni-based alloys. Many published papers have demonstrated the feasibility of reducing the bonding temperature and preventing the formation of harmful products by introducing an appropriate interlayer [21]. Among the various kinds of interlayers, Ni has been widely employed as the interlayer during the diffusion bonding process [22–24] because of its good compatibility with other alloys and its plastic deformation capability [25]. Lin et al. [26] investigated the diffusion bonding of titanium/zirconium/molybdenum alloy (TZM) and Nb-Zr alloys with and without Ni interlayers, and the results showed that a reliable joint can be formed at a low joining temperature by using a Ni interlayer. He et al. [27] reported that the production of hard and brittle phases was prevented, and the highest shear strength was 148MPa when a Ni interlayer was sandwiched between a Ti alloy and a stainless steel web. In the study described in this paper, the Ni3Al-based alloys were direct diffusion bonded to themselves with and without Ni interlayers. The effects of bonding temperature and time on the microstructure and mechanical performance of the joint were studied in detail. In addition, the reasons for the coarsening of γ´ precipitates during the bonding process and the formation mechanism of the bonding interface were investigated and will be discussed in this paper. 2. Experiments The nominal composition of the Ni3Al-based alloy is listed in Table 1. The typical microstructure and XRD pattern of a Ni3Al-based alloy after electropolishing are shown in Fig. 1; and the alloy was composed of dual phase γ+γ´ structure, eutectic γ-γ´ structure, and some carbide. However, the carbides gradually corrode off during the electropolishing process, as seen in Fig. 1(a). Fig. 1(b) shows that the
microstructure of the dual phase zone can be characterized by gray block γ´ precipitates embedded within a γ matrix; nanoscale γ´ precipitates were also observed in the γ matrix. In addition, an obvious interface could be observed between the γ+γ´ structure and the γ-γ´ structure, as shown in Fig. 1(c). As seen in Fig. 1(d), the presence of Ni3Al and NiAl diffraction peaks indicated that the regions analyzed by XRD were dual phase γ+γ´ area and eutectic γ-γ´ area [28]. The energy-dispersive X-ray spectroscopy (EDS) chemical composition analysis of each typical area is listed in Table 2. It can be seen that from γ´ precipitates in the dual phase zone to the interface and eutectic structure, that the Al concentration gradually increases. Table 1 Nominal composition of Ni3Al-based alloy (at%).
Al
Cr
Mo
C
18.07
8.01
7.82
0.35
Si 0.11
W
B
Mn
Ti
<0.01
0.09
<0.01
0.01
Fe
Ni
11.48 54.07
Fig. 1. Microstructure in SE mode and XRD pattern of Ni3Al-based alloy: (a) typical microstructure after electropolishing; (b) microstructure of dual phase area; (c) magnification of
area 1; (d) XRD pattern. Table 2 Element content of each point marked in Fig. 1 (at%). Spots
Al
Mo
Ti
Cr
Fe
Ni
A
16.63
1.03
0.31
9.18
12.03
60.81
B
24.98
0.54
0.32
3.28
6.35
64.54
C
33.61
0.18
0.30
2.18
6.89
56.84
The dimensions of the samples used for diffusion bonding were 10 × 10 × 4mm3 and 7 × 7 × 4mm3. The sample surfaces to be joined were ground using SiC sandpaper up to 3000#. The Ni coating on the sample surface was prepared by electroplating; Ni foil with a purity of 99.8% and Ni3Al-based alloy were used as the anode and cathode, respectively, and the composition of the electrolyte and the experimental parameters are shown in Table 3. Prior to diffusion bonding, all samples were ultrasonically cleaned in acetone. Three different diffusion bonding conditions were prepared: (I) direct diffusion bonding, (II) diffusion bonding using 30µm Ni foil as an interlayer, and (III) diffusion bonding using an electroplated Ni coating as an interlayer; the corresponding schematic sketches are shown in Fig. 2. For the diffusion bonding, the assemblies were heated to 800°C at a rate of 20°C/min with a dwell time of 10min. Next, the assemblies were heated to 950-1100°C at 10°C/min and then maintained for 10-60min. The samples were cooled to 400°C at 5°C/min and subsequently furnace-cooled to room temperature. It should be noted that a pressure of 20MPa was applied to the samples during the entire bonding process. For microstructural characterizations, the samples were cut along the cross section by a wire electrical discharge machining. Then the cross section was ground manually and electropolished with
a
solution
composed
of
60vol%CH3(CH2)3OH,
30vol%CH3OH,
and
10vol%HClO4. The three-dimensional surface morphology of the specimens prepared under different conditions was characterized by atomic force microscopy (AFM 5500).
The interfacial microstructure of diffusion bonded joints was characterized by scanning electron microscope (SEM, JSM-7800F) in secondary electron (SE) mode and back-scattering electron (BSE) mode. The chemical composition of various products in the joint was analyzed by energy dispersive spectrometer (EDS). The morphology of coarsened γ´ precipitates was investigated by transmission electron microscopy (TEM, JEM-2100F) with an accelerating voltage of 300kV in bright field mode. The samples used for TEM observations were prepared by electropolishing with a solution of 5vol%HClO3 + 95vol%C2H5OH at a temperature of -30°C with the voltage of 40V. Moreover, to measure the bonding performance of the joints, shear tests were performed by a testing machine (MTS E45.105) at a fixed rate of 0.2mm/min. Four samples were used for shear testing under different bonding conditions to ensure data repeatability. The morphology and composition of the fracture surface after shear testing were examined by the SEM and by X-ray diffraction (XRD, BRUKER D8 Advance), and the kind of X-ray radiation was Cu-Kα radiation source. The γ´ precipitation was quantitatively measured by Image J software.
Fig. 2. Assembly schematic sketches of three different diffusion bonding methods. Table 3 Bath composition and electroplating parameters. Bath composition .
NiSO4 6H2O NiCl2.6H2O H3BO3 CH3(CH2)11OSO3Na
Concentration (g/l)
Electroplating condition
300 40 30 0.2
Current density: 0.015A/cm2 pH: 4 Temperature: 36°C
3. Results and discussion 3.1. Surface morphology of the Ni3Al-based alloy The three-dimensional surface morphologies of the samples prepared by grinding with 3000# SiC sandpaper and electroplating Ni are shown in Fig. 3, separately. The ground surface was relatively flat and some shallow parallel scratches could be observed. The surface roughness of the specimens prepared by grinding and by electroplating is listed in Table 4. The roughness value of the Ni coating was higher than that of the ground surface.
Fig. 3. 3D surface morphologies of samples prepared by different treatment methods: (a) ground by 3000# SiC sandpaper; (b) electroplated Ni coating. Table 4 Different surface roughnesses of samples prepared by grinding and by electroplating. Surface condition
a
Surface roughness (µm) a
Ground by 3000# SiC sandpaper
Ra : 0.289
Rqb: 0.112
Rmaxc: 1.281
Electroplated Ni
Ra: 0.509
Rq: 0.162
Rmax: 1.329
Arithmetic mean deviation of the absolute value of the distances from each point of the contour
to the middle position. b
c
Root mean square deviation of the distance from each point of the contour to the middle position. Maximum distance between the highest and lowest points of the contour.
After electroplating, the characterization of the Ni coating was carried out as shown in Fig. 4. It can be seen from Fig. 4(a) that a Ni coating with a uniform thickness was prepared on the Ni3Al-based alloy surface. Based on the analysis of element
distribution along the red line in Fig. 4(a), the thickness of the Ni coating was found to be about 3µm, as seen in Fig. 4(b). Fig. 4(c) shows the SEM morphology of the Ni coating, which was mainly composed of Ni particles with irregular shape. In order to further confirm the presence of a Ni coating, XRD analysis was performed, as shown in Fig. 4(d).
Fig. 4. Characterization of Ni coating: (a) BSE micrograph of Ni coating; (b) line distribution of the elements along the red line in (a); (c) SEM micrograph of Ni coating; (d) XRD pattern.
3.2. Microstructure of Ni3Al-based alloy direct diffusion bonded joint In order to observe the microstructural evolution of Ni3Al-based alloy joints, direct diffusion bonding was performed at different joining temperatures, as seen in Fig. 5. The bonded joint did not form sound metallurgical bonding, and there were many unbonded areas at the interface, as shown in Fig. 5(a). It is well known that the initial stage of diffusion bonding involves establishing good contact between the two surfaces that are to be bonded. At low bonding temperatures, the micro-plastic deformation of the surfaces to be joined was restricted by the anomalous strength
behavior of the Ni3Al-based alloy that is due to the presence of a high volume fraction of L12 type ordered γ´ phase in the superalloy [29,30]. Therefore, micro-voids were observed at the bonding interface because of the insufficient contact between the rough surfaces [31]. When the bonding temperature was 1000°C, it can be seen (by comparing with the joint bonded at 950°C) that the unbonded areas decreased significantly, indicating that the micro-plastic deformation of the asperities was improved. Meanwhile, the higher temperature accelerated the atomic diffusion across the interface, which was beneficial to obtain reliable joints; when the bonding temperature was 1050°C, only a small number of micro-voids remained, as demonstrated in Fig. 5(c). However, coarsening of γ´ precipitates in the Ni3Al-based alloy was observed. By further increasing the bonding temperature to 1100°C, as shown in Fig. 5(d), a sound joint was formed. Moreover, the bonding interface disappeared in some areas, and the γ´ precipitates near the bonding interface were integrated with each other. This phenomenon can be attributed to the promotion of atomic diffusion as the joining temperature increases. In addition, the coarsening of γ´ precipitates in the matrix was further aggravated. The morphology of the γ´ precipitates was transformed from its initial cubic form into a strip-like geometry. The average width of γ´ precipitation in the joints diffusion bonded at the 1000°C (Fig. 5b), 1050°C (Fig. 5c) and 1100°C (Fig. 5d) was counted to be approximate 0.71µm, 0.94µm and 1.13µm, respectively. It can be seen that the average width of γ´ precipitation after diffusion bonding was larger than that of the base metal (0.53µm).
Fig. 5. Microstructure of the direct diffusion bonding joints bonded at: (a) 950°C; (b) 1000°C; (c) 1050°C; (d) 1100°C.
Fig. 6 shows the direct diffusion bonding joints of the Ni3Al-based alloy at different holding times. Similar to the influence of the diffusion bonding temperature, the unbonded areas at the interface were gradually decreased by enhancing the holding time from 10min to 60min.
Fig. 6. Microstructural evolution of Ni3Al-based alloy joints formed by direct diffusion bonding at different times with a temperature of 1100°C and a pressure of 20MPa: (a) 10min; (b) 20min; (c) 40min; (d) 60min.
To observe the coarsening phenomenon of γ´ precipitates and explore the formation mechanism of the bonding interface, TEM analysis was performed. Fig. 7(a-c) shows the morphologies of γ´ precipitates in the joint bonded at 1100°C for 60min. It can be seen that coalescence of partial γ´ precipitates occurred, and the morphology of the γ´ precipitates was transformed from the initial cubic shape to a strip-like geometry (as marked by the red dotted line). Fig. 7(d-f) respectively shows the composition analysis of each point in Fig. 7c, and it can be seen from Fig. 7e that the composition of this point is Ni-based solid solution. Compared to the cuboidal γ´ phase, it can be seen that the Ni concentration in the coarsened γ´ precipitates increased from 68.03% to 71.41% while Al decreased from 16.69% to 12.46%. It has been reported that Al atoms require less energy to form vacancies than Ni atoms [32]. Moreover, the diffusion rate of Al atoms is higher than that of Ni atoms in Ni-based single-crystal
superalloys [5,33]. Therefore, the reason for the decrease of Al content in γ´ precipitates neighboring the bonding interface may be that Al atoms diffused from the substrate to the interface to be joined, filling up the voids and finally forming the bonding interface.
Fig. 7. TEM analysis of the joint bonded at 1100°C for 60min under 20MPa: (a-c) bright-field images; (d-f) EDS results of the marked locations in (c); (g) SAED pattern of coarsened γ´ precipitates.
When bonded at 1100°C for 60min at 20MPa, the formation of small γ´ precipitates occurred at the interface between the eutectic structure and the dual phase structure in the base metal, as seen in Fig. 8. The formation of small γ´ precipitates might be due
to the concentration gradient of Al content between the dual phase structure and the interface, as analyzed in Section 2. Based on the above analysis, it can be concluded that the content of Al was reduced in the dual phase zone because of its diffusion from the substrate to the interface zone. Therefore, Al was transported from the eutectic zone with high Al content to the dual phase zone in the form of decomposition γ´ precipitates to maintain the balance of the system.
Fig. 8. The decomposition phenomenon of γ´ precipitates: (a) the entire microstructure; (b) magnification of area b; (c) magnification of area c.
3.3. Mechanical properties of Ni3Al-based alloy direct diffusion bonded joint Fig. 9 shows the effect of bonding temperature and time on the shear strength and interface bonding ratio of the diffusion bonded joints. For the diffusion bonding of similar materials, the interface bonding ratio is generally considered to be a meaningful criterion to evaluate the mechanical performance of the bonded joints [34]. It should be noted in Fig. 9(a-b) that there is a positive relationship between the shear strength and the bonding ratio. This can be explained by the fact that the diffusion rate of atoms was improved by the increase of the joining parameters, and this accelerated the closing of the unbonded areas. Therefore, the shear strength was gradually increased with the increase of bonding temperature and time. The highest shear strength and interface bonding ratio (689MPa and 95%, respectively) were achieved when the samples were bonded at 1100°C for 60min under 20MPa.
Fig. 9. Effect of (a) joining temperature and (b) holding time on the shear strength and interface bonding ratio of Ni3Al-based alloy joints.
Fig. 10 presents the SEM images of the fracture morphologies of the joints that were diffusion bonded at different temperatures. When the joining temperature was low, such as 950°C, copious voids were observed at the diffusion zone, as demonstrated in Fig. 5(a). Therefore, the fracture surface was smooth and the black area was the unbonded zone. The fracture mainly occurred at the bonding interface. When the bonding temperature was 1100°C, cracks propagated mainly in the Ni3Al substrate and partially at the bonding interface during the shear test. The fracture morphology at the interface was mainly exhibited shallow dimples and cleavage fractures. However, the fracture occurred in the substrate was primarily brittle fracture because the presence of quasi-cleavage fracture characterization.
Fig. 10. Fracture morphologies and corresponding schematic diagrams of fracture paths of the joints bonded at (a-c) 950°C; (d-f) 1100°C.
3.4. Diffusion bonding of Ni3Al-based alloy using a 30µm-Ni foil When the direct diffusion bonding temperature of the Ni3Al-based alloy was 1100°C, severe coarsening of the γ´ precipitates occurred. The morphology of the γ´ precipitates was transformed from cubic to strip-like. It should be noted that the mechanical performance of Ni-based alloys is affected by the size, morphology, and volume fraction of γ´ precipitates [35,36]. Therefore, the morphological change of the γ´ phase caused by the high bonding temperature would affect the creep resistance and service life of the Ni-based alloy [37,38]. To mitigate the impact of the coarsening of the γ´ precipitates, it is necessary to reduce the bonding temperature. Therefore, a Ni foil with a thickness of 30µm was used as an interlayer. Fig. 11 shows the interfacial microstructure and distribution of the elements within the joints that were diffusion bonded at different temperatures with 30µm thick Ni foil. The joints were well bonded without micro-voids for all three temperatures after the addition of the Ni interlayer with good plastic deformation ability. Fig. 11(d-f) illustrates the line distribution of
the elements across the joining interface. The thickness of the residual Ni interlayer decreased from 20µm to 8µm when the joining temperature was increased from 950°C to 1000°C. Subsequently, the pure Ni foil completely disappeared because of the increased elemental interdiffusion between the Ni foil and the base metal when the joining temperature was increased to 1050°C. In addition, the concentration of Fe, Al, Mo, and Cr gradually increased from the joint center to the base metal, but the variation in the Ni content was the opposite because of the concentration gradient.
Fig. 11. Microstructural observations and corresponding element distribution in the samples bonded with 30µm Ni foil for 60min under 20MPa at: (a, d) 950°C; (b, e) 1000°C; (c, f) 1050°C.
Fig. 12 demonstrates the shear strength of the joints that were diffusion bonded at different joining temperatures with a 30µm thick Ni interlayer for 60min under 20MPa. Compared to direct diffusion bonding, the shear strength of the joint bonded at 950°C and 1000°C using the Ni foil as the interlayer increased from 80MPa to 349MPa and from 256MPa to 465MPa, respectively. Upon further increasing the bonding temperature to 1050°C, the shear strength did not change markedly and remained at 488MPa.
Fig. 12. The effect of bonding temperature on the shear strength of the joint bonded with 30µm thick Ni foil.
Fig. 13 demonstrates the SEM fracture morphologies and XRD pattern of the joints diffusion bonded at different bonding temperatures. When Ni foil was inserted between the Ni3Al substrates, the fracture surface presented a lamellar structure, as seen in Fig. 13(a). Moreover, the disconnected areas that were clearly observed on the fracture surface of the joint that had been direct diffusion bonded at 950°C vanished. Shallow dimples gradually appeared on the fracture surface as a result of increasing the joining temperature, as demonstrated in Fig. 13(b). This is because the fact that the degree of interdiffusion between the substrate and the interlayer was improved by the increase of bonding temperature from 950°C to 1000°C. Upon further increasing the bonding temperature to 1050°C, the pure Ni foil disappeared, and the number of dimples increased, as shown in Fig. 13(c). The lattice constant of the phase to be identified was calculated based on the XRD pattern. When the Ni interlayer was used for diffusion bonding at 950℃, 1000℃ and 1050℃, the average lattice constants of the phase on the fracture surface were 0.35301nm, 0.35252nm and 0.35287nm, respectively, which was basically equal to the lattice constant of Ni (0.3524nm). According to the EDS analysis and XRD pattern shown in Table 5, the rupture location of the joints was mostly concentrated in the diffusion zone.
Fig. 13. The fracture morphologies of the joints bonded with a 30µm thick Ni interlayer at different bonding temperatures, and the corresponding XRD pattern of the ruptured surface: (a) 950°C; (b) 1000°C; (c) 1050°C; (d) XRD pattern. Table 5 Chemical composition for each point in Fig. 13(a-c) (at%). Position
Al
Mo
Ti
Cr
Fe
Ni
Possible phase
A
10.06
1.01
0.13
8.45
12.88
67.48
Ni(s,s)
B C D E F
7.03 2.07 0.84 6.41 6.40
0.29 0.16 0.20 0.22 0.25
0.25 0.27 0.64 0.36 0.30
3.53 0.33 0.21 1.38 1.01
3.76 0.88 0.67 2.41 2.34
85.14 96.29 97.44 89.22 89.69
Ni(s,s) Ni(s,s) Ni(s,s) Ni(s,s) Ni(s,s)
3.5. Diffusion bonding of Ni3Al-based alloy using a 3µm-Ni coating Although the pure Ni interlayer completely disappeared by interdiffusion when bonded at 1050°C, the performance of the joint was still limited because of the difference in microstructure between the bonding interface and base metal. One solution to this problem would be to reduce the thickness of the Ni interlayer. Therefore, a Ni coating with a thickness of about 3µm was electroplated onto the
substrate surface. Fig. 14(a) shows the microstructure of the joint bonded at 1050°C under 20MPa for 60min with this Ni coating. According to the analysis of element distribution along the diffusion zone, there is no significant difference in composition between the diffusion zone and the base metal. This suggests that the Ni coating completely disappeared and formed the same microstructure as the substrate. In contrasted to the joint bonded at 1050°C with 30µm Ni foil, the shear strength was significantly increased when the 3µm Ni coating was introduced for diffusion bonding. The maximum shear strength of the joint was 617MPa, which basically reached the level of direct diffusion bonding at 1100°C.
Fig. 14. The microstructure in SE mode and element distribution of the joint bonded at 1050°C with Ni coating: (a) interfacial microstructure; (b) elemental distribution along the white line in (a).
The fracture morphologies of the joints formed by direct diffusion bonding were characterized by brittle fracture as the main fracture mode, as seen in Fig. 8. However, it can be seen from Fig. 15(a) that the fracture surface of the joint bonded with the Ni coating was characterized by alternating dimples and flat morphologies. For the joint bonded with Ni coating, the calculated lattice constant of the phase on the fracture surface was 0.35665nm, which was very close to that of Ni3Al (0.3572nm). Moreover, the appearance of superlattice diffraction (110) peak also indicated the existence of L12 ordered Ni3Al phase [39]. According to the EDS results and XRD analysis, the
composition of the fracture surface was Ni3Al.
Fig. 15. The fracture morphology of the joint bonded at 1050°C with the Ni coating and the corresponding XRD pattern of the ruptured surface: (a) typical fracture morphology; (b) magnification of area 1; (c) magnification of area 2; (d) XRD pattern. Table 6 Chemical composition at the locations shown in Fig. 15(b) and (c) (at%). Position
Al
Mo
Ti
Cr
Fe
Ni
Possible phase
A B
22.83 16.69
0.75 0.70
0.22 0.39
4.99 6.75
7.80 10.54
63.42 64.92
Ni3Al Ni3Al
Conclusions (1) A maximum shear strength of 689MPa and an interface bonding ratio of 95% were achieved for the Ni3Al-based alloy joint that was direct diffusion bonded at 1100°C for 60min. The fracture position of the joint was transferred from the bonding interface to the substrate due to the increase bonding temperature and time. The fracture morphology was characterized primarily by quasi-cleavage fracture. (1) In addition, the formation mechanism of the bonding interface and the phenomenon of the coarsening of the γ´ precipitates were controlled by the diffusion of Al. (2) In order to alleviate the coarsening of γ´ precipitates caused by the high bonding temperature, pure Ni foil was introduced to reduce the bonding temperature. The shear strength of the joints bonded at 950°C and 1000°C with this Ni foil interlayer was significantly increased. However, when bonded at 1050°C, the performance of the joint was still limited by the microstructure heterogeneity between the base metal and the diffusion zone. The maximum shear strength was 488MPa and the fracture occurred in the Ni diffusion layer. The fracture morphology was dimple-like, which could be evidence of ductile rupture. (3) When bonded at 1050°C with a 3µm electroplated Ni coating, the shear strength of the joint basically reached the level of the direct diffusion bonded joint bonded at 1100°C. Moreover, the degree of coarsening of the γ´ precipitates was reduced. The fracture morphology changed from the main cleavage fracture under direct diffusion bonding to the alternating dimples and facets with the addition of the Ni coating.
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Highlights: 1. Direct diffusion bonding of Ni3Al-based alloys was carried out. 2. The formation mechanism of the direct diffusion bonded joint was revealed. 3. A Ni interlayer was introduced to reduce the bonding temperature.
Declaration of interests ☐ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☒The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S0925-8388(19)34570-0
DOI:
https://doi.org/10.1016/j.jallcom.2019.153324
Reference:
JALCOM 153324
To appear in:
Journal of Alloys and Compounds
Received Date: 6 June 2019 Revised Date:
2 December 2019
Accepted Date: 6 December 2019
Please cite this article as: Z.W. Yang, J. Lian, J. Wang, X.Q. Cai, Y. Wang, D.P. Wang, Z.M. Wang, Y.C. Liu, Diffusion bonding of Ni3Al-based alloy using a Ni interlayer, Journal of Alloys and Compounds (2020), doi: https://doi.org/10.1016/j.jallcom.2019.153324. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Yang Zhenwen: Conceptualization, Writing - Review & Editing Lian Jie: Investigation, Writing - Original Draft Wang Jing: Validation Cai Xiaoqiang: Formal analysis, Methodology Wang Ying: Project administration, Funding acquisition Wang Dongpo: Supervision Wang Zumin: Resources Liu Yongchang: Resources
Diffusion bonding of Ni3Al-based alloy using a Ni interlayer Z.W. Yang, J. Lian, J. Wang, X.Q. Cai, Y. Wang*, D.P. Wang, Z.M. Wang, Y.C. Liu Tianjin Key Lab of Advanced Joining Technology, School of Materials Science and Engineering, Tianjin University, Tianjin 300350, China
Abstract Ni3Al-based alloys were direct diffusion bonded to themselves at 950-1100°C for 10-60min under a pressure of 20MPa. The effects of the joining parameters on shear strength and interface bonding ratio of the direct diffusion bonded joints were studied in detail. The maximum joint strength achieved was 689MPa, when the joint was bonded at 1100°C for 60min under 20MPa, corresponding to an interface bonding ratio of 95%. However, the fracture surface of the bonded joint was characterized mainly by cleavage fracture, indicating that the fracture mode was brittle rupture. The formation mechanism of the direct diffusion bonded joint and the coarsening phenomenon of γ´ precipitates in the Ni3Al-based alloy were revealed. To inhibit the coarsening of γ´ caused by high bonding temperature, a Ni interlayer was introduced. When bonded at 1050°C with a 30µm thick Ni foil, the pure Ni interlayer completely vanished because of interdiffusion with the Ni3Al-based alloy; however, the performance of the joint was still inferior to the base metal because of the heterogeneity in the microstructure of the diffusion zone. Therefore, the fracture occurred mainly at the diffusion zone. When bonded at 1050°C with a 3µm thick electroplated Ni coating, the microstructure of the diffusion zone was consistent with that of the base metal because of sufficient interdiffusion. The shear strength basically reached the level of the direct diffusion bonded joint at 1100°C. The fracture morphology changed from cleavage fracture to alternating dimples and facets after introducing the electroplated Ni coating. Keywords: Intermetallics; Ni interlayer; Diffusion bonding; Mechanical properties *Corresponding author. Tel./Fax: +86 022 85356744 Email: [email protected] (Y. Wang)
1. Introduction In the past few decades, a large number of researchers have concentrated on intermetallic-based alloys and compounds as alternative materials in turbine engines [1–3]. Compared with the traditional high temperature structural materials, intermetallic-based alloys have the advantages of lighter weight, higher strength and good service performance at high temperature [4–6]. In particular, Ni3Al-based alloys have received much attention, and are considered as primary substitutes for Ni-based alloys [7–10]. Moreover, Ni-based alloys are used frequently as parts of complex components. Therefore, the opportunities for Ni3Al-based alloys as high temperature structural materials used in engineering fields may be significantly increased by the development of a suitable bonding method. Ojo et al. [11] reported that there was a serious tendency for cracks to be generated in the fusion and heat-affected zones when laser welding was used for joining Ni3Al-based alloys. Montazeri et al. [12] investigated the phenomenon of liquation cracking in laser-welded IN738LC superalloy joints, and the results suggested that the existence of Cr-Mo rich borides in the substrate increases the possibility of liquation cracking. In addition, the high temperature performance of the brazed joints was weakened because of formation of low re-melting temperature products, which limits the application of this method to bond Ni-based superalloy parts with high service performance requirements [13]. For diffusion bonding, melting of the base metal is not involved in the joining process. Moreover, the performance and microstructure of the obtained joints can approach that of to the base metal [14]. However, only a few papers [15,16] have concentrated on joining Ni-based alloys by diffusion bonding. The critical problem associated with this process is that the triggering of atomic self-diffusion at the interface needs higher bonding temperatures, pressures, and times because of the absence of a concentration
gradient in comparison with dissimilar materials joining [17–20]; and this may degrade the performance of Ni-based alloys. Many published papers have demonstrated the feasibility of reducing the bonding temperature and preventing the formation of harmful products by introducing an appropriate interlayer [21]. Among the various kinds of interlayers, Ni has been widely employed as the interlayer during the diffusion bonding process [22–24] because of its good compatibility with other alloys and its plastic deformation capability [25]. Lin et al. [26] investigated the diffusion bonding of titanium/zirconium/molybdenum alloy (TZM) and Nb-Zr alloys with and without Ni interlayers, and the results showed that a reliable joint can be formed at a low joining temperature by using a Ni interlayer. He et al. [27] reported that the production of hard and brittle phases was prevented, and the highest shear strength was 148MPa when a Ni interlayer was sandwiched between a Ti alloy and a stainless steel web. In the study described in this paper, the Ni3Al-based alloys were direct diffusion bonded to themselves with and without Ni interlayers. The effects of bonding temperature and time on the microstructure and mechanical performance of the joint were studied in detail. In addition, the reasons for the coarsening of γ´ precipitates during the bonding process and the formation mechanism of the bonding interface were investigated and will be discussed in this paper. 2. Experiments The nominal composition of the Ni3Al-based alloy is listed in Table 1. The typical microstructure and XRD pattern of a Ni3Al-based alloy after electropolishing are shown in Fig. 1; and the alloy was composed of dual phase γ+γ´ structure, eutectic γ-γ´ structure, and some carbide. However, the carbides gradually corrode off during the electropolishing process, as seen in Fig. 1(a). Fig. 1(b) shows that the
microstructure of the dual phase zone can be characterized by gray block γ´ precipitates embedded within a γ matrix; nanoscale γ´ precipitates were also observed in the γ matrix. In addition, an obvious interface could be observed between the γ+γ´ structure and the γ-γ´ structure, as shown in Fig. 1(c). As seen in Fig. 1(d), the presence of Ni3Al and NiAl diffraction peaks indicated that the regions analyzed by XRD were dual phase γ+γ´ area and eutectic γ-γ´ area [28]. The energy-dispersive X-ray spectroscopy (EDS) chemical composition analysis of each typical area is listed in Table 2. It can be seen that from γ´ precipitates in the dual phase zone to the interface and eutectic structure, that the Al concentration gradually increases. Table 1 Nominal composition of Ni3Al-based alloy (at%).
Al
Cr
Mo
C
18.07
8.01
7.82
0.35
Si 0.11
W
B
Mn
Ti
<0.01
0.09
<0.01
0.01
Fe
Ni
11.48 54.07
Fig. 1. Microstructure in SE mode and XRD pattern of Ni3Al-based alloy: (a) typical microstructure after electropolishing; (b) microstructure of dual phase area; (c) magnification of
area 1; (d) XRD pattern. Table 2 Element content of each point marked in Fig. 1 (at%). Spots
Al
Mo
Ti
Cr
Fe
Ni
A
16.63
1.03
0.31
9.18
12.03
60.81
B
24.98
0.54
0.32
3.28
6.35
64.54
C
33.61
0.18
0.30
2.18
6.89
56.84
The dimensions of the samples used for diffusion bonding were 10 × 10 × 4mm3 and 7 × 7 × 4mm3. The sample surfaces to be joined were ground using SiC sandpaper up to 3000#. The Ni coating on the sample surface was prepared by electroplating; Ni foil with a purity of 99.8% and Ni3Al-based alloy were used as the anode and cathode, respectively, and the composition of the electrolyte and the experimental parameters are shown in Table 3. Prior to diffusion bonding, all samples were ultrasonically cleaned in acetone. Three different diffusion bonding conditions were prepared: (I) direct diffusion bonding, (II) diffusion bonding using 30µm Ni foil as an interlayer, and (III) diffusion bonding using an electroplated Ni coating as an interlayer; the corresponding schematic sketches are shown in Fig. 2. For the diffusion bonding, the assemblies were heated to 800°C at a rate of 20°C/min with a dwell time of 10min. Next, the assemblies were heated to 950-1100°C at 10°C/min and then maintained for 10-60min. The samples were cooled to 400°C at 5°C/min and subsequently furnace-cooled to room temperature. It should be noted that a pressure of 20MPa was applied to the samples during the entire bonding process. For microstructural characterizations, the samples were cut along the cross section by a wire electrical discharge machining. Then the cross section was ground manually and electropolished with
a
solution
composed
of
60vol%CH3(CH2)3OH,
30vol%CH3OH,
and
10vol%HClO4. The three-dimensional surface morphology of the specimens prepared under different conditions was characterized by atomic force microscopy (AFM 5500).
The interfacial microstructure of diffusion bonded joints was characterized by scanning electron microscope (SEM, JSM-7800F) in secondary electron (SE) mode and back-scattering electron (BSE) mode. The chemical composition of various products in the joint was analyzed by energy dispersive spectrometer (EDS). The morphology of coarsened γ´ precipitates was investigated by transmission electron microscopy (TEM, JEM-2100F) with an accelerating voltage of 300kV in bright field mode. The samples used for TEM observations were prepared by electropolishing with a solution of 5vol%HClO3 + 95vol%C2H5OH at a temperature of -30°C with the voltage of 40V. Moreover, to measure the bonding performance of the joints, shear tests were performed by a testing machine (MTS E45.105) at a fixed rate of 0.2mm/min. Four samples were used for shear testing under different bonding conditions to ensure data repeatability. The morphology and composition of the fracture surface after shear testing were examined by the SEM and by X-ray diffraction (XRD, BRUKER D8 Advance), and the kind of X-ray radiation was Cu-Kα radiation source. The γ´ precipitation was quantitatively measured by Image J software.
Fig. 2. Assembly schematic sketches of three different diffusion bonding methods. Table 3 Bath composition and electroplating parameters. Bath composition .
NiSO4 6H2O NiCl2.6H2O H3BO3 CH3(CH2)11OSO3Na
Concentration (g/l)
Electroplating condition
300 40 30 0.2
Current density: 0.015A/cm2 pH: 4 Temperature: 36°C
3. Results and discussion 3.1. Surface morphology of the Ni3Al-based alloy The three-dimensional surface morphologies of the samples prepared by grinding with 3000# SiC sandpaper and electroplating Ni are shown in Fig. 3, separately. The ground surface was relatively flat and some shallow parallel scratches could be observed. The surface roughness of the specimens prepared by grinding and by electroplating is listed in Table 4. The roughness value of the Ni coating was higher than that of the ground surface.
Fig. 3. 3D surface morphologies of samples prepared by different treatment methods: (a) ground by 3000# SiC sandpaper; (b) electroplated Ni coating. Table 4 Different surface roughnesses of samples prepared by grinding and by electroplating. Surface condition
a
Surface roughness (µm) a
Ground by 3000# SiC sandpaper
Ra : 0.289
Rqb: 0.112
Rmaxc: 1.281
Electroplated Ni
Ra: 0.509
Rq: 0.162
Rmax: 1.329
Arithmetic mean deviation of the absolute value of the distances from each point of the contour
to the middle position. b
c
Root mean square deviation of the distance from each point of the contour to the middle position. Maximum distance between the highest and lowest points of the contour.
After electroplating, the characterization of the Ni coating was carried out as shown in Fig. 4. It can be seen from Fig. 4(a) that a Ni coating with a uniform thickness was prepared on the Ni3Al-based alloy surface. Based on the analysis of element
distribution along the red line in Fig. 4(a), the thickness of the Ni coating was found to be about 3µm, as seen in Fig. 4(b). Fig. 4(c) shows the SEM morphology of the Ni coating, which was mainly composed of Ni particles with irregular shape. In order to further confirm the presence of a Ni coating, XRD analysis was performed, as shown in Fig. 4(d).
Fig. 4. Characterization of Ni coating: (a) BSE micrograph of Ni coating; (b) line distribution of the elements along the red line in (a); (c) SEM micrograph of Ni coating; (d) XRD pattern.
3.2. Microstructure of Ni3Al-based alloy direct diffusion bonded joint In order to observe the microstructural evolution of Ni3Al-based alloy joints, direct diffusion bonding was performed at different joining temperatures, as seen in Fig. 5. The bonded joint did not form sound metallurgical bonding, and there were many unbonded areas at the interface, as shown in Fig. 5(a). It is well known that the initial stage of diffusion bonding involves establishing good contact between the two surfaces that are to be bonded. At low bonding temperatures, the micro-plastic deformation of the surfaces to be joined was restricted by the anomalous strength
behavior of the Ni3Al-based alloy that is due to the presence of a high volume fraction of L12 type ordered γ´ phase in the superalloy [29,30]. Therefore, micro-voids were observed at the bonding interface because of the insufficient contact between the rough surfaces [31]. When the bonding temperature was 1000°C, it can be seen (by comparing with the joint bonded at 950°C) that the unbonded areas decreased significantly, indicating that the micro-plastic deformation of the asperities was improved. Meanwhile, the higher temperature accelerated the atomic diffusion across the interface, which was beneficial to obtain reliable joints; when the bonding temperature was 1050°C, only a small number of micro-voids remained, as demonstrated in Fig. 5(c). However, coarsening of γ´ precipitates in the Ni3Al-based alloy was observed. By further increasing the bonding temperature to 1100°C, as shown in Fig. 5(d), a sound joint was formed. Moreover, the bonding interface disappeared in some areas, and the γ´ precipitates near the bonding interface were integrated with each other. This phenomenon can be attributed to the promotion of atomic diffusion as the joining temperature increases. In addition, the coarsening of γ´ precipitates in the matrix was further aggravated. The morphology of the γ´ precipitates was transformed from its initial cubic form into a strip-like geometry. The average width of γ´ precipitation in the joints diffusion bonded at the 1000°C (Fig. 5b), 1050°C (Fig. 5c) and 1100°C (Fig. 5d) was counted to be approximate 0.71µm, 0.94µm and 1.13µm, respectively. It can be seen that the average width of γ´ precipitation after diffusion bonding was larger than that of the base metal (0.53µm).
Fig. 5. Microstructure of the direct diffusion bonding joints bonded at: (a) 950°C; (b) 1000°C; (c) 1050°C; (d) 1100°C.
Fig. 6 shows the direct diffusion bonding joints of the Ni3Al-based alloy at different holding times. Similar to the influence of the diffusion bonding temperature, the unbonded areas at the interface were gradually decreased by enhancing the holding time from 10min to 60min.
Fig. 6. Microstructural evolution of Ni3Al-based alloy joints formed by direct diffusion bonding at different times with a temperature of 1100°C and a pressure of 20MPa: (a) 10min; (b) 20min; (c) 40min; (d) 60min.
To observe the coarsening phenomenon of γ´ precipitates and explore the formation mechanism of the bonding interface, TEM analysis was performed. Fig. 7(a-c) shows the morphologies of γ´ precipitates in the joint bonded at 1100°C for 60min. It can be seen that coalescence of partial γ´ precipitates occurred, and the morphology of the γ´ precipitates was transformed from the initial cubic shape to a strip-like geometry (as marked by the red dotted line). Fig. 7(d-f) respectively shows the composition analysis of each point in Fig. 7c, and it can be seen from Fig. 7e that the composition of this point is Ni-based solid solution. Compared to the cuboidal γ´ phase, it can be seen that the Ni concentration in the coarsened γ´ precipitates increased from 68.03% to 71.41% while Al decreased from 16.69% to 12.46%. It has been reported that Al atoms require less energy to form vacancies than Ni atoms [32]. Moreover, the diffusion rate of Al atoms is higher than that of Ni atoms in Ni-based single-crystal
superalloys [5,33]. Therefore, the reason for the decrease of Al content in γ´ precipitates neighboring the bonding interface may be that Al atoms diffused from the substrate to the interface to be joined, filling up the voids and finally forming the bonding interface.
Fig. 7. TEM analysis of the joint bonded at 1100°C for 60min under 20MPa: (a-c) bright-field images; (d-f) EDS results of the marked locations in (c); (g) SAED pattern of coarsened γ´ precipitates.
When bonded at 1100°C for 60min at 20MPa, the formation of small γ´ precipitates occurred at the interface between the eutectic structure and the dual phase structure in the base metal, as seen in Fig. 8. The formation of small γ´ precipitates might be due
to the concentration gradient of Al content between the dual phase structure and the interface, as analyzed in Section 2. Based on the above analysis, it can be concluded that the content of Al was reduced in the dual phase zone because of its diffusion from the substrate to the interface zone. Therefore, Al was transported from the eutectic zone with high Al content to the dual phase zone in the form of decomposition γ´ precipitates to maintain the balance of the system.
Fig. 8. The decomposition phenomenon of γ´ precipitates: (a) the entire microstructure; (b) magnification of area b; (c) magnification of area c.
3.3. Mechanical properties of Ni3Al-based alloy direct diffusion bonded joint Fig. 9 shows the effect of bonding temperature and time on the shear strength and interface bonding ratio of the diffusion bonded joints. For the diffusion bonding of similar materials, the interface bonding ratio is generally considered to be a meaningful criterion to evaluate the mechanical performance of the bonded joints [34]. It should be noted in Fig. 9(a-b) that there is a positive relationship between the shear strength and the bonding ratio. This can be explained by the fact that the diffusion rate of atoms was improved by the increase of the joining parameters, and this accelerated the closing of the unbonded areas. Therefore, the shear strength was gradually increased with the increase of bonding temperature and time. The highest shear strength and interface bonding ratio (689MPa and 95%, respectively) were achieved when the samples were bonded at 1100°C for 60min under 20MPa.
Fig. 9. Effect of (a) joining temperature and (b) holding time on the shear strength and interface bonding ratio of Ni3Al-based alloy joints.
Fig. 10 presents the SEM images of the fracture morphologies of the joints that were diffusion bonded at different temperatures. When the joining temperature was low, such as 950°C, copious voids were observed at the diffusion zone, as demonstrated in Fig. 5(a). Therefore, the fracture surface was smooth and the black area was the unbonded zone. The fracture mainly occurred at the bonding interface. When the bonding temperature was 1100°C, cracks propagated mainly in the Ni3Al substrate and partially at the bonding interface during the shear test. The fracture morphology at the interface was mainly exhibited shallow dimples and cleavage fractures. However, the fracture occurred in the substrate was primarily brittle fracture because the presence of quasi-cleavage fracture characterization.
Fig. 10. Fracture morphologies and corresponding schematic diagrams of fracture paths of the joints bonded at (a-c) 950°C; (d-f) 1100°C.
3.4. Diffusion bonding of Ni3Al-based alloy using a 30µm-Ni foil When the direct diffusion bonding temperature of the Ni3Al-based alloy was 1100°C, severe coarsening of the γ´ precipitates occurred. The morphology of the γ´ precipitates was transformed from cubic to strip-like. It should be noted that the mechanical performance of Ni-based alloys is affected by the size, morphology, and volume fraction of γ´ precipitates [35,36]. Therefore, the morphological change of the γ´ phase caused by the high bonding temperature would affect the creep resistance and service life of the Ni-based alloy [37,38]. To mitigate the impact of the coarsening of the γ´ precipitates, it is necessary to reduce the bonding temperature. Therefore, a Ni foil with a thickness of 30µm was used as an interlayer. Fig. 11 shows the interfacial microstructure and distribution of the elements within the joints that were diffusion bonded at different temperatures with 30µm thick Ni foil. The joints were well bonded without micro-voids for all three temperatures after the addition of the Ni interlayer with good plastic deformation ability. Fig. 11(d-f) illustrates the line distribution of
the elements across the joining interface. The thickness of the residual Ni interlayer decreased from 20µm to 8µm when the joining temperature was increased from 950°C to 1000°C. Subsequently, the pure Ni foil completely disappeared because of the increased elemental interdiffusion between the Ni foil and the base metal when the joining temperature was increased to 1050°C. In addition, the concentration of Fe, Al, Mo, and Cr gradually increased from the joint center to the base metal, but the variation in the Ni content was the opposite because of the concentration gradient.
Fig. 11. Microstructural observations and corresponding element distribution in the samples bonded with 30µm Ni foil for 60min under 20MPa at: (a, d) 950°C; (b, e) 1000°C; (c, f) 1050°C.
Fig. 12 demonstrates the shear strength of the joints that were diffusion bonded at different joining temperatures with a 30µm thick Ni interlayer for 60min under 20MPa. Compared to direct diffusion bonding, the shear strength of the joint bonded at 950°C and 1000°C using the Ni foil as the interlayer increased from 80MPa to 349MPa and from 256MPa to 465MPa, respectively. Upon further increasing the bonding temperature to 1050°C, the shear strength did not change markedly and remained at 488MPa.
Fig. 12. The effect of bonding temperature on the shear strength of the joint bonded with 30µm thick Ni foil.
Fig. 13 demonstrates the SEM fracture morphologies and XRD pattern of the joints diffusion bonded at different bonding temperatures. When Ni foil was inserted between the Ni3Al substrates, the fracture surface presented a lamellar structure, as seen in Fig. 13(a). Moreover, the disconnected areas that were clearly observed on the fracture surface of the joint that had been direct diffusion bonded at 950°C vanished. Shallow dimples gradually appeared on the fracture surface as a result of increasing the joining temperature, as demonstrated in Fig. 13(b). This is because the fact that the degree of interdiffusion between the substrate and the interlayer was improved by the increase of bonding temperature from 950°C to 1000°C. Upon further increasing the bonding temperature to 1050°C, the pure Ni foil disappeared, and the number of dimples increased, as shown in Fig. 13(c). The lattice constant of the phase to be identified was calculated based on the XRD pattern. When the Ni interlayer was used for diffusion bonding at 950℃, 1000℃ and 1050℃, the average lattice constants of the phase on the fracture surface were 0.35301nm, 0.35252nm and 0.35287nm, respectively, which was basically equal to the lattice constant of Ni (0.3524nm). According to the EDS analysis and XRD pattern shown in Table 5, the rupture location of the joints was mostly concentrated in the diffusion zone.
Fig. 13. The fracture morphologies of the joints bonded with a 30µm thick Ni interlayer at different bonding temperatures, and the corresponding XRD pattern of the ruptured surface: (a) 950°C; (b) 1000°C; (c) 1050°C; (d) XRD pattern. Table 5 Chemical composition for each point in Fig. 13(a-c) (at%). Position
Al
Mo
Ti
Cr
Fe
Ni
Possible phase
A
10.06
1.01
0.13
8.45
12.88
67.48
Ni(s,s)
B C D E F
7.03 2.07 0.84 6.41 6.40
0.29 0.16 0.20 0.22 0.25
0.25 0.27 0.64 0.36 0.30
3.53 0.33 0.21 1.38 1.01
3.76 0.88 0.67 2.41 2.34
85.14 96.29 97.44 89.22 89.69
Ni(s,s) Ni(s,s) Ni(s,s) Ni(s,s) Ni(s,s)
3.5. Diffusion bonding of Ni3Al-based alloy using a 3µm-Ni coating Although the pure Ni interlayer completely disappeared by interdiffusion when bonded at 1050°C, the performance of the joint was still limited because of the difference in microstructure between the bonding interface and base metal. One solution to this problem would be to reduce the thickness of the Ni interlayer. Therefore, a Ni coating with a thickness of about 3µm was electroplated onto the
substrate surface. Fig. 14(a) shows the microstructure of the joint bonded at 1050°C under 20MPa for 60min with this Ni coating. According to the analysis of element distribution along the diffusion zone, there is no significant difference in composition between the diffusion zone and the base metal. This suggests that the Ni coating completely disappeared and formed the same microstructure as the substrate. In contrasted to the joint bonded at 1050°C with 30µm Ni foil, the shear strength was significantly increased when the 3µm Ni coating was introduced for diffusion bonding. The maximum shear strength of the joint was 617MPa, which basically reached the level of direct diffusion bonding at 1100°C.
Fig. 14. The microstructure in SE mode and element distribution of the joint bonded at 1050°C with Ni coating: (a) interfacial microstructure; (b) elemental distribution along the white line in (a).
The fracture morphologies of the joints formed by direct diffusion bonding were characterized by brittle fracture as the main fracture mode, as seen in Fig. 8. However, it can be seen from Fig. 15(a) that the fracture surface of the joint bonded with the Ni coating was characterized by alternating dimples and flat morphologies. For the joint bonded with Ni coating, the calculated lattice constant of the phase on the fracture surface was 0.35665nm, which was very close to that of Ni3Al (0.3572nm). Moreover, the appearance of superlattice diffraction (110) peak also indicated the existence of L12 ordered Ni3Al phase [39]. According to the EDS results and XRD analysis, the
composition of the fracture surface was Ni3Al.
Fig. 15. The fracture morphology of the joint bonded at 1050°C with the Ni coating and the corresponding XRD pattern of the ruptured surface: (a) typical fracture morphology; (b) magnification of area 1; (c) magnification of area 2; (d) XRD pattern. Table 6 Chemical composition at the locations shown in Fig. 15(b) and (c) (at%). Position
Al
Mo
Ti
Cr
Fe
Ni
Possible phase
A B
22.83 16.69
0.75 0.70
0.22 0.39
4.99 6.75
7.80 10.54
63.42 64.92
Ni3Al Ni3Al
Conclusions (1) A maximum shear strength of 689MPa and an interface bonding ratio of 95% were achieved for the Ni3Al-based alloy joint that was direct diffusion bonded at 1100°C for 60min. The fracture position of the joint was transferred from the bonding interface to the substrate due to the increase bonding temperature and time. The fracture morphology was characterized primarily by quasi-cleavage fracture. (1) In addition, the formation mechanism of the bonding interface and the phenomenon of the coarsening of the γ´ precipitates were controlled by the diffusion of Al. (2) In order to alleviate the coarsening of γ´ precipitates caused by the high bonding temperature, pure Ni foil was introduced to reduce the bonding temperature. The shear strength of the joints bonded at 950°C and 1000°C with this Ni foil interlayer was significantly increased. However, when bonded at 1050°C, the performance of the joint was still limited by the microstructure heterogeneity between the base metal and the diffusion zone. The maximum shear strength was 488MPa and the fracture occurred in the Ni diffusion layer. The fracture morphology was dimple-like, which could be evidence of ductile rupture. (3) When bonded at 1050°C with a 3µm electroplated Ni coating, the shear strength of the joint basically reached the level of the direct diffusion bonded joint bonded at 1100°C. Moreover, the degree of coarsening of the γ´ precipitates was reduced. The fracture morphology changed from the main cleavage fracture under direct diffusion bonding to the alternating dimples and facets with the addition of the Ni coating.
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Highlights: 1. Direct diffusion bonding of Ni3Al-based alloys was carried out. 2. The formation mechanism of the direct diffusion bonded joint was revealed. 3. A Ni interlayer was introduced to reduce the bonding temperature.
Declaration of interests ☐ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☒The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: