Brazing graphite to hastelloy N superalloy using pure-Au filler metal: Bonding mechanism and joint properties

Materials and Design 104 (2016) 1–9

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Brazing graphite to hastelloy N superalloy using pure-Au filler metal: Bonding mechanism and joint properties Yanming He a, Jianguo Yang a,b,⁎, Hanyang Shen a, Limei Wang a, Zengliang Gao a a b

Institute of Process Equipment and Control Engineering, Zhejiang University of Technology, Hangzhou 310014, PR China State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin 150001, PR China

a r t i c l e

i n f o

Article history: Received 26 February 2016 Received in revised form 28 April 2016 Accepted 29 April 2016 Available online 30 April 2016 Keywords: Graphite Hastelloy N alloy Brazing Microstructure Mechanical property

a b s t r a c t The graphite was joined to Hastelloy N alloy using Au foil at 980–1100 °C for 0.5 h. The effect of brazing temperature on microstructure and mechanical properties of the joints was investigated. The typical microstructure for the joint is given below: graphite/Zone 1/Zone 2/Zone 3/Zone 4/Hastelloy N alloy. Zone 1 is mainly composed of Au (s.s) + Ni (s.s), in which some larger blocky Mo2C are embedded. Zone 2 is constituted by Au (s.s) and Ni (s.s). Many Mo-C carbides are precipitated in Zone 3 with Mo2C in the grain interiors and tiny MoC at the grain boundaries. In Zone 4, there are still tiny MoC at the grain boundaries but no Mo2C can be observed. Zones 1 and 2 are formed in a liquid state while Zones 3 and 4 occur due to the diffusion reaction of Mo and C in a solid state. The brazing temperatures used have a limited influence on the joint shear strength. The minimum average value is 22.5 MPa, which is equal to 77% of the graphite's intrinsic strength. A naturally hierarchical structure in relieving the thermal stresses is produced in the joints, contributing to the excellent bonding. © 2016 Elsevier Ltd. All rights reserved.

1. Introduction The graphite materials exhibit excellent performances, such as low density, high melting point, good lubrication, excellent thermal and electrical conductivity, outstanding resistance to thermal shock and fatigue [1]. The application of graphite, however, is restricted due to its intrinsic low mechanical strength. The problem can be overcome by joining the graphite to metals, such as Ni-based alloy. Brazing is the most promising way to join the carbon-based materials [2]. This method shows an encouraging advantage in joining dissimilar materials, particularly the joining of nonmetals to metals [3–5]. Dai et al. [3] reported that a reliable joining of ZrO2 ceramic and TiAl alloy could be achieved when they were brazed with Ag-Cu alloy and the highest shear strength reached 48.4 MPa. Sun et al. [4] introduced the Ag-Cu-Ti alloy to join the SiO2f/SiO2 composite with Invar, and found that the surface of the SiO2f/ SiO2 composite modified by few-layer graphene could significantly improve its wettability. A vacuum-tight Al2O3–5A05 Al alloy joint for application in neutron sensors could be obtained by the aid of two steps, which involved active metallization of the Al2O3 surface, followed by diffusion brazing of metallized Al2O3 and 5A05 Al [5]. In this work, the graphite was joined to Hastelloy N alloy using the brazing. The Ni-MoCr based Hastelloy alloys show a strong resistance to a wide variety of chemical environments, e.g., strong oxidizers, formic and sulfuric acids, hot contaminated media, chlorine, sea water and brine solutions. ⁎ Corresponding author at: Institute of Process Equipment and Control Engineering, Zhejiang University of Technology, Hangzhou 310014, PR China. E-mail address: [email protected] (J. Yang).

http://dx.doi.org/10.1016/j.matdes.2016.04.093 0264-1275/© 2016 Elsevier Ltd. All rights reserved.

The alloy serials attract interests from nuclear industries, aerospace, chemical processing, oil and gas industries [6]. The graphite/Hastelloy N alloy joint shows potential applications in energy and petrochemical industries [7]. The wettability problem should be considered during brazing since most metals can't wet the graphite [2,8–9]. The addition of an activator (Ti, Cr and Zr) into the braze is a common way to improve the wettability. The activator can react with the graphite to form a stable phase, leading to a good joining of the graphite to metals [8,10,11]. Surface modification is another key way to enhance the wettability. After modification, the graphite will be wetted favorably by molten metals [9,12]. The residual stress arising from the coefficient of thermal expansion (CTE) mismatch is another critical problem in joining the graphite to metals [13–14]. The CTE of graphite is usually lower than that of metals and considerable residual stress is easily generated in the joint. This will lead to a low joint bonding strength or even premature failure. Asthana and Singh [15] found that a sound Hastelloy/C-C composite joint could not be obtained using the MBF-20 or MBF-30 (Ni-based filler). This appears to be due to a CTE mismatch between the joined substrates (αgraphite = 2.3 × 10−6 K−1, αHastelloy N = 12.3 × 10−6 K−1). In the present work, Au foil was first introduced to join the graphite with Hastelloy N alloy. It was chosen based on following considerations: Firstly, Au will melt with the Hastelloy N through a contact reaction during brazing, producing liquid in the joint. Secondly, Au has an excellent ability of plastic deformation, which will relieve the residual stresses generated. A reliable joint with a lower level of residual stresses will be expected to be realized. At last, the commercial Ag-Cu-Ti alloy is widely used to join the nonmetals to metals, and this kind of filler

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alloy shows good wettability to most nonmetals [16–19]. Zhou et al. [16] described that the C/C composite could be successfully brazed with TC4 alloy using nano-Al2O3 strengthened Ag-Cu-Ti composite filler. The maximum shear strength for the joints reached 27.8 MPa. However, the application temperature for the joints brazed with Ag-Cu-Ti is below 500 °C. The Au and Au-based alloys can maintain stability at a temperature up to 800 °C [20]. It can be expected that the graphite/ Hastelloy N alloy joints brazed with Au can steadily work at 800 °C. On the basis of above analysis, the effect of brazing temperatures (980–1100 °C) on microstructural evolution of the joints was investigated. The bonding strength was evaluated, and the joining mechanism was also revealed. 2. Materials and methods An average shear strength for the graphite used in this work is 29.4 MPa. The Hastelloy N alloy is a commercial material with following composition: Ni-17.29Mo-6.96Cr-3.96Fe-0.627Mn-0.466Si0.0488C (wt.%). Au foil with a thickness of 25 μm was introduced as the filler. The graphite was cut into samples with a dimension of 4 mm × 4 mm × 4 mm, while the Hastelloy N alloy was sliced into 10 mm × 5 mm × 4 mm. The bonding surfaces (4 mm × 4 mm) were ground using SiC paper, down to a grit size of 800. Last stage of surface preparation was using diamond paste with a particle size of 1 μm. All the materials were ultrasonically cleaned in acetone for 0.5 h prior to assembly. Au foil with a size of 4 mm × 4 mm was sandwiched between them, and a normal load of 0.01 MPa was applied on the assembly. During brazing, the assembly was heated to the brazing temperature (980–1100 °C) under vacuum, isothermally held for 0.5 h at the temperature, and then cooled to room temperature with a cooling rate of 5 °C/min. The obtained joints were mounted, sectioned, polished, and examined using scanning electron microscopy (SEM, FEI Quanta 200F) coupled with an energy dispersive X-ray spectroscopy (EDS, Bruker Nano XFlash Detector 5010). The phases in the joint were identified by X-ray diffraction (XRD, Bruker D8 Advance). The strength of the lap joint was measured by a shear test (MTS, CMT4204). At least three samples were tested for each joining condition. 3. Results 3.1. Microstructure of the graphite/Hastelloy N alloy joints Fig. 1 shows the typical microstructure of the graphite/Hastelloy N alloy joint brazed with Au foil at 1060 °C for 0.5 h. It can be seen from the figure that a good wetting and intimate contact are achieved at the substrates/braze interfaces. There are four typical zones in the joint, labeled as Zone 1, Zone 2, Zone 3 and Zone 4. The Zone 1 is close to the graphite while Zone 4 is located in the Hastelloy N. In comparison with the Zones 1 and 2, the Zones 3 and 4 are much wider. The boundary

of Zone 1 to Zone 4 was defined based on their constitution, which would be interpreted in the following chapter. Fig. 2 depicts the magnified BSE image at the graphite/braze interface and concentration profiles of relevant elements across the interface. It shows that no obvious reaction layer is found at the interface (Fig. 2a). When the scanning line gets in touch with the graphite, the concentration of Au and Ni decreases to the minimum, while the concentration of C increases to the maximum (Fig. 2b). No element accumulation shows that the interfacial bonding was achieved by diffusion rather than a direct chemical reaction. The width of diffusion layer, as shown in Fig. 2(b), is approximately 1 μm. The detailed microstructure for Zone 1 is shown in Fig. 3. The zone is primarily made up of the white phases, gray phases and dark-gray phases. The white phases, as indicated in Fig. 3(e) and (f), are enriched with Au and Ni and they are actually Au (s.s). Ni and Au are accumulated in the gray phases (Fig. 3e and f), and they are Ni (s.s). The dark-gray phases embedded in Au (s.s) and Ni (s.s) are concentrated with Mo and Cr (Fig. 3c and d). The composition analysis shows they are composed of 68 wt.% Mo, 11 wt.% Cr, 9 wt.% C, 9 wt.% Ni, 2 wt.% Au and 1 wt.% Fe. Coupled with the XRD results shown in Fig. 8, they are determined to be the Mo2C. The magnified microstructure for Zone 2 is described in Fig. 4.The Zone 2 is close to the Zone 1 and mainly consists of the white phases, gray phases and black-gray phases. The white phases are composed of 82 wt.% Au, 9 wt.% Ni, 5 wt.% C, 2 wt.% Cr,1 wt.% Mo and 1 wt.% Fe, which should also be Au (s.s). The gray phases have the constitution of 65 wt.% Ni, 14 wt.% Au, 9 wt.% C, 4 wt.% Fe, 4 wt.% Cr, 3 wt.% Mo and 1 wt.% Si, while the black-gray phases are made up of 60 wt.% Ni, 18 wt.% Au, 12 wt.% C, 4 wt.% Fe, 4 wt.% Cr and 2 wt.% Mo. Both of them should be Ni (s.s). That is to say, the Zone 2 is basically constituted by Au (s.s) and Ni (s.s). Fig. 5 shows the microstructure for Zone 3. Some larger blocky particles can be found in the zone. Note that Hastelloy N is composed of Ni (s.s) and embedded MoC particles [21]. These larger particles are the original MoC. In addition, many lath-like or rod-like precipitates occur in the grain interiors, as shown in Fig. 5(a) and (b). From Fig. 5(c) it can be seen that Mo, Ni, Cr and C are enriched in them. The EDS was also performed for the rod-like precipitates (~ 1 μm), as shown in Fig. 6. They are mostly composed of 64 wt.% Mo, 12 wt.% Ni, 12 wt.% Cr, 11 wt.% C, and 1 wt.% Fe. However, it is hard to identify their type merely based on the EDS results. In conjunction with the XRD result (Fig. 8), they are actually the Mo2C. In fact, the width for most precipitates is less than 1 μm. Note that the diameter measured by EDS is probably of the order of ~1 μm, which means that most precipitates are below the spatial resolution limit of EDS. In such a case the composition information is also gathered from the regions around them. Note that the neighboring regions are actually Ni (s.s). That is the critical reason why a higher amount of Ni is also detected in these precipitates. At the grain boundaries, lots of tiny precipitates can also be inspected (Fig. 5a–b). They are basically made up of 43 wt.% Mo,

Fig. 1. The SEM micrograph obtained in the back-scattered electron (BSE) mode shows that there are four typical zones in the graphite/Hastelloy N alloy joint brazed with Au foil at 1060 °C for 0.5 h.

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Fig. 2. (a) Magnified BSE image at the graphite/braze interface and (b) the concentration profiles of related elements across the scanning line shown in (a).

37 wt.% Ni, 8 wt.% Cr, 8 wt.% C, 3 wt.% Si and 1 wt.% Fe. It is noteworthy that up to 3 wt.% Si can be found in them. Since Si is an important element in promoting the formation of MoC in the Hastelloy N [21], Si enrichment gives a hint that the precipitates at the grain boundaries should be the MoC. It should be mentioned that Au is not observed both in the Ni (s.s) and precipitates, which means that the precipitates at the grain boundaries and in the interiors were formed by a solidstate reaction. Actually, impurities and nonmetal elements, such as Si and C, tend to agglomerate at the grain boundaries [22]. The accumulation of Si at the grain boundaries would promote the formation of MoC. While in the grain interiors, Si with its larger radius would diffuse in the fcc Ni crystals by vacancy diffusion (Ni 124 pm; Si 134 pm; C 86 pm). Compared with interstitial diffusion, the vacancy diffusion is hard to happen [23]. Lack of Si hindered the formation of MoC in the grain interiors. In such a case the Mo2C were produced.

Fig. 7 describes the microstructure for Zone 4. Besides original MoC, the grain boundaries are decorated by a continuous network of smaller blocky particles after brazing. They are made up of 45 wt.% Ni, 35 wt.% Mo, 8 wt.% Cr, 6 wt.% C, 3 wt.% Si and 3 wt.% Fe, similar to that of the original MoC. These tiny particles are also the MoC. Their occurrence is ascribed to the enrichment of Si and C at the grain boundaries. Fig. 8 shows the XRD results for the graphite/Hastelloy N alloy joint. The joint was cut at the graphite side close to the bonding interface, and then ground until Zone 1, Zone 3 and Zone 4 were exposed. As expected, Au and Ni appear in Zone 1. Apart from them, the Mo2C and C can also be identified. Ni and Mo2C are detected in Zone 3. At last, in Zone 4 Ni and MoC can be found. The XRD results coincide well with the aforementioned microstructural analysis. It can be seen from Fig. 8 that the peaks of Ni in Zone 1 deviate from those in Zone 3. The same problem also appears in Zones 3 and 4. The deviation of Ni peaks should be

Fig. 3. (a) BSE micrograph showing the detailed microstructure in Zone 1, and (b–f) relevant elements distribution maps.

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Fig. 4. Magnified BSE image for Zone 2. The figure shows that this zone mainly consists of the white phases, gray phases and black-gray phases.

Fig. 6. Magnified BSE image for Zone 3. The white phases indicated by arrows were performed by EDS analysis.

ascribed to solution of elements. Note that many elements, such as Au, Mo, Cr and Fe, were dissolved in Ni during brazing. Take the Zones 1 and 3 for example, Ni (s.s) in Zone1 are made up of 58 wt.% Ni, 19 wt.% Au, 10 wt.% C, 6 wt.% Mo, 4 wt.% Cr and 3 wt.% Fe. However, in Zone 3 they consist of 74 wt.% Ni, 9 wt.% C, 6 wt.% Cr, 5 wt.% Mo, 5 wt.% Fe and 1 wt.% Si. The composition discrepancy makes the peaks of Ni deviate, which was also reported in [6,24]. Based on the above analysis, Zone 1 is mainly made up of Au (s.s) and Ni (s.s), in which a few Mo2C are embedded. Zone 2 is composed of Au (s.s) and Ni (s.s). Besides Ni (s.s) and original MoC, a continuous network of tiny MoC

particles started to appear at the grain boundaries in Zone 3, while many lath-like or rod-like Mo2C were precipitated in the grain interiors. The tiny MoC still emerged at the grain boundaries in Zone 4, but no Mo2C could be formed in the interiors. It is worthy to be noted that the formation of Zones 1 and 2 was involved the liquid, while the Zones 3 and 4 were produced in a solid state. The formation process for the whole joint is given below: When the heating temperature reaches 955 °C during brazing, Au foil melts with neighboring Hastelloy N, producing the liquid. The zone close to the graphite is namely Zone 1 while the zone beside the

Fig. 5. (a–b) BSE micrographs showing the microstructure in Zone 3 and (c) elemental distribution across the scanning line in (b).

Y. He et al. / Materials and Design 104 (2016) 1–9

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Fig. 7. BSE images for Zone 3 and Zone 4: (a) position 1; (b) position 2; (c) position 3; (d) In Zone 4 the grain boundaries are decorated by a continuous network of tiny MoC.

Hastelloy N is defined as Zone 2. At the Hastelloy N/braze interface, an increasing amount of Hastelloy N substrates will be gradually dissolved with the heating proceeding. When Ni concentration reaches the equilibrium, however, subsequent dissolution becomes hard to proceed. In this way Zones 1 and 2 can act as a barrier, impeding more Hastelloy N dissolving. In the cooling, Ni-rich phases will preferentially nucleate and grow at the Hastelloy N/braze interface. Therefore Ni (s.s) are first solidified in Zone 2, followed by Au (s.s). Other atoms, such as Au, Mo and C, will be expelled towards Zone 1 during the process. In this zone Ni (s.s) are likewise first solidified, followed by Au (s.s). But in the meantime, the C can react with Mo to bring the Mo2C when a local composition is satisfied. It should be noticed that the solubility of C in molten Au reaches 4.7 at.% [25]. That's to say, the C atoms released from graphite will be dissolved in the liquid. In fact, the C with a smaller radius can

Fig. 8. X-ray diffraction patterns for different zones in the graphite/Hastelloy N alloy joint.

diffuse freely in the liquid or solid (Au 179 pm; Ni 124 pm; C 86 pm). They are small enough to fit into interstitial positions in the fcc Ni (s.s) and migrate through the interstitial diffusion. It therefore makes sense that the C atoms will pass through the Zones 1 and 2 to diffuse towards the Hastelloy N substrates. At the grain boundaries, the enrichment of C, Si and Mo will produce tiny MoC particles. While in the grain interiors, the C reacts with Mo to bring lath-like or rod-like Mo2C. In this way Zone 3 is coming into being. Far from the graphite substrates, the diffusion ability of C decreases. In such a case the tiny MoC can also be precipitated at the grain boundaries in Zone 4, but no Mo2C are formed in the grain interiors.

3.1.1. Effect of brazing temperature on the microstructure and mechanical properties of the graphite/Hastelloy N alloy joints Fig. 9 illustrates the microstructure in the graphite/Hastelloy N alloy joint brazed with Au foil at different brazing temperatures. The wellbonded joints are obtained for all the temperatures used. In addition, all the joints can be divided into four zones and the temperatures used affect the width of each zone obtained. As depicted in Fig. 10, the width for Zones 1 and 2 exhibits a negligible variation with elevating the temperature during brazing. However, Zones 3 and 4 become wider remarkably. The phenomenon should be attributed to the hindering effect of Zones 1 and 2, as already mentioned. However, the C atoms would still diffuse irrespective of the hindering effect of Zones 1 and 2. From an atomic perspective, the diffusion is just a stepwise migration of atoms from one lattice site to another. The temperature is the critical factor in determining the diffusion rate of atoms, and a higher temperature usually favors the diffusion. Therefore a higher temperature introduced would accelerate the diffusion of C atoms, producing a wider Zones 3 and 4 in the joint. The effect of brazing temperature on the joint average shear strength is depicted in Fig. 11. It shows that the average strength first increases and then decreases with increasing the brazing temperature. The maximum average strength with a value of 26 MPa is obtained when the

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Fig. 9. BSE micrographs showing the microstructure in the graphite/Hastelloy N alloy joint brazed with Au foil at different temperatures for 0.5 h: (a) 980 °C; (b) 1020 °C; (c) 1060 °C and (d) 1100 °C.

temperature is 1060 °C, which is equal to 88% of the graphite's intrinsic shear strength. The minimum average strength still reaches 22.5 MPa (980 °C). In a word, all the temperatures introduced in the work can ensure receiving a high-quality joint.

Wettability should be seriously considered in joining the dissimilar materials. In order to achieve a strong bonding, an excellent wetting to the joined substrates must be fulfilled. As far as the graphite/ Hastelloy N joint is concerned, the wetting of graphite should be paid

serious attention since this kind of material is characterized by stable electron configurations and strong interatomic bonds. The general framework for characterizing wettability needs to introduce the concept of a contact angle θ. When the θ is below 90°, the solid can be wetted by the liquid. As a general rule, there are two different modes to promote the wettability. One is interfacial reaction, which can produce a continuous or discrete reaction layer at the solid/liquid interface. The chemical reaction will decrease the system's free energy and hence promote the wettability [26–28]. The other is physical dissolution (Van der Waals interfacial interaction). To an extent, dissolution of substrates can also reduce the surface tension and achieve the wetting [29–30]. During brazing, Au foil diffused and melted with the close contact Hastelloy N substrates when the heating temperature reached 955 °C.

Fig. 10. Effect of brazing temperature on the width of each zone obtained in the joint.

Fig. 11. Effect of brazing temperature on the average shear strength of the joint.

4. Discussion 4.1. Wettability of the graphite substrates

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In such a case the graphite substrates were directly exposed towards the molten Au and Ni. The solubility of C in the molten Au reaches 4.7 at.% [25]. However, no stable Au-C compounds can be found between them. The Ni-C phase diagram is a simple eutectic type with no stable intermediate phases [31]. The eutectic composition and temperature are 10 at.% C and 1329 °C, respectively. Even though no stable carbides occur in Ni-C system, the formation of metastable Ni carbide may be possible due to the interaction of molten Ni with C. Wang et al. [32] found that the Ni3C, which has a positive Gibbs free energy of formation from Ni and C, may occur at the graphite/Ni interface under the condition of C supersaturation. This action will modify the surface and interfacial energies, leading to a good wettability with relatively low contact angles (68–90°) of Ni on the graphite [15]. Besides, Cr in the Hastelloy N alloy would also be dissolved into the molten braze coupled with the melting of Au during brazing. Note that the Hastelloy N has 7 wt.% Cr. The addition of Cr in molten braze could also improve the wettability, because the active Cr shows a high affinity towards the carbon [33]. In this work, however, no obvious reaction layer is detected at the graphite/braze interface. As mentioned above, up to 3 wt.% C can be inspected in Au (s.s) and Ni (s.s). In fact, dissolution of graphite substrates contributed to the wettability here [34]. Overall, the wettability between the graphite and braze was achieved basically through the physical interaction.

4.2. Precipitates formation in the Hastelloy N substrates As already noted, Mo-C carbides can be found in the Hastelloy N substrates after brazing. At least six kinds of phases have been identified in Mo-C system [9], and the only thermodynamically stable phases at room temperature are Mo2C and MoC. Note that Si element will promote the formation of MoC in the Hastelloy N [21]. The enrichment of Si at the grain boundaries would facilitate the formation of MoC during brazing. While in the grain interiors, the excessive Mo would be beneficial to bring the Mo2C, since the Hastelloy N has 16 wt.% Mo. The occurrence of Mo2C in the alloy steels has also been reported [35]. It should be noted that the binary Mo-C is a complex system and the Mo2C has two stable structures: orthorhombic (Pbcn) α-Mo2C and hexagonal (P63/mmc) β-Mo2C. Both of them exhibit ABAB stacking sequence of Mo planes with C in octahedral sites. The only difference is the order/disorder distribution of C atoms in the octahedral sites. Compared with the α-Mo2C, the β-Mo2C has a more disordered distribution of C [36]. Thermodynamically, both of them will occur due to the negative enthalpy of formation for α-Mo2C (−2.28 eV/unit cell) and β-Mo2C (− 1.36 eV/unit cell) [37]. However, only β-Mo2C is detected in the work on the basis of the XRD results shown in Fig. 8. Besides the thermodynamics, however, the concentration of C would also affect the type of Mo2C obtained. In general, abundant C would accelerate its diffusion and lead to a more disordered distribution of C in the octahedral sites. The β-Mo2C were hence produced. Moreover, the higher temperatures (980–1100 °C) used in the work favored the diffusion of C atoms, leading to the β-Mo2C prevailing in the Hastelloy N substrates beside the brazing seam. Since the Hastelloy N has 7 wt.% Cr, the C may also react with Cr to bring the Cr-C compounds in the Hastelloy N during brazing. In the Cr-C system, Cr carbides have many crystalline structures: CrC (space group Fm3m), Cr3C (space group Pnma), Cr3C2 (space group Pnma), Cr7C3 (space group Pnma), and Cr23C6 (space group Fm3m) [38]. Among them, the Cr3C2, Cr7C3 and Cr23C6 are common phases while the CrC and Cr3C belong to metastable carbides [39]. From thermodynamics [40], the Cr3C2, Cr7C3 and Cr23C6 will also be formed in the Hastelloy N. However, only Mo-C compounds are observed after brazing. The absence of Cr-C carbides might be attributed to the relative content of Mo and Cr within the alloy. It should be noticed that the Hastelloy N has 17 wt.% Mo and 7 wt.% Cr. The relatively lower content of Cr might suppress the formation of Cr-C carbides.

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4.3. Residual stresses in the joints The residual stress caused by difference of thermal expansion rate between the joined materials is a significant factor in determining the quality of joint. The stress distribution in a joint depends on many factors, such as the material system, bonding condition and geometry of the joint, which will make its estimation difficult. Several works [41–43] have reported that residual strains in a joint can be measured using diffraction methods. The strain measurement by diffraction benefits from the variation of the crystalline lattice spacing with stress (either tensile or compressive). For every grain it extends (or contracts) when the material is placed under tensile (or compressive) stresses. The subtle change will be captured as a shift in the angle at which the diffraction peaks appear. After obtaining the strains, the stresses can also be calculated [41]. The strain behavior at the surface of a joint can be experimentally measured by X-ray sources. On the other hand, the inner strain field can be determined by neutron diffraction [42]. The facility for neutron diffraction, however, is not very common. Another effective approach to predict the stresses is the finite element method (FEM). The method can analyze the magnitude and distribution of the residual stresses along a joint based on input of particular properties of materials to be joined [43]. However, its accuracy is often questioned, particularly because assumptions must be made regarding the properties of the filler. It is noteworthy that the mechanical strength of a brazed joint usually depends upon the strength of interfacial bonding in conjunction with residual stresses. As described in Fig. 11, the graphite/Hastelloy N joints always fracture in the graphite when they were joined at different temperatures. The fact shows that a strong bonding was achieved at the graphite/braze interface. The residual stresses, therefore, should have a critical influence in determining the joint strength. As stated above, the temperatures introduced could always obtain a high-quality joint, which means that the joints with the lower residual stresses were realized. The following factors were believed to contribute to the lower residual stresses: Firstly, the graphite was wetted through physical interaction. The wettability mode should be beneficial to the joint strength since no reaction layer was formed at the interface. Besides, the molten braze could infiltrate the pores open to the bonding surface of graphite during brazing (Fig. 9). The mechanical interlocking was hence coming into being at the interface, improving the interfacial bonding strength. As for the nonmetal/metal interface, the chemical reaction usually happens and reaction products are accordingly formed in order to achieve the wetting. The problem with interfacial reactions, however, is that the resulting layer of reaction products tends to provide sources of crack initiation. This behavior may have a deleterious effect on the interfacial properties, especially when its thickness exceeds about 1.0 μm [44]. Moreover, the reaction layer with an excessive thickness will also cause great stress in the joint, since the interfacial reaction products are new phases to the joined substrates. Secondly, many blocky Mo2C occur in Zone 1 during brazing. Note that the Mo2C has a combination of favorable properties, such as high hardness, low CTE, excellent thermal stability [45]. Particularly, the CTE for Mo2C, Au and Ni are 7.8 × 10− 6 K− 1, 14.2 × 10−6 K− 1 and 13.0 × 10−6 K−1, respectively. Incorporation of Mo2C in Au (s.s) and Ni (s.s) would thus control the CTE and strengthen the braze. The residual stresses arising from CTE mismatch were accommodated and the joint shear strength was accordingly enhanced. As a matter of fact, the addition of hard particles, such as the SiC [46], TiN [47] and W [48], into the braze is a common way to mitigate the residual stresses. We once introduced the SiC or Mo particles modified braze to join the Si3N4 ceramic. A strong bonding can be achieved when the content of particles is optimized in the composite filler [46]. The reinforcements can be directly incorporated [46–48] or in-situ synthetized [49] during brazing. As for the braze doped with particles, the wettability between the particles and braze needs to be considered and bad wettability depresses the incorporation effect. In this work, the Mo2C were directly

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coming into being from the molten braze during brazing and a defectfree Mo2C/braze interface was realized (Fig. 3). Lastly, lots of Mo-C carbides also occur in the Hastelloy N substrates beside the brazing seam during brazing. The occurrence of Mo-C made the alloy resemble a metal matrix composite (MMC), in which Ni was matrix while Mo-C acted as reinforcements. Based on this the graphite was actually joined to a MMC. It is noteworthy that the Mo-C carbides usually have a lower CTE (Mo2C 7.8 × 10−6 K−1, Ni 13.0 × 10−6 K−1). Incorporation of the Mo-C in the alloy would decrease its CTE. Compared with the Hastelloy N, the MMC had a lower CTE. Consequently the CTE mismatch between the joined materials was lowered. The CTE mismatch is usually proportional to the residual stresses. For full elastic conditions, the thermal stresses are simply estimated by a well-known equation [50]: σ = [EM · EG / (EM + EG)] · ΔT · Δα, where E is Young's modulus and α is linear CTE, the subscript M and G stand for metal and graphite. Through the equation, a lower CTE mismatch in a joint will produce the lower level stresses. That's to say, the occurrence of Mo-C in the Hastelloy N made the joint become a naturally hierarchical structure in accommodating the stresses. In summary, the above three factors cooperated together to produce the graphite/Hastelloy N joint with the lower thermal stresses. A strong bonding was finally realized while different brazing temperatures were introduced. 5. Conclusions The typical microstructure for the graphite/Hastelloy N alloy joint is given below: graphite/Zone 1/Zone 2/Zone 3/Zone 4/Hastelloy N alloy. No obvious reaction layer is found at the graphite/braze interface, and the graphite is wetted through physical interaction. Zone 1 is composed of Au (s.s), Ni (s.s) and scattered Mo2C. Zone 2 is constituted by Au (s.s) and Ni (s.s). Both of them are produced in a liquid state. Lots of Mo-C carbides are precipitated in Zone 3 (MoC at the grain boundaries and Mo2C in the grain interiors). In Zone 4 the tiny MoC still occurs at the grain boundaries but no Mo2C can be found. The occurrence of Zones 3 and 4 is ascribed to the diffusion reaction of Mo and C. The graphite/Hastelloy N alloy joint consists of four zones at different brazing temperatures and each zone has similar constitution. The width for Zones 1 and 2 varies less with increasing the brazing temperature, but Zones 3 and 4 are widened remarkably owing to strong diffusion ability of C atoms at a higher temperature. The brazing temperatures introduced have a limited influence on the joint shear strength. The maximum average value is 26 MPa obtained at 1060 °C, which reaches 88% of the graphite's intrinsic strength. In addition, the minimum average strength is 22.5 MPa obtained at 980 °C. A naturally hierarchical structure in relaxing the residual stresses is produced in the joints, contributing to the strong bonding. Acknowledgements The financial support of the National Natural Science Foundation of China under Grant Nos. 51405439 and 51475426 is highly acknowledged. References [1] Y.Q. Li, Z.D. Zhang, C.Q. Den, Y.S. Su, Investigation of brazing structure of bulk graphite to a W-Re substrate, Mater. Charact. 44 (2000) 425–429. [2] T.T. Ikeshoji, Brazing of carbon-carbon (C/C) composites to metals, Advances in brazing (2013) 394–420. [3] X.Y. Dai, J. Cao, J.K. Liu, S. Su, J.C. Feng, Effect of holding time on microstructure and mechanical properties of ZrO2/TiAl joints brazed by Ag-Cu filler metal, Mater. Des. 87 (2015) 53–59. [4] Z. Sun, L.X. Zhang, J.L. Qi, Z.H. Zhang, C.L. Tian, J.C. Feng, Brazing of SiO2f/SiO2 composite modified with few-layer graphene and invar using AgCuTi alloy, Mater. Des. 88 (2015) 51–57. [5] Y. Wang, Z.W. Yang, L.X. Zhang, D.P. Wang, J.C. Feng, Low-temperature diffusion brazing of actively metallized Al2O3 ceramic tube and 5A05 aluminum alloy, Mater. Des. 88 (2015) 328–337.

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