TiAl brazed joint

Materials Science and Engineering A 528 (2011) 5135–5140

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Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea

Effect of Si3 N4 -particles addition in Ag–Cu–Ti filler alloy on Si3 N4 /TiAl brazed joint X.G. Song a,b , J. Cao a,∗ , Y.F. Wang a , J.C. Feng a a b

State Key Lab of Advanced Welding Production Technology, Harbin Institute of Technology, Harbin 150001, China HeiLongjiang Institute of Technology, Harbin 150001, China

a r t i c l e

i n f o

Article history: Received 12 October 2010 Received in revised form 26 February 2011 Accepted 6 March 2011 Available online 12 March 2011 Keywords: Si3 N4 ceramics TiAl intermetallics Brazing Composite filler alloy Interfacial microstructure

a b s t r a c t A novel composite filler alloy was developed by introducing Si3 N4p (p = particles) into Ag–Cu–Ti filler alloy. The brazing of Si3 N4 ceramics and TiAl intermetallics was carried out using this composite filler alloy. The typical interfacial microstructure of brazed joints was: TiAl/AlCu2 Ti reaction layer/Ag(s,s) + Al4 Cu9 + Ti5 Si3p + TiNp /TiN + Ti5 Si3 reaction layer/Si3 N4 . Effects of Si3 N4p content in composite filler alloy on the interfacial microstructure and joining properties were investigated. The distribution of Ti5 Si3p and TiNp compounds in Ag-based solid solution led to the decrease of the mismatch of the coefficient of thermal expansion (CTE) and the Young’s modulus between Si3 N4 and TiAl substrate. The maximum shear strength of 115 MPa was obtained when 3 wt.% Si3 N4p was added in the composite filler alloy. The fracture analysis showed that the addition of Si3 N4p could improve the mechanical properties of the joint. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Advanced ceramics have attracted great interest due to their outstanding properties in recent years [1–3]. Especially, silicon nitride (Si3 N4 ) ceramics are being widely used in structural applications as replacement for conventional metallic materials because of their desirable properties [4,5]. The successful performance of such applications depends on the quality and reliability of the ceramicto-ceramic and ceramic-to-metal joints. Various bonding methods including brazing, diffusion bonding, partial transient-liquid-phase bonding and glass adhesive bonding have been developed in the last few decades [6–11]. Especially, brazing has become the most effective method due to its convenience and cost-effectiveness. However, there are two problems for brazing ceramics to metals. Firstly, the wettability of traditional filler alloys on the ceramics is poor, and secondly the high residual stresses generated in the interfacial region of brazed joints due to the differences in physicchemical properties between ceramics and metals can degrade the joint properties [12]. The first problem can be solved by the employment of active filler alloys, where the active elements (Ti, Zr, etc.) react with the ceramics resulting in excellent wettability. The second problem is mainly caused by the mismatch of the coefficient of thermal expansion (CTE) and the Young’s modulus among the ceramics, the brazing seams and the metals. The residual stresses

tend to result in fracture at low, or even zero loads. Therefore, it is important to reduce or eliminate the residual stresses in the interfacial region to improve the joining quality. In order to release the residual stresses, the fabrication method of metal-based composite was introduced into the investigation on joining process in this study. The metal-based composite reinforced by particles or fibers has shown significantly improved materials properties including increased strength and fracture toughness. Especially, the CTE can be reduced and the Young’s modulus can be increased remarkably [13–15]. In recent years, by means of adding some particles or fibers with low CTE into filler alloys, the mismatch of CTE and Young’s modulus between ceramics and brazing seams was reduced, and thus the joining strength was improved [16–19]. The results reported by Gurdial Blugan et al. showed that an appropriate amount of reinforcing SiC particles in the IncusilABA filler metal effectively improved the properties of the brazed joints [20]. In the present work, Si3 N4 ceramics were brazed to TiAl intermetallics using a novel composite filler alloy, which was modified Ag–Cu–Ti filler alloy by adding Si3 N4p . In addition, the typical interfacial microstructure was characterized, and the effects of the Si3 N4p contents on the interfacial microstructure and shear strength of Si3 N4 /TiAl brazed joints were analyzed in detail. 2. Experimental

∗ Corresponding author. Tel.: +86 451 86418882; fax: +86 451 86418146. E-mail address: cao [email protected] (J. Cao). 0921-5093/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2011.03.032

The Si3 N4 ceramics used in this work were hot-pressed with small amounts of Al2 O3 and Y2 O3 as sintering additive.

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Table 1 Properties of the joining materials. Materials

Density (g/cm3 )

E (GPa)

˛ (×10−6 K−1 )

Strength (MPa)

Si3 N4 TiAl Ag–Cu–Ti

3.23 3.78 9.2

380 ± 20 170 ± 10 70 ± 5

3.1 ± 0.2 10.8 ± 0.5 18.2 ± 0.8

860–920a 505–538b –

a b

Three points bending strength. Tensile strength.

Fig. 1. SEM micrograph of the filler powders: (a) the original Ag–Cu–Ti filler alloy and (b) the composite filler alloy.

The raw ceramics were cut to blocks with the dimension of 6 mm × 6 mm × 5 mm using a diamond cutting machine. The dimensions of TiAl(Ti-46Al-2Cr-2Nb at.%) specimens for metallographic observation and strength test were 8 mm × 8 mm × 2 mm and 20 mm × 8 mm × 2 mm, respectively. The main properties of the base materials and filler metal are listed in Table 1. Si3 N4p with an average diameter of 150 nm were added into Ag–Cu–Ti (70Ag-27.5Cu-2.5Ti (wt.%)) filler alloy powders. Then the mixture was milled for 2 h in an argon atmosphere using a QM-SB planetary ball mill to prepare the new composite filler alloy. Fig. 1 shows the morphology of the original Ag–Cu–Ti and the as-prepared composite filler alloy. It can be seen that some Si3 N4p were adhered on the surfaces of the original Ag–Cu–Ti powders. Five series of the composite filler alloys were prepared and the contents of Si3 N4p in composite filler were 0%, 1.5%, 3%, 4.5%, and 6% (wt.%), respectively. Prior to joining, the brazing surfaces of the TiAl and Si3 N4 samples were ground on SiC grit papers and then polished using diamond pastes. All of the polished samples were ultrasonically cleaned in acetone and dried by air blowing. The composite filler alloy was placed between the brazing couples. At the beginning of brazing process, the furnace was heated to 1023 K at a rate of 30 K/min, and then the temperature continued to increase to 1153 K at a rate of 10 K/min. Subsequently, the brazing specimens were held for 5 min at 1153 K and then cooled down to 473 K at a rate of 5 K/min. Finally, the furnace was cooled down spontaneously to room temperature in the furnace. During the brazing process, the vacuum was kept at (1.3–2.0) × 10−3 Pa and a pressure of 20 kPa was applied to ensure proper contact. The interfacial microstructure was characterized employing scanning electron microscopy (SEM) equipped with energy dis-

Fig. 2. Schematic of the shear test.

persive spectrometer (EDS). The room temperature shear tests were performed at a constant speed of 0.5 mm/min using a universal testing machine (Instron1186). The schematic illustration of the shear test is shown in Fig. 2. For each set of experimental data, at least five samples were used to average the joint strength. In order to identify the interfacial compounds accurately, the analysis was performed using an X-ray diffraction (XRD, JDX3530 M) spectrometer equipped with Cu-K␣ radiation on a selected

Table 2 EDS results of chemical compositions at each spot in Figs. 3 and 4 (at.%). Spots

Ag

Cu

Ti

Al

Si

N

Possible phases

A B C D E F

2.63 82.61 85.03 3.68 5.62 6.32

49.63 9.72 5.60 54.49 1.28 51.85

23.37 2.47 0.85 5.23 23.22 7.55

23.18 2.36 3.58 23.46 2.36 22.22

0.21 1.45 2.51 8.32 26.53 6.16

0.98 1.39 2.43 4.82 40.99 5.90

AlCu2 Ti Ag(s,s) Ag(s,s) Al4 Cu9 Ti5 Si3P + TiNP + Si3 N4P Al4 Cu9

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Fig. 3. Backscattered electron images (BEIs) of interfacial microstructure of Si3 N4 /TiAl joints brazed at 1153 K for 5 min using composite filler alloy with Si3 N4p contents of (a) 0%; (b) 1.5%; (c) 3%; (d) 4.5%; and (e) 6%; (wt.%).

Fig. 4. The microstructure in the selected region of the Si3 N4 /TiAl brazed joints (a) high magnification BEI of the rectangular region in Fig. 3(c); (b) agglomeration of residual Si3 N4p ; and (c) Ag–Cu eutectic structure.

fracture surfaces which cracked at the brazed seam after shear test. 3. Results and discussion 3.1. Typical microstructure of the Si3 N4 /TiAl joints Fig. 3 shows the typical interfacial microstructure of the Si3 N4 /TiAl brazed joints. It is notable that the defect free brazed joints were achieved. A continuous reaction layer (grey layer in Fig. 3(a)) formed adjacent to TiAl substrate when Ag–Cu–Ti filler alloy was used. According to the EDS results of spot A in Table 2 and the previous studies [21–23], the phase in this reaction layer can be identified as AlCu2 Ti. A thin layer consisting of TiN and Ti5 Si3 compounds can be observed between the bright Ag-based solid solution and Si3 N4 substrate, as demonstrated in Refs. [24–26]. When the composite filler alloys were used, the interfacial microstructure changed obviously, as shown in Fig. 3(b)–(e). With the addition of Si3 N4p , the thickness of AlCu2 Ti reaction layer decreased considerably and the reaction layer of TiN + Ti5 Si3 became indistinct. The continuous bright Ag-based solid solution disappeared, and some new fine compounds came forth in Ag-based solid solution. The distribution of these fine compounds varied with the changing of Si3 N4p content. Fig. 4(a) shows the high magnification view of the rectangular region in Fig. 3(c). It is clearly seen that three kinds of compounds formed during the brazing process. According to both the EDS results (Table 2) and the XRD patterns obtained

from the fracture surface of brazed joints (shown in Fig. 5), it is confirmed that the large grey compounds, small grey and black compounds are Al4 Cu9 , Ti5 Si3p and TiNp , respectively. Compared with the AlCu2 Ti reaction layer shown in Fig. 3(a), the formation of Al4 Cu9 compounds is characterized because of the exhaustion of active Ti by Si3 N4p during the brazing process. And it can be seen from Fig. 4(a) that the Al4 Cu9 compounds precipitated and grew

Fig. 5. XRD pattern of the fracture surface after shear test for specimens brazed using composite filler added 3 wt.% Si3 N4p at 1153 K for 5 min.

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around the fine Ti5 Si3p and TiNp as nucleating sites. As analyzed above, the typical interfacial microstructure of the brazed joint was TiAl/AlCu2 Ti/Ag(s,s) + Al4 Cu9 + Ti5 Si3p + TiNp /TiN + Ti5 Si3 /Si3 N4 . 3.2. Effect of Si3 N4p content on interfacial microstructure The composite filler alloy began to melt when the specimens were heated to the Ag–Cu eutectic temperature. Subsequently, Ti in both composite filler and TiAl substrate dissolved in the molten filler alloy. Active Ti in molten filler alloy diffused towards the Si3 N4 substrate and enriched on its surface. The chemical reaction between Ti and Si3 N4 ceramics occurred at the elevated temperature. Then TiN and Ti5 Si3 compounds formed at the interface, which played an important role on the joining of Si3 N4 ceramics and TiAl intermetallics. The reaction mechanism between Ti and Si3 N4 has been discussed in Ref. [26]. However, in the present study, not only the Si3 N4 substrate, but also the Si3 N4p in composite filler alloy took part in the reaction with active Ti in molten filler alloy. When the composite filler alloys with different contents of Si3 N4p were applied, the sizes, amounts and distribution of the compounds in Ag-based solid solution differed greatly. When composite filler alloy with 1.5 wt.% Si3 N4p was adopted, blocks of bright Ag-based solid solution formed due to the insufficient content of Si3 N4p , as shown in Fig. 3(b). Desired interfacial microstructure was obtained when 3 wt.% Si3 N4p was added into composite filler alloy, as shown in Fig. 3(c). The homogeneous microstructure (shown in Fig. 4(a)) consisted of Al4 Cu9 compounds (2–5 ␮m), fine Ti5 Si3p and TiNp (200–500 nm) in Ag-based solid solution, which was similar to the microstructure of metal-based composite reinforced by particles. With the content of Si3 N4p increased to 4.5 wt.% or 6 wt.%, the excessive Si3 N4p made the active Ti in molten brazing alloy insufficient relatively. The brazed seam can be divided into zone I and II as shown in Fig. 3(d) and (e). In zone I, the content of active Ti in molten brazing alloy was higher due to the dissolution of TiAl substrate. Therefore, the Si3 N4p reacted with Ti adequately, which led to the formation of similar microstructure as seen in Fig. 3(c). Moreover, the consumption of Ti by Si3 N4p in zone I decreased the amount of Ti diffusing towards zone II. In addition, the diffusion of Ti towards zone II was restricted by the excessive Si3 N4p in zone I. So the active Ti in zone II mainly derived from the original Ag–Cu–Ti filler alloy. Due to the insufficient Ti in zone II, an amount of Si3 N4p remained and aggregated after brazing process, as shown in Fig. 4(b). The Ag–Cu eutectic structure rather than Ti–Cu or Al–Cu compounds formed due to the small amounts of Ti or Al in zone II, as shown in Fig. 4(c). The comparison between Fig. 3(d) and (e) revealed that the thickness of zone I decreased from 100 ␮m to 50 ␮m when the Si3 N4p content increased from 4.5% to 6% (wt.%). The phenomena confirmed the restriction effect of Si3 N4p in molten filler alloy on the diffusion of active Ti during the brazing process.

Fig. 6. Effect of Si3 N4p content on CTE and Young’s modulus of the brazed seams.

where ˛ is the CTE of brazed seam, ˛m and ˛f are the CTE of Ag-based solid solution and the compounds particles respectively, Vf is the volume fraction of reinforcing particles in Ag-based solid solution.



E = Em 1 +

Vf 15(1 − vm ) · 8 − 10vm 1 − Vf



where E is the Young’s modulus of brazed seam, Em and vm are the Young’s modulus and Poisson’s ratio of Ag, respectively. From formula (1) and (2), it can be seen that the addition of reinforcing particles with low CTE and high Young’s modulus into the matrix can effectively reduce the CTE and increase the Young’s modulus of the composite. And the volume fraction of the reinforcing particles plays a strong influence on the properties of the composite. With the increase of the volume fraction, the CTE decreases and the Young’s modulus increases gradually respectively. And thus, the CTE of brazed seams, which consisted of Ag-based composite reinforced by some fine compounds with low CTE such as Ti5 Si3P (CTE = 11.0 × 10−6 K−1 ) [18], TiNp (CTE = 11.0 × 10−6 K−1 ) [18] and remnant Si3 N4P (CTE = 3.1 × 10−6 K−1 ), could be reduced considerably. The effect of Si3 N4p content on the calculated CTE and Young’s modulus of brazed seam is shown in Fig. 6. It can be seen that the increase of Si3 N4p content leads to the decrease of the CTE, while the Young’s modulus is monotone increased. When the Si3 N4P content exceeded 1.5 wt.%, the CTE of brazed seam could be reduced to about 10 × 10−6 K−1 or less. By comparing the CTE of Si3 N4 (3.1 × 10−6 K−1 ), TiAl (10.8 × 10−6 K−1 ), and the brazed seam (16.8 × 10−6 K−1 ) obtained using Ag–Cu–Ti filler alloy with-

3.3. Effect of Si3 N4p content on CTE and Young’s modulus of the brazed seam As analyzed above, the interfacial microstructure of the joints obviously changed due to the addition of Si3 N4p into Ag–Cu–Ti filler. A typical interfacial structure consisted of fine Al4 Cu9 compound, Ti5 Si3p and TiNp in Ag-based solid solution, which was similar to metal-based composite reinforced by particles. Based on the investigation on the composite, the CTE and Young’s modulus of the brazed seam can be calculated according to the following formula (1) and (2) respectively [27–29].



˛ = ˛m 1 −

   Vf

+

˛f Vf

(1)

(2)

Fig. 7. Effect of Si3 N4p content on shear strength of brazed joints.

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Fig. 8. Fractography of Si3 N4 /TiAl joints brazed with different Si3 N4p contents (a) 0 wt.% and (b) 3 wt.%.

out Si3 N4P addition, it can be concluded that the CTE mismatch between Si3 N4 and TiAl was reduced effectively with the addition of Si3 N4P . Similarly, the Si3 N4P addition in filler alloy could also reduce the Young’s modulus mismatch between Si3 N4 and TiAl. 3.4. Effect of Si3 N4p content on shear strength of brazed joints Fig. 7 illustrates the shear strengths of Si3 N4 /TiAl joints using composite filler alloys at 1153 K for 5 min. Compared with joint using Ag–Cu–Ti filler metal, the shear strengths of the joint were improved when the composite filler alloy were adopted. The maximum shear strength of 115 MPa was attained when the content of Si3 N4p was 3 wt.%, which is approximately 53 MPa (85%) higher than that using the traditional Ag–Cu–Ti filler alloy alone. When Si3 N4p content was further increased to 4.5 wt.% or 6 wt.%, the shear strength decreased sharply as shown in Fig. 7. The experimental results can be explained as following. Desired interfacial microstructure was obtained when Si3 N4p content was 3 wt.% (shown in Fig. 3(c)), which was beneficial to the joining properties. When Si3 N4p content was less than 3 wt.%, blocks of bright Ag-based solid solution formed after brazing due to the insufficient Si3 N4p in composite filler metal, as shown in Fig. 3(b). The heterogeneous microstructure has negative effects on the joining quality. In addition, when Si3 N4p content was more than 3 wt.%, the interfacial microstructure of brazed joint was heterogeneous due to poor distribution of Al4 Cu9 compounds, Ag–Cu eutectic structure and the agglomeration of remnant Si3 N4p , which also degraded the joining properties. As analyzed above, the Si3 N4P addition in filler alloy reduced the CTE and Young’s modulus mismatch in the joint, which resulted in the release of the residual stress and improvement of the joining quality. However, excessive reinforcing particles deteriorated the fluidity of the liquid composite filler alloy [18]. Simultaneously, the consumption of Ti by the excessive Si3 N4p near the Si3 N4 substrate reduced the active Ti content sharply, which brought insufficient reactions between the Ti and Si3 N4 substrate. This effect resulted in a negative impact on wettability and connectivity of the brazing layer to Si3 N4 substrate. Therefore, a suitable Si3 N4p content (3 wt.% in this study) was essential and reasonable. The fracture after shear test was investigated as shown in Fig. 8. Failure of the Si3 N4 /TiAl joints using traditional Ag–Cu–Ti filler alloy always occurred in the ceramic substrate near the braze interface

and a bowed crack path was observed in Si3 N4 substrate, as shown in Fig. 8(a). This result indicated that high residual stresses were generated in the ceramics substrate close to the interface, and the residual stresses led to the decrease of joining properties. Different failure modes were observed in the joints using composite filler alloy. Fig. 8(b) shows the fractography of joint using composite filler alloy containing Si3 N4p of 3 wt.%. Cracks propagated in the brazed seam and finally ruptured in Si3 N4 substrate. The failure mode suggested that the residual stresses decreased because the improved microstructure was obtained using the composite filler alloy. 4. Conclusion Reliable brazed joints of Si3 N4 ceramics to TiAl intermetallics were successfully produced using a novel composite filler alloy modified by adding Si3 N4p into the Ag–Cu–Ti filler alloy. The typical interfacial microstructure of the brazed joints was TiAl/AlCu2 Ti reaction layer/Ag(s,s) + Al4 Cu9 + Ti5 Si3p + TiNp /TiN + Ti5 Si3 reaction layer/Si3 N4 . The addition of Si3 N4p led to the obvious change of the microstructure and the optimal microstructure was obtained when the Si3 N4p content was 3 wt.%. The distribution of fine compounds in Ag-based solid solution decreased the mismatch of CTE and the Young’s modulus in the joints, which had positive effects on the joining properties. This result showed agreements with fracture analysis. With the increase of Si3 N4p content, the shear strength of brazed joints increased and then decreased. The maximum value reached 115 MPa when the content of Si3 N4p was 3 wt.%. Acknowledgement The authors gratefully acknowledge the financial support from project 50805038 supported by National Natural Science Foundation of China. References [1] [2] [3] [4] [5]

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