- Email: [email protected]
Ti6Al4V joints
Ceramics International 45 (2019) 18119–18123
Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Short communication
In situ TiB-network-reinforced Al2O3/Ti6Al4V joints Yang Weiqi, Yang Xunuo, Dai Wei, Chen Liutong, Lin Jincheng, Ao Run, Ma Xianfeng, Xing Lili
∗
T
Sino-French Institute of Nuclear Engineering and Technology, Sun Yat-sen University, Zhuhai, 519082, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Brazing Electroless plating Whisker reinforced composites Microstructure TiB network
A three-dimensional network architecture of TiB whiskers were in situ synthesized in Al2O3/Ti6Al4V joints by using Ni–B coated Cu foam interlayer. Boron was extracted from Ni–B alloy at low braze temperature to form a thin flexible boride layer comprised by TiB and Ni3B nano-particles. Subsequently, a network structure of TiB whiskers was developed from the interlaced boride layers at high temperature. The underlying mechanisms were addressed based on microstructural evolution. The uncoordinated deformation of the network microstructure, stress adjustments and crack-deflection effects of TiB whiskers contributed to the high shear strength of joints.
1. Introduction Engineering ceramics represent credible materials capable of serving in severe environments. In the vast majority of applications, the engineering ceramic needs to be joined to metal to constitute a composite component [1]. Brazing is a common approach to achieve metallurgical bonding of ceramic and metal with good hermeticity and high temperature performance [2]. However, a main issue for this technique is the high residual stresses caused by the mismatch in coefficient of thermal expansion (CTE) between the two materials [3,4], which could be a hidden danger leading to premature failure of component under very low loads. A simple way to improve the stress state in ceramic/metal joints is by incorporating low CTE reinforcements into braze alloy, such as ceramic particles [5] or refractory metal particles [6]. Recently, in-situ TiB methods flourish in the research of brazing [7–11]. Compared with conventional ex-situ methods, in-situ methods offer stronger interface bonding between reinforcements and matrix. Additionally, TiB whiskers with high-aspect-ratio lower the CTE of braze alloy more effectively and show a potent strengthening effect [7]. Although a number of studies have been carried out to reveal the reinforcing effects of TiB, few concerns the effect of its distribution on the joint performance. It has been a common practice to pursue a homogeneous distribution of TiB in seam. But according to the investigations on metal matrix composites (MMCs) [12], a homogeneous distribution of reinforcements usually results in very low damage tolerance due to impurities (such as oxides) introduced in powder metallurgy process. Conversely, multi-scale hierarchical structures with metallic phases around stiffer phases show superior performance [13,14]. Following this path, we reported a new brazing method for Al2O3/Ti6Al4V joints reinforced by ∗
3-dimensional in-situ TiB network. The electroless plating process was carried out to prepare Ni–B alloy coated Cu foam composite interlayer, which was used as boron source to implement the design of TiB distribution. The evolution of the network microstructure and the mechanical properties of the joints were studied. 2. Materials and experimental procedures The pure copper foam (89% porosity, hole sizes of ∼50 μm, thickness of ∼200 μm, supplied by Taili Foam Metal Co., Ltd.) with opencelled network structure was cut into 6 × 6 mm2 and activated in 5% HNO3 aqueous solution for about 2 min. After chemical etching, the copper foams were rinsed with deionised water, then got coated in a Ni–B electroless solution under stirring. The chemical composition of the electroless solution is shown in Table 1. The coating temperature is 90–95 °C. Around 2 μm thick Ni–B deposits were obtained after 30 min of immersion. The polycrystalline Al2O3 ceramic (94.9 wt% Al2O3, density 3.9 g/ cm3), supplied by Hua Quan Electric Co., Ltd., was sawn to 6 × 6 × 3 mm3 by diamond wire cutter. The Ti6Al4V alloy (TC4), supplied by Baotai Metal Co., Ltd., was cut to 7 × 15 × 2 mm3 by wirecut electric discharge machine. The brazing alloy is Ag–Cu eutectic foil (72 wt% Ag, 100 μm thick) purchased from Zhengzhou Research Institute of Mechanical Engineering. Before brazing, the surfaces of materials to be joined were ground to grit 1000 by abrasive paper, then ultrasonically cleaned in acetone for 5 min. The braze interlayer AgCu foil/Ni–B coated Cu foam/AgCu foil (hereinafter "composite interlayer") were sandwiched between Al2O3 ceramic and Ti6Al4V alloy with a normal load of 3 kPa. The schematic illustration of Ni–B coating and joint assembly is shown in Fig. 1a. The brazing process was carried
Corresponding author. E-mail addresses: [email protected], [email protected] (X. Lili).
https://doi.org/10.1016/j.ceramint.2019.05.264 Received 18 April 2019; Received in revised form 23 May 2019; Accepted 23 May 2019 Available online 24 May 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Ceramics International 45 (2019) 18119–18123
Y. Weiqi, et al.
3. Results and discussion
Table 1 Chemical composition of the electroless nickel plating bath. Bath composition
Concentration (g/L)
Hexahydrated nickel chloride (NiCl2·6H2O) Sodium hydroxide (NaOH) Ethylene diamine (NH2–CH2–CH2–NH2) Sodium borohydride (NaBH4)
24 39 54 0.49
out in a vacuum furnace with vacuum better than 3 × 10−3 Pa. The assembly was heated to brazing temperature at 10 °C/min, isothermally held for 10 min, and then cooled to room temperature at 10 °C/min. Two typical brazing temperatures, namely 860 °C and 900 °C, were specifically studied, at which the seam was Ag–Cu dominated and Ti–Cu dominated, respectively [7]. The heat treatment of Ni–B coated Cu foam was performed so as to study its possible microstructure change during heating in brazing process. The sample was first heated in a vacuum tube furnace to 700 °C at 10 °C/min, then the heating elements were removed and the sample was quenched in argon atmosphere in room temperature. The Ni–B coated Cu foam and the one after heat treatment were tested by X-ray diffraction (XRD, PANalytical). The microstructure of the joints was characterized by scanning electron microscope (SEM, FEI Quanta 400FEG) and transmission electron microscopy (TEM, JEOL JEM3200FS). The sample for TEM was prepared by a focused ion beam system (FIB, FEI Scios). The shear strength of joints was examined using a universal testing machine (Instron-1186). The scheme of shear testing was reported in Ref. [8] and the strength was calculated by dividing the maximum load by the surface of the joint. The tested values for each condition were an average of at least 3 measurements. The fracture of the joints was observed by SEM and digital microscope (Keyence VHX6000).
Fig. 1b shows the original morphology of Cu foam. The open-cell network structure of Cu foam made it possible to be filled by aqueous solution or molten alloy, which facilitated plating and brazing process. The electroless plating on Cu foam produced an uniform Ni–B layer exhibiting cauliflower-like structure (Fig. 1c). But due to the CTE mismatch, crack and peeling of the Ni–B layer can be clearly found. The XRD pattern (Fig. 1d) of the as deposited layer shows a low single broad peak indicative of the amorphous nature of the coating [15]. When heated to 700 °C (Fig. 1d), the coating transformed to well-crystallized Ni and Ni3B. Fig. 2a shows the microstructure of Al2O3/Ti6Al4V joint brazed at 860 °C with the composite interlayer. The Cu foam losing protection of Ni coating totally dissolved in the Ag–Cu liquid and produced grey phases extensive distributed in the white Ag solid solution (Agss). From a zoom-in SEM image (Fig. 2b), the remaining Ni coating (point A) was identified by EDS (Table 2). It's noteworthy that some black "lines" appear around the Niss and separate the Ag–Cu alloy into several regions. A focused view (Fig. 2c) of the "line" shows the detailed microstructure: on one side, the line boundary is coarse and the alloy is doped with dispersive nano-particles or needle shape phases, on the other side, the line boundary is smooth and the alloy is clear. In order to identify the compositions of the "line", characterization by TEM was performed. The bright field image (Fig. 2d and e) shows that a number of nanoparticles gathered to constitute the quasi-continuous linear structure. The selected area electron diffraction (SAED) patterns confirmed the nano-particles were comprised by two phases, namely TiB and Ni3B, and the metal matrix was CuNi2Ti. The formation of the boride "line" indicates the segregation and reaction of boron from Ni–B alloy have happened at relatively low temperature. These procedures are efficient not only in
Fig. 1. a) Schematic diagram of electroless plating and brazing process; b) Morphology of Ni–B coated Cu foam; c) Magnified image of Ni–B coated Cu foam; d) XRD patterns of as deposited Cu foam and heat-treated Cu foam. 18120
Ceramics International 45 (2019) 18119–18123
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Fig. 2. a) Microstructure of Al2O3/Ti6Al4V joint brazed at 860 °C. b) Backscattered electron image of the braze alloy. c) The magnified image of nano-boride layer. d) Bright field image of the boride layer. e) A focused view of the selected region in d). Table 2 EDS analysis of the zones identified in Figs. 2 and 3. Points
A B C D E F G
Elements (at. %)
Possible Phase
Ag
Cu
Ti
Ni
Al
V
1 85 93 3 6 3 1
8 10 6 92 87 21 26
– 1 1 – 3 65 49
81 4 – 3 – 7 16
10 – – 2 4 4 5
– – – – – – 3
(Ni, Al, Cu)ss Agss Agss Cuss Cuss Ti2(Cu0.75Ni0.25) Ti(Cu0.6Ni0.4)
thermodynamics but also in kinetics. As the Gibbs energy of TiB is much lower than Ni3B (ΔGo(TiB) = −156 kJ/mol, ΔGo(Ni3B) = −81 kJ/mol, at 900 °C [16]), boron atoms in Ni–B alloy tended to diffuse into AgCu liquid containing low content of Ti dissolved from TC4 alloy. Fast diffusion of boron atoms in Ni also could be achieved due to its high diffusion coefficient (DBNi = 6.24 × 10−8 m2/s, at 900 °C [17]). When
boron atoms dissolved in Ag–Cu liquid, abundant TiB nucleated and depleted Ti in local liquid alloy. With further accumulation of boron, Ni3B recrystallized close to TiB nucleus to constitute a layer whose cross-sections appeared as the black "lines" in Fig. 2b and c. The mobility of those nano particles made the boride layers presented flexible in the flowing braze alloy, which could be widely observed in Fig. 2b. Additionally, due to the close contact of the nanoparticles (as shown in Fig. 2e), the boride layers hindered atom diffusion to some extent. The EDS analysis (Table 2) on the microstructure of both sides of the boride layer (Fig. 2c) demonstrates the Ni fractions in Agss (point B) and Cuss (point D) nearby the remaining Ni coating are higher than those in the other side (point C and E), while the Ti fractions are just in the opposite trend. Because Ni dramatically suppressed the activity of Ti [18], the nucleation and growth of TiB only happened at the Ti-rich part. The unilateral growth of TiB and the expansion tendency of boride layers from the remaining Niss can be clearly found as indicated in Fig. 2b. Fig. 3 shows the microstructure of the joint brazed at 900 °C. The width of the seam is significantly decreased compared with the one brazed at 860 °C, suggesting a part of Ag–Cu liquid has been squeezed
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Fig. 3. Backscattered electron image of Al2O3/Ti6Al4V joint brazed at 900 °C. The inset is the magnified image of TiB network.
out of the seam. It's observed that a quasi-network of TiB whiskers with irregularly shaped "mesh" (as indicated in yellow dash lines) formed in the whole seam. The matrix of the seam is mainly comprised by a grey phase indentified as Ti2(Cu0.75Ni0.25) (point F) [19] and dispersively distributed Agss. Apparently, the remaining Ni coating has totally dissolved in the subsequent Ti–Cu melts at high temperature and finally increased the average Ni content in metal matrix. The inset of Fig. 3 shows the magnified image of TiB network, in which clusters of nanoTiB whiskers, together with some larger ones, randomly distributed in a narrow region. This network structure of TiB evolved from the boride layers produced at the early stage of brazing. When the joint was heated to high temperature, the majority of TiB initially formed at low temperature (as shown in Fig. 2c) was squeezed out of the seam with Ag–Cu liquid. Meanwhile, with compression of the seam, the boride layers interweaved together to form a three-dimensional network structure which served as boron source for further in-situ TiB reaction. Because the dissolution of Ti from Ti6Al4V alloy increased at high temperature, the fluidity of braze melt decreased. The newly produced TiB whiskers nearly stayed where the boron source lay, thus appearing as a network distribution. The EDS results show that the matrix (point G) of TiB aggregated region is rich in Ni, which is consistent with the fact that the boride layer is partly comprised by Ni3B. The shear strength of Al2O3/Ti6Al4V joints brazed with composite interlayer at 860 °C was quite low (only 23 ± 6.7 MPa) due to the insufficient reactions on ceramic interface and voids in seam. But as the temperature increased to 900 °C, the strength reached 99.9 ± 15.8 MPa, 71.9% higher than the one (58.1 ± 17.9 MPa) with AgCu eutectic interlayer. Fig. 4a shows the load-displacement curves of the two joints brazed at 900 °C. At the maximum load, the joint with AgCu eutectic interlayer ruptured abruptly and the fracture presented a bowl-shape morphology, indicating large stress concentration in ceramic [20]. Comparatively, the curve of the joint with composite interlayer shows a gradual drop with many steps after the maximum load. Cracks propagated along the joining interface and then deflected to ceramic (Fig. 4a). A detailed SEM observation (Fig. 4b) on this fracture identifies the TiB aggregated regions presented as network architecture, in which the exposed whiskers as well as the pores, caused by pullout and crack-deflection effects, can be clearly found (Fig. 4c).
Due to the uncoordinated deformation between the metal matrix and TiB network during shear test, the micro fracture exhibits a rough surface: the TiB-free regions either protrude or go deep lower than the TiB aggregated regions. This fracture mode together with the crackdeflection effect of TiB are expected to have contributed to the controlled failure of the joints [12]. Similar observations were made with Ti and Al MMCs possessing network microstructure [21,22]. Additionally, the improvement of joint strength may be also attributed to the stress adjustment of TiB network. As TiB whiskers have low CTE (αTiB = 8 × 10−6 K−1 [7]) and high contiguity in their aggregated region, the deformation of metal matrix induced by temperature could be restrained by the TiB network with micron scale mesh, thus the stress concentration in ceramic was alleviated.
4. Conclusion In summary, we proposed a simple approach to fabricate TiB network reinforced Al2O3/Ti6Al4V joints by using Ni–B coated copper foam interlayer. At low braze temperature, boron was extracted from Ni–B alloy in the form of Ni3B and TiB. The two phases in nanoscale constituted flexible boride layers which interweave together after the extrusion of AgCu liquid at high temperature. A quasi-network structure of TiB whiskers were subsequently developed from the interlaced boride layers. The joints prepared by this approach show significant improvement in shear strength. The uncoordinated deformation of the network microstructure as well as the crack deflection of TiB helped to toughen the joints.
Acknowledgement The authors gratefully acknowledge the financial support from National Natural Science Foundation of China (NSFC, Grant number 51605499 and 11504438), Fundamental Research Funds for the Central Universities (171gpy35 and 171gpy33) and Department of Education of Guangdong Province (2016KQNCX005).
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Fig. 4. a) Load-displacement curves of Al2O3/TC4 joints brazed with composite interlayer and AgCu eutectic interlayer at 900 °C. The right side is optical pseudo-3D fractures of the two joints. b) The fracture of Al2O3/TC4 joint brazed with composite interlayer. c) Magnified image of TiB network in b).
References [1] J.A. Fernie, R.A.L. Drew, K.M. Knowles, Joining of engineering ceramics, Int. Mater. Rev. 54 (2009) 283–331. [2] O.M. Akselsen, Advances in brazing of ceramics, J. Mater. Sci. 27 (1992) 1989–2000. [3] Z.W. Yang, C.L. Wang, Y. Han, Y.T. Zhao, Y. Wang, D.P. Wang, Design of reinforced interfacial structure in brazed joints of C/C composites and Nb by pre-oxidation surface treatment combined with in situ growth of CNTs, Carbon 143 (2019) 494–506. [4] R. Asthana, M. Singh, Joining of ZrB2-based ultra-high-temperature ceramic composites using Pd-based braze alloys, Scripta Mater. 61 (2009) 257–260. [5] G. Blugan, J. Kuebler, V. Bissig, J. Janczak-Rusch, Brazing of silicon nitride ceramic composite to steel using SiC-particle-reinforced active brazing alloy, Ceram. Int. 33 (2007) 1033–1039. [6] Y.M. He, J. Zhang, Y. Sun, C.F. Liu, Microstructure and mechanical properties of the Si3N4/42CrMo steel joints brazed with Ag-Cu-Ti plus Mo composite filler, J. Eur. Ceram. Soc. 30 (2010) 3245–3251. [7] W.Q. Yang, T.S. Lin, P. He, M. Zhu, C.B. Song, D.C. Jia, J.C. Feng, Microstructural evolution and growth behavior of in situ TiB whisker array in ZrB2-SiC/Ti6Al4V brazing joints, J. Am. Ceram. Soc. 96 (2013) 3712–3719. [8] P. He, W.Q. Yang, T.S. Lin, D.C. Jia, J.C. Feng, Y. Liu, Diffusion bonding of ZrB2-SiC/Nb with in situ synthesized TiB whiskers array, J. Eur. Ceram. Soc. 32 (2012) 4447–4454. [9] M. Yang, T. Lin, P. He, Y. Huang, In situ synthesis of TiB whisker reinforcements in the joints of Al2O3/TC4 during brazing, Mater. Sci. Eng. A 528 (2011) 3520–3525. [10] J. Ba, H. Li, B.X. Ren, B.X. Qi, X.H. Zheng, R. Ning, J.L. Qi, J. Cao, W. Cai, J.C. Feng, In situ formation of TiB whiskers to reinforce SiO2-BN/Ti6Al4V brazed joints, Ceram. Int. 45 (2019) 8054–8057.
[11] J.M. Shi, L.X. Zhang, H. Liu, Z. Sun, J.C. Feng, Vacuum brazing of SiBCN ceramic and TC4 alloy using TiB2 reinforced AgTi composite filler, Vacuum 156 (2018) 108–114. [12] L.Q. Huang, L.H. Wang, M. Qian, J. Zou, High tensile-strength and ductile titanium matrix composites strengthened by TiB nanowires, Scripta Mater. 141 (2017) 133–137. [13] L.J. Huang, L. Geng, H.X. Peng, Microstructurally inhomogeneous composites: is a homogeneous reinforcement distribution optimal? Prog. Mater. Sci. 71 (2015) 93–168. [14] K. Lu, The future of metals, Science 328 (2010) 319–320. [15] W.T. Evans, M. Schlesinger, The effect of solution pH and heat-treatment on the properties of electroless nickel boron films, J. Electrochem. Soc. 141 (1994) 78–82. [16] E.T. Turkdogan, Physical Chemistry of High Temperature Technology, Academic Press, New York, 1980. [17] W.D. Wang, S.H. Zhang, X.L. He, Diffusion of boron in alloys, Acta Metall. Mater. 43 (1995) 1693–1699. [18] R. Arroyave, T.W. Eagar, Metal substrate effects on the thermochemistry of active brazing interfaces, Acta Mater. 51 (2003) 4871–4880. [19] W.J. Zhu, L.I. Duarte, C. Leinenbach, Experimental study and thermodynamic assessment of the Cu-Ni-Ti system, Calphad-Comput. Coupling Ph, Diagrams Thermochem. 47 (2014) 9–22. [20] J.W. Park, P.F. Mendez, T.W. Eagar, Strain energy distribution in ceramic-to-metal joints, Acta Mater. 50 (2002) 883–899. [21] B. Kaveendran, G.S. Wang, L.J. Huang, L. Geng, Y. Luo, H.X. Peng, In situ (Al3Zrp +Al2O3np)/2024Al metal matrix composite with controlled reinforcement architecture fabricated by reaction hot pressing, Mater. Sci. Eng. A 583 (2013) 89–95. [22] L.J. Huang, S. Wang, Y.S. Dong, Y.Z. Zhang, F. Pan, L. Geng, H.X. Peng, Tailoring a novel network reinforcement architecture exploiting superior tensile properties of in situ TiBw/ Ti composites, Mater. Sci. Eng. A 545 (2012) 187–193.
18123
Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Short communication
In situ TiB-network-reinforced Al2O3/Ti6Al4V joints Yang Weiqi, Yang Xunuo, Dai Wei, Chen Liutong, Lin Jincheng, Ao Run, Ma Xianfeng, Xing Lili
∗
T
Sino-French Institute of Nuclear Engineering and Technology, Sun Yat-sen University, Zhuhai, 519082, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Brazing Electroless plating Whisker reinforced composites Microstructure TiB network
A three-dimensional network architecture of TiB whiskers were in situ synthesized in Al2O3/Ti6Al4V joints by using Ni–B coated Cu foam interlayer. Boron was extracted from Ni–B alloy at low braze temperature to form a thin flexible boride layer comprised by TiB and Ni3B nano-particles. Subsequently, a network structure of TiB whiskers was developed from the interlaced boride layers at high temperature. The underlying mechanisms were addressed based on microstructural evolution. The uncoordinated deformation of the network microstructure, stress adjustments and crack-deflection effects of TiB whiskers contributed to the high shear strength of joints.
1. Introduction Engineering ceramics represent credible materials capable of serving in severe environments. In the vast majority of applications, the engineering ceramic needs to be joined to metal to constitute a composite component [1]. Brazing is a common approach to achieve metallurgical bonding of ceramic and metal with good hermeticity and high temperature performance [2]. However, a main issue for this technique is the high residual stresses caused by the mismatch in coefficient of thermal expansion (CTE) between the two materials [3,4], which could be a hidden danger leading to premature failure of component under very low loads. A simple way to improve the stress state in ceramic/metal joints is by incorporating low CTE reinforcements into braze alloy, such as ceramic particles [5] or refractory metal particles [6]. Recently, in-situ TiB methods flourish in the research of brazing [7–11]. Compared with conventional ex-situ methods, in-situ methods offer stronger interface bonding between reinforcements and matrix. Additionally, TiB whiskers with high-aspect-ratio lower the CTE of braze alloy more effectively and show a potent strengthening effect [7]. Although a number of studies have been carried out to reveal the reinforcing effects of TiB, few concerns the effect of its distribution on the joint performance. It has been a common practice to pursue a homogeneous distribution of TiB in seam. But according to the investigations on metal matrix composites (MMCs) [12], a homogeneous distribution of reinforcements usually results in very low damage tolerance due to impurities (such as oxides) introduced in powder metallurgy process. Conversely, multi-scale hierarchical structures with metallic phases around stiffer phases show superior performance [13,14]. Following this path, we reported a new brazing method for Al2O3/Ti6Al4V joints reinforced by ∗
3-dimensional in-situ TiB network. The electroless plating process was carried out to prepare Ni–B alloy coated Cu foam composite interlayer, which was used as boron source to implement the design of TiB distribution. The evolution of the network microstructure and the mechanical properties of the joints were studied. 2. Materials and experimental procedures The pure copper foam (89% porosity, hole sizes of ∼50 μm, thickness of ∼200 μm, supplied by Taili Foam Metal Co., Ltd.) with opencelled network structure was cut into 6 × 6 mm2 and activated in 5% HNO3 aqueous solution for about 2 min. After chemical etching, the copper foams were rinsed with deionised water, then got coated in a Ni–B electroless solution under stirring. The chemical composition of the electroless solution is shown in Table 1. The coating temperature is 90–95 °C. Around 2 μm thick Ni–B deposits were obtained after 30 min of immersion. The polycrystalline Al2O3 ceramic (94.9 wt% Al2O3, density 3.9 g/ cm3), supplied by Hua Quan Electric Co., Ltd., was sawn to 6 × 6 × 3 mm3 by diamond wire cutter. The Ti6Al4V alloy (TC4), supplied by Baotai Metal Co., Ltd., was cut to 7 × 15 × 2 mm3 by wirecut electric discharge machine. The brazing alloy is Ag–Cu eutectic foil (72 wt% Ag, 100 μm thick) purchased from Zhengzhou Research Institute of Mechanical Engineering. Before brazing, the surfaces of materials to be joined were ground to grit 1000 by abrasive paper, then ultrasonically cleaned in acetone for 5 min. The braze interlayer AgCu foil/Ni–B coated Cu foam/AgCu foil (hereinafter "composite interlayer") were sandwiched between Al2O3 ceramic and Ti6Al4V alloy with a normal load of 3 kPa. The schematic illustration of Ni–B coating and joint assembly is shown in Fig. 1a. The brazing process was carried
Corresponding author. E-mail addresses: [email protected], [email protected] (X. Lili).
https://doi.org/10.1016/j.ceramint.2019.05.264 Received 18 April 2019; Received in revised form 23 May 2019; Accepted 23 May 2019 Available online 24 May 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Ceramics International 45 (2019) 18119–18123
Y. Weiqi, et al.
3. Results and discussion
Table 1 Chemical composition of the electroless nickel plating bath. Bath composition
Concentration (g/L)
Hexahydrated nickel chloride (NiCl2·6H2O) Sodium hydroxide (NaOH) Ethylene diamine (NH2–CH2–CH2–NH2) Sodium borohydride (NaBH4)
24 39 54 0.49
out in a vacuum furnace with vacuum better than 3 × 10−3 Pa. The assembly was heated to brazing temperature at 10 °C/min, isothermally held for 10 min, and then cooled to room temperature at 10 °C/min. Two typical brazing temperatures, namely 860 °C and 900 °C, were specifically studied, at which the seam was Ag–Cu dominated and Ti–Cu dominated, respectively [7]. The heat treatment of Ni–B coated Cu foam was performed so as to study its possible microstructure change during heating in brazing process. The sample was first heated in a vacuum tube furnace to 700 °C at 10 °C/min, then the heating elements were removed and the sample was quenched in argon atmosphere in room temperature. The Ni–B coated Cu foam and the one after heat treatment were tested by X-ray diffraction (XRD, PANalytical). The microstructure of the joints was characterized by scanning electron microscope (SEM, FEI Quanta 400FEG) and transmission electron microscopy (TEM, JEOL JEM3200FS). The sample for TEM was prepared by a focused ion beam system (FIB, FEI Scios). The shear strength of joints was examined using a universal testing machine (Instron-1186). The scheme of shear testing was reported in Ref. [8] and the strength was calculated by dividing the maximum load by the surface of the joint. The tested values for each condition were an average of at least 3 measurements. The fracture of the joints was observed by SEM and digital microscope (Keyence VHX6000).
Fig. 1b shows the original morphology of Cu foam. The open-cell network structure of Cu foam made it possible to be filled by aqueous solution or molten alloy, which facilitated plating and brazing process. The electroless plating on Cu foam produced an uniform Ni–B layer exhibiting cauliflower-like structure (Fig. 1c). But due to the CTE mismatch, crack and peeling of the Ni–B layer can be clearly found. The XRD pattern (Fig. 1d) of the as deposited layer shows a low single broad peak indicative of the amorphous nature of the coating [15]. When heated to 700 °C (Fig. 1d), the coating transformed to well-crystallized Ni and Ni3B. Fig. 2a shows the microstructure of Al2O3/Ti6Al4V joint brazed at 860 °C with the composite interlayer. The Cu foam losing protection of Ni coating totally dissolved in the Ag–Cu liquid and produced grey phases extensive distributed in the white Ag solid solution (Agss). From a zoom-in SEM image (Fig. 2b), the remaining Ni coating (point A) was identified by EDS (Table 2). It's noteworthy that some black "lines" appear around the Niss and separate the Ag–Cu alloy into several regions. A focused view (Fig. 2c) of the "line" shows the detailed microstructure: on one side, the line boundary is coarse and the alloy is doped with dispersive nano-particles or needle shape phases, on the other side, the line boundary is smooth and the alloy is clear. In order to identify the compositions of the "line", characterization by TEM was performed. The bright field image (Fig. 2d and e) shows that a number of nanoparticles gathered to constitute the quasi-continuous linear structure. The selected area electron diffraction (SAED) patterns confirmed the nano-particles were comprised by two phases, namely TiB and Ni3B, and the metal matrix was CuNi2Ti. The formation of the boride "line" indicates the segregation and reaction of boron from Ni–B alloy have happened at relatively low temperature. These procedures are efficient not only in
Fig. 1. a) Schematic diagram of electroless plating and brazing process; b) Morphology of Ni–B coated Cu foam; c) Magnified image of Ni–B coated Cu foam; d) XRD patterns of as deposited Cu foam and heat-treated Cu foam. 18120
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Fig. 2. a) Microstructure of Al2O3/Ti6Al4V joint brazed at 860 °C. b) Backscattered electron image of the braze alloy. c) The magnified image of nano-boride layer. d) Bright field image of the boride layer. e) A focused view of the selected region in d). Table 2 EDS analysis of the zones identified in Figs. 2 and 3. Points
A B C D E F G
Elements (at. %)
Possible Phase
Ag
Cu
Ti
Ni
Al
V
1 85 93 3 6 3 1
8 10 6 92 87 21 26
– 1 1 – 3 65 49
81 4 – 3 – 7 16
10 – – 2 4 4 5
– – – – – – 3
(Ni, Al, Cu)ss Agss Agss Cuss Cuss Ti2(Cu0.75Ni0.25) Ti(Cu0.6Ni0.4)
thermodynamics but also in kinetics. As the Gibbs energy of TiB is much lower than Ni3B (ΔGo(TiB) = −156 kJ/mol, ΔGo(Ni3B) = −81 kJ/mol, at 900 °C [16]), boron atoms in Ni–B alloy tended to diffuse into AgCu liquid containing low content of Ti dissolved from TC4 alloy. Fast diffusion of boron atoms in Ni also could be achieved due to its high diffusion coefficient (DBNi = 6.24 × 10−8 m2/s, at 900 °C [17]). When
boron atoms dissolved in Ag–Cu liquid, abundant TiB nucleated and depleted Ti in local liquid alloy. With further accumulation of boron, Ni3B recrystallized close to TiB nucleus to constitute a layer whose cross-sections appeared as the black "lines" in Fig. 2b and c. The mobility of those nano particles made the boride layers presented flexible in the flowing braze alloy, which could be widely observed in Fig. 2b. Additionally, due to the close contact of the nanoparticles (as shown in Fig. 2e), the boride layers hindered atom diffusion to some extent. The EDS analysis (Table 2) on the microstructure of both sides of the boride layer (Fig. 2c) demonstrates the Ni fractions in Agss (point B) and Cuss (point D) nearby the remaining Ni coating are higher than those in the other side (point C and E), while the Ti fractions are just in the opposite trend. Because Ni dramatically suppressed the activity of Ti [18], the nucleation and growth of TiB only happened at the Ti-rich part. The unilateral growth of TiB and the expansion tendency of boride layers from the remaining Niss can be clearly found as indicated in Fig. 2b. Fig. 3 shows the microstructure of the joint brazed at 900 °C. The width of the seam is significantly decreased compared with the one brazed at 860 °C, suggesting a part of Ag–Cu liquid has been squeezed
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Fig. 3. Backscattered electron image of Al2O3/Ti6Al4V joint brazed at 900 °C. The inset is the magnified image of TiB network.
out of the seam. It's observed that a quasi-network of TiB whiskers with irregularly shaped "mesh" (as indicated in yellow dash lines) formed in the whole seam. The matrix of the seam is mainly comprised by a grey phase indentified as Ti2(Cu0.75Ni0.25) (point F) [19] and dispersively distributed Agss. Apparently, the remaining Ni coating has totally dissolved in the subsequent Ti–Cu melts at high temperature and finally increased the average Ni content in metal matrix. The inset of Fig. 3 shows the magnified image of TiB network, in which clusters of nanoTiB whiskers, together with some larger ones, randomly distributed in a narrow region. This network structure of TiB evolved from the boride layers produced at the early stage of brazing. When the joint was heated to high temperature, the majority of TiB initially formed at low temperature (as shown in Fig. 2c) was squeezed out of the seam with Ag–Cu liquid. Meanwhile, with compression of the seam, the boride layers interweaved together to form a three-dimensional network structure which served as boron source for further in-situ TiB reaction. Because the dissolution of Ti from Ti6Al4V alloy increased at high temperature, the fluidity of braze melt decreased. The newly produced TiB whiskers nearly stayed where the boron source lay, thus appearing as a network distribution. The EDS results show that the matrix (point G) of TiB aggregated region is rich in Ni, which is consistent with the fact that the boride layer is partly comprised by Ni3B. The shear strength of Al2O3/Ti6Al4V joints brazed with composite interlayer at 860 °C was quite low (only 23 ± 6.7 MPa) due to the insufficient reactions on ceramic interface and voids in seam. But as the temperature increased to 900 °C, the strength reached 99.9 ± 15.8 MPa, 71.9% higher than the one (58.1 ± 17.9 MPa) with AgCu eutectic interlayer. Fig. 4a shows the load-displacement curves of the two joints brazed at 900 °C. At the maximum load, the joint with AgCu eutectic interlayer ruptured abruptly and the fracture presented a bowl-shape morphology, indicating large stress concentration in ceramic [20]. Comparatively, the curve of the joint with composite interlayer shows a gradual drop with many steps after the maximum load. Cracks propagated along the joining interface and then deflected to ceramic (Fig. 4a). A detailed SEM observation (Fig. 4b) on this fracture identifies the TiB aggregated regions presented as network architecture, in which the exposed whiskers as well as the pores, caused by pullout and crack-deflection effects, can be clearly found (Fig. 4c).
Due to the uncoordinated deformation between the metal matrix and TiB network during shear test, the micro fracture exhibits a rough surface: the TiB-free regions either protrude or go deep lower than the TiB aggregated regions. This fracture mode together with the crackdeflection effect of TiB are expected to have contributed to the controlled failure of the joints [12]. Similar observations were made with Ti and Al MMCs possessing network microstructure [21,22]. Additionally, the improvement of joint strength may be also attributed to the stress adjustment of TiB network. As TiB whiskers have low CTE (αTiB = 8 × 10−6 K−1 [7]) and high contiguity in their aggregated region, the deformation of metal matrix induced by temperature could be restrained by the TiB network with micron scale mesh, thus the stress concentration in ceramic was alleviated.
4. Conclusion In summary, we proposed a simple approach to fabricate TiB network reinforced Al2O3/Ti6Al4V joints by using Ni–B coated copper foam interlayer. At low braze temperature, boron was extracted from Ni–B alloy in the form of Ni3B and TiB. The two phases in nanoscale constituted flexible boride layers which interweave together after the extrusion of AgCu liquid at high temperature. A quasi-network structure of TiB whiskers were subsequently developed from the interlaced boride layers. The joints prepared by this approach show significant improvement in shear strength. The uncoordinated deformation of the network microstructure as well as the crack deflection of TiB helped to toughen the joints.
Acknowledgement The authors gratefully acknowledge the financial support from National Natural Science Foundation of China (NSFC, Grant number 51605499 and 11504438), Fundamental Research Funds for the Central Universities (171gpy35 and 171gpy33) and Department of Education of Guangdong Province (2016KQNCX005).
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Fig. 4. a) Load-displacement curves of Al2O3/TC4 joints brazed with composite interlayer and AgCu eutectic interlayer at 900 °C. The right side is optical pseudo-3D fractures of the two joints. b) The fracture of Al2O3/TC4 joint brazed with composite interlayer. c) Magnified image of TiB network in b).
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