Effect of Brazing Conditions on Microstructure and Mechanical Properties of Al 2 O 3 /Tie6Ale4V Alloy Joints Reinforced by TiB Whiskers

Transcription

1 Available online at SciVerse ScienceDirect J. Mater. Sci. Technol., 2013, 29(10), 961e970 Effect of Brazing Conditions on Microstructure and Mechanical Properties of Al 2 O 3 /Tie6Ale4V Alloy Joints Reinforced by TiB Whiskers Minxuan Yang 1,2), Peng He 1), Tiesong Lin 1)* 1) State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin , China 2) Beijing Power Machinery Research Institute, Beijing , China [Manuscript received April 4, 2012, in revised form December 20, 2012, Available online 7 June 2013] Al 2 O 3 and Tie6Ale4V alloy were brazed with AgeCueTi þ B fillers in different brazing conditions. Effects of brazing temperature, holding time and additive Ti content on joints microstructure and shear strength were investigated by scanning electron microscopy, energy dispersive spectrometry, X-ray diffraction, transmission electron microscopy and shear testing. Results indicate that TiCu and Ti(Cu,Al) decrease, but Ti 2 Cu and Ti 2 (Cu,Al) increase in brazing seam with increasing brazing temperature, holding time and additive Ti content. Area consisting of Ti 3 (Cu,Al) 3 O and TiO near Al 2 O 3 becomes gradually discontinuous from continuity when brazing temperature rises or holding time extends. As Ti additive content increases, TiO is absent near Al 2 O 3 ; area consisting of only Ti 3 (Cu,Al) 3 O thickens. TiB whiskers are in situ synthesized by Ti and B atoms during brazing process. The brazing temperature, holding time and additive Ti content on joints microstructure influence the joints shear strength directly. The shear strength of joints, obtained at 850 C holding for 10 min, reaches the maximum of 78 MPa. According to the experimental results, phase diagram and thermodynamics calculation, the interface evolution mechanism of the Al 2 O 3 /Tie6Ale4V alloy joint was analyzed. KEY WORDS: Active brazing; Addition of B and Ti; Al 2 O 3 ;Tie6Ale4V alloy 1. Introduction With the excellent resistances to heat and wear, as well as superior physical, chemical and mechanical properties, Al 2 O 3 has become a promising structure material for aeronautic applications [1].Tie6Ale4V alloy can be also widely used for aeronautic application, owing to its superplasticity, low weight and high mechanical resistance [2]. To acquire the interesting properties of them, Al 2 O 3 and Tie6Ale4V alloy need to be joined. Brazing is very important for joining ceramics and metals. However, there are two problems in brazing ceramics and metals: ceramic surface is difficult to wet by brazing filler, and joint residual stress is caused by mismatch of coefficient of thermal expansion (CTE) between ceramic and brazing seam or between ceramic and substrate metal [3]. As a preferred brazing method, active brazing has improved wettability of brazing filler * Corresponding author. Tel.: þ ; Fax: þ ; address: hitjoining@hit.edu.cn (T. Lin) /$ e see front matter Copyright Ó 2013, The editorial office of Journal of Materials Science & Technology. Published by Elsevier Limited. All rights reserved. on ceramic surface [4]. In order to decrease the mismatch of CTE between ceramics and brazing fillers, inserting soft interlayer or adding low CTE materials into brazing fillers is commonly used. Two approaches can be adopted for adding low CTE materials: one is directly adding; the other is adding by in situ synthesizing [5e8]. By comparison, in situ synthesizing low CTE materials in brazing seam during brazing is better than the method of directly adding. Moreover, present authors have investigated the brazing of Al 2 O 3 and Tie6Ale4V alloy by in situ synthesizing TiB in joint during brazing process, and achieved good results [9]. The reliability and integrity of ceramicemetal composite components depend on the properties of ceramicemetal joints created by brazing process. Brazing temperature and holding time have great effect on the microstructure and mechanical properties of joint, because different reaction products will form with different brazing temperature and holding time [10e22]. Feng et al. [18] have discussed the effect of brazing temperature and holding time on the microstructure and shear strength of SiO 2 /Tie6Ale4V alloy joints. It has been found that the maximum shear strength was 110 MPa when the joint was joined at 970 C for 10 min. Joint shear strength was correlative to the Ti 4 O 7 þ TiSi 2 reaction layer. Kim and Weil [17] have studied the influence of brazing temperature on the

2 962 M. Yang et al.: J. Mater. Sci. Technol., 2013, 29(10), 961e970 microstructure and mechanical properties of air-brazed aluminum joints. They found that higher joining temperature and longer holding time could result in higher bend strengths: the maximum average bend strength of 135 MPa was obtained by joining at 1200 C for 100 h. Elrefaey and Tillmann [13] have used Incusil-ABA to braze commercially pure titanium in a high-vacuum furnace. Brazing temperature and holding time were also critical factors to control the microstructure and the mechanical properties of joints. A brazing temperature of 750 C provided the best results in terms of joint strength. The change of joint strength with brazing parameters was to a large extent dependent on a thickness of the reaction layer between substrate and brazed alloy. Consequently, the purpose of present study is to evaluate the effect of brazing temperature, holding time and additive amount of Ti powders on the joint microstructure and shear strength. Furthermore, the interface evolution mechanism of the Al 2 O 3 / Tie6Ale4V alloy joint has been basically analyzed, based on the brazing temperature- and holding time-dependent compositional change. 2. Experimental The substrate materials used in the experiment were Al 2 O 3 and Tie6Ale4V alloy. Al 2 O 3 was cut with diamond discs into specimens with a size of 5.0 mm 5.0 mm 3.0 mm for microstructure examination and 4.0 mm 4.0 mm 3.0 mm for shear testing. Tie6Ale4V alloy was cut into the specimens with a size of 10.0 mm 8.0 mm 1.2 mm by wire electro-discharge machining. The surfaces to be brazed were sanded with #800 SiC papers and then Al 2 O 3 and Tie6Ale4V alloy specimens were dipped into acetone and cleaned with an ultrasonic washer for 30 min. AgeCueTi þ B filler was manufactured by commercially available Age26.4Cue4.5Ti (wt%) powders (w50 mm indiam- eter, 99.5 wt%) and B powders (w5 mm in diameter, 95 wt%) at room-temperature in ethanol atmosphere by wet mixing in a mortar. In situ synthesizing 20 vol.% TiB in joint corresponded to the quantity of additive B powders during brazing. In addition, the brazing fillers with different Ti content were manufactured by AgeCu eutectic powders (w50 mm in diameter, 99.5 wt%), Ti powders (w50 mm in diameter, 99.5 wt%) and B powders by adding suitable binder mixture into paste, and then the mixed paste was applied between Al 2 O 3 and Tie6Ale4V alloy by coating on the surface of Tie6Ale4V alloy (w100e200 mm), as schematically shown in Fig. 1(a). After that the assembly was brazed at 850, 880, 900, 930 and 950 C for dwell duration of 10 min, and 900 C for 0, 5, 20, and 30 min, respectively, in a vacuum furnace with the vacuum of Pa. During brazing, in order to ensure the brazed surfaces were clean, the assembly was first heated to 450 C at a rate of 20 C/min, and held for 10 min to volatilize the binder. Then the temperature was continuously increased to specified temperature at a rate of 10 C/min and held for required time. Finally, the assembly was cooled down to 400 Catarateof5 C/min and then subjected to furnace cooling. The polished cross-sections of brazed joints were examined to characterize the interfacial microstructure by scanning electron microscopy (SEM, S-3400), energy dispersive spectrometry (EDS) equipped with SEM. The X-ray diffraction (XRD, D/max- RB) was used to identify the metallographic phases of each reaction layer, which paralleled to Al 2 O 3 /Tie6Ale4V alloy joint by layered stripping. The transmission electron microscopy Fig. 1 Schematics of Al 2 O 3 and Tie6Ale4V alloy brazing (a) and shear strength experiment (b). (TEM, Philips-CM12) was used to confirm the needle-like phases. Joints shear strength at room-temperature was examined with a universal testing machine (Instron-1186) with a displacement rate of 0.5 mm/min. The specimens were fixed in a Fig. 2 Microstructure of (a) Al 2 O 3 /Tie6Ale4V alloy joint brazed with AgeCueTi þ B, (b) the center of joint interface.

3 M. Yang et al.: J. Mater. Sci. Technol., 2013, 29(10), 961e Fig. 3 Microstructure of the Al 2 O 3 /Tie6Ale4V alloy joint brazed with AgeCueTi þ B filler at different temperatures for 10 min: (a) 850 C, (b) 880 C, (c) 930 C and (d) 950 C. special fixture, as illustrated in Fig. 1(b). At least five specimens were tested for each experimental condition. 3. Results and Discussion 3.1. Effect of brazing temperature on joint microstructure Fig. 2 shows the microstructure of joint brazed at 900 Cfor 10 min with AgeCueTi þ B filler. According to previous work [9], the interface of joint is constituted by four areas (from Al 2 O 3 to Tie 6Ale4V alloy): Ti 3 (Cu,Al) 3 O (Area I), Ti 2 Cu þ Ti 2 (Cu,Al) (Area II), Ag(s.s) þ TiCu þ Ti(Cu,Al) þ Ti 2 Cu þ Ti 2 (Cu,Al) þ TiB (Area III) and Ti(s.s) þ Ti 2 Cu (Area IV). Table 1 EDS results of phases AeM inal 2 O 3 /Tie6Ale4V alloy joint brazed at different temperatures Phase O Al Ag Ti Cu Possible phase A Ti 3 (Cu,Al) 3 O B e 1.75 e TiCu C e 3.48 e Ti 2 Cu D e e Ti(s.s)þTi 2 Cu E Ti 3 (Cu,Al) 3 O F e Ti 2 Cu G e Ag(s.s) H e Ti 2 (Cu,Al) I e Ti(Cu,Al) J e Ti 2 Cu K e Ti 2 Cu L e Ti 2 (Cu,Al) M e Ti 2 (Cu,Al) Fig. 4 TEM bright field image of TiB.

4 964 M. Yang et al.: J. Mater. Sci. Technol., 2013, 29(10), 961e970 Fig. 3 shows microstructure of Al 2 O 3 /Tie6Ale4V alloy joints brazed at temperature of 850, 880, 930 and 950 C, respectively, for holding time of 10 min. It shows that the microstructure of every joint obtained at different brazing temperature is very similar, except at 850 C. Table 1 shows the EDS analysis results of the microconstituent of joints obtained at 850, 880, 930 and 950 C, similar to those at 900 C [9]. The black needle-like phases distributed in joint is too tiny to be confirmed by EDS, which was detected by TEM as shown in Fig. 4. It can be concluded that the black needle-like phases are TiB. Carim and Mohr [23] have thought that in addition to Ti 3 Cu 3 O, TiO appeared near Al 2 O 3. They have presented the reaction of Ti 3 Cu 3 O þ 2O (alumina) / 3Cu þ 3TiO through brazing Al 2 O 3 with Ti 3 Cu 3 O interlayer. According to a line analysis carried out by EDS at the interface near Al 2 O 3 obtained at 930 C, as shown in Fig. 5, the amount of Ti and O is higher than that of other elements. It can be speculated that TiO compounds appear near Al 2 O 3 when brazing temperature is higher than 900 C, because Fig. 6 XRD results of every area after striping layer by layer: (a) mixing layer of Area I and II, (b) Area II, (c) Area III. Al 2 O 3 can release much more amount of Al and O into the brazing filler with increasing brazing temperature. Combining with the result of XRD, TiO also can be detected in Area I as shown in Fig. 6(a). Area I gradually thins out and becomes discontinuous. It can be attributed to the fact that Ti 3 (Cu,Al) 3 O Fig. 5 (a) Microstructure of Al 2 O 3 /Tie6Ale4V alloy interface (T ¼ 930 C, t ¼ 10 min), (b) magnification of area a on (a), (c) qualitative compositional line analysis. The line on (b) indicates the place where line analysis was performed. Fig. 7 Microstructure of Al 2 O 3 /Tie6Ale4V alloy interface (T ¼ 950 C, t ¼ 10 min).

5 M. Yang et al.: J. Mater. Sci. Technol., 2013, 29(10), 961e reacts with O to produce TiO, and a larger brittle and hard TiO layer may break up itself [24]. This is why cracking occurs in Al 2 O 3 side when the brazing temperature rises to 950 C, as shown in Fig. 7. Both Area II and IV broaden as the brazing temperature increases from 880 to 950 C, because Ti and Al atoms increasingly diffuse into the brazing seam and toward Al 2 O 3 from Tie6Ale4V alloy. The formation amount of Ti 2 Cu and Ti 2 (Cu,Al) intermetallics increases in joint. Therefore, Area II consisting of Ti 2 Cu and Ti 2 (Cu,Al) expands and grows over to the center of brazing seam. Meanwhile, Area IV broadens, as the brazing temperature rises, due to the increase of Tie6Ale4V alloy dissolution and the diffusion of Cu atoms into Tie6Ale4V alloy. 20 vol.% TiB whiskers were in situ synthesized in joint, and thus the range of distribution area of TiB whiskers (Area III) was constant. However, with increasing brazing temperature, Ti(Cu,Al) displaced TiCu in Area III, and then Ti(Cu,Al) and TiCu disappeared gradually; finally a large number of Ti 2 (Cu,Al) and Ti 2 Cu formed in Area III, due to the increase of Ti and Al content in joint. The microstructure of Al 2 O 3 /Tie6Ale4V alloy joint brazed at 850 C is obviously different from that at other temperatures from 880 to 950 C, but similar to that of Al 2 O 3 /Al 2 O 3 joint [25]. The microstructure of joint obtained at 850 CisAl 2 O 3 /Ti 3 (Cu,Al) 3 O/ Ag(s.s) þ Cu(s.s) þ TiB/TiCu/Ti 2 Cu/Ti(s.s) þ Ti 2 Cu/Tie6Ale 4Valloy, and TiB whiskers distribute in Ag solid solution (Ag(s.s)) and Cu solid solution (Cu(s.s)). It is led by the diffusion of Ti and Al, from Tie6Ale4V alloy to brazing seam, decreases at low brazing temperature. Cu atoms from brazing filler diffuse toward Al 2 O 3 and Tie6Ale4V alloy, respectively, by the concentration gradient. Combining with the interaction of Ti and Cu atoms, the diffusion of Cu toward Tie6Ale4V alloy is faster than that toward Al 2 O 3. Therefore, the structure of TiCu/Ti 2 Cu/ Ti(s.s) þ Ti 2 Cu/Tie6Ale4V alloy forms. Also Cu(s.s) forms in brazing seam owing to the decrease of consumption of Cu. However, limited pores or voids form in the joint, as shown in Fig. 8 Microstructure of the Al 2 O 3 /Tie6Ale4V alloy joint brazed with AgeCueTi þ B filler at 900 C for: (a) and (b) 0 min, (c) and (d) 5 min, (e) 20 min, (f) 30 min.

6 966 M. Yang et al.: J. Mater. Sci. Technol., 2013, 29(10), 961e970 Fig. 3(a), because low temperature leads to the decrease of fluidity and filling power of melted brazing filler Effect of holding time on joint microstructure Fig. 8 shows the microstructure of Al 2 O 3 /Tie6Ale4V alloy joints obtained at 900 C for different holding time. It indicates that the microstructures of joint transform from non-uniformity to uniformity with increasing holding time. Because atomic diffusion is limited when the holding time is too short. For the holding time is 0 or 5 min, with the interaction of Ti, Cu and Ag, Ag(s.s) forms in joint as big block, and the microstructure of Al 2 O 3 /Ti 3 (Cu,Al) 3 O/ Ti 2 Cu þ Ti 2 (Cu,Al)/Ag(s.s) þ TiCu þ Ti(Cu,Al) þ Ti 2 Cu þ Ti 2 (Cu,Al) þ TiB/Ti(s.s) þ Ti 2 Cu/Tie6Ale4Valloy only partially exists. For the holding time is 10 min or more, as shown in Fig. 8(e) and (f), the microstructure of joint becomes uniform. Similarly, the amount of TiCu and Ti(Cu,Al) decreases, but that of Ti 2 Cu and Ti 2 (Cu,Al) increases in joint with increasing holding time. Area I becomes discontinuous, as the holding time prolongs, because Ti 3 (Cu,Al) 3 O formed near Al 2 O 3 reacts fully with O released from Al 2 O 3 when the duration time is long. Area II consisting of Ti 2 Cu and Ti 2 (Cu,Al) broadens, and the amount of Ti 2 (Cu,Al) increases with increasing holding time. It indicates that long holding time can also make the dissolution of Ti and Al atoms increase, and the brittleness and hardness of Area II increase. Therefore, the significant cracks appear in Area II as the holding time extends to 30 min. The residual stress of joint releases difficultly, due to the increase of brittleness and hardness of the area. Meanwhile, the increase of a thickness of brazing seam means that the dissolution of Tie6Ale 4V alloy increases, and the amount of Ti and Al in brazing seam also increases. Also, given the more dissolution of Ti and Al atoms with longer holding time, TiCu and Ti(Cu,Al) transform to Ti 2 Cu and Ti 2 (Cu,Al) in Area III. And the generation amount of Ti 2 (Cu,Al) especially increases. The range of Area III does not change yet, since TiB whiskers content is certain. Ti(Cu,Al) still forms in Area III without turning into Ti 2 (Cu,Al). The above change of joint microstructure with increasing holding time is similar to that with the rising of brazing temperature, but the effect of holding time on joint microstructure is less severely than that of brazing temperature. It makes cracking only occur in Area II, where most of Ti 2 (Cu,Al) phases concentrate on Area II. Because the increase in both brazing temperature and holding time promotes the dissolution of Tie6Ale4V alloy, thereinto, the brazing temperature effect is much stronger than holding time effect [26] Effect of the amount of additive Ti powders on joint microstructure The diffusion of Ti from Tie6Ale4V alloy to brazing seam is of the essence, according to the effects of brazing temperature and holding time on microstructure of joints. For this reason, it is necessary to investigate the effect of Ti content on Al 2 O 3 /Tie 6Ale4V alloy joint microstructure. Fig. 9 provides the morphology of Al 2 O 3 /Tie6Ale4V alloy joints brazed with four compositions of brazing filler: 0, 5, 10 and 15 wt% Ti powders, respectively and in situ synthesizing TiB whiskers in these joints are 20 vol.%. From Fig. 9(aed), Area I consisting of only Ti 3 (Cu,Al) 3 O thickens when the Ti content increases from 0 to 15 wt%. Ti 3 (Cu,Al) 3 O does not react with Al 2 O 3 to form TiO with increasing Ti content. It shows that the reaction between Ti 3 (Cu,Al) 3 O and Al 2 O 3 only occurs at appropriate brazing temperature and holding time. Ti content has no effect on that. Area II first increases and then decreases as Ti content increases. The reason why Area II increases is the increase of Fig. 9 Microstructure of the Al 2 O 3 /Tie6Ale4V alloy joint brazed at 900 C for 10 min with AgeCu þ B filler and different Ti: (a) 0 wt %, (b) 5 wt%, (c) 10 wt%, (d) 15 wt%.

7 M. Yang et al.: J. Mater. Sci. Technol., 2013, 29(10), 961e Fig. 10 Schematic drawing of the formation of the microstructure of Al 2 O 3 /Tie6Ale4V alloy: (a) atom diffusion and formation of Ti 3 (Cu,Al) 3 O and TiB; (b) dissolution of Tie6Ale4V alloy, atom diffusion and formation of Ti 2 Cu; (c) atom diffusion become uniform, Ti 2 Cu growth remain; (d) atoms keep on diffusing, Ti(Cu,Al) 3 O growth remain; (e) complete solidification of brazing seam.

8 968 M. Yang et al.: J. Mater. Sci. Technol., 2013, 29(10), 961e970 Ti 2 Cu and Ti 2 (Cu,Al) formation amount with increasing Ti content. And the reason why Area II decreases is the brazing fillers fabricated by (1 x)agecu/xti proportion, namely that as the amount of additive Ti powders in brazing filler increases, the amount of AgeCu powders will decrease. Cu content decreases and no longer satisfies the reaction with Ti. Thus Area II narrows down for the decrease of formation amount of TieCu compounds. Meanwhile, the amount of Ag(s.s) formed in Area III increases, because the consumption of Cu increases. As Ti content increases, the exclusive interaction of Ti and Cu to Ag weakens. Besides, it is also worth noting that TiB 2 particles remain in Area III as shown in Fig. 9(a). It demonstrates that the reaction TiB 2 þ Ti / 2TiB occurs when the Ti content is excessive in brazing seam [9,16]. Area IV thins out gradually for the decrease in dissolution of Tie6Ale4V alloy, because of the decrease in concentration gradient of Ti between brazing seam and Tie6Ale4V alloy Microstructure evolution model of brazed joint A conceptual interface evolution model for Al 2 O 3 / AgCuTi þ B/Tie6Ale4V alloy joint is presented in Fig. 10 based on the experimental results, phase diagrams and thermodynamics calculations as shown in Fig. 11 [9]. The whole reaction process can be divided into five stages. Stage one (Fig. 10(a)): according to AgeCueTi phase diagram, the system has a eutectic temperature of 780 C. When the temperature is lower than 780 C, brazing filler powders only become softer and contact each other under the pressure of Pa with increasing temperature. Stage two (Fig. 10(b)): when the temperature is elevated to the 780 C, liquid AgeCueTi eutectic forms in the brazing filler. With increasing heating temperature, the amount of liquid phase increases; meanwhile, Ti and Al atoms from Tie6Ale4V alloy as well as B atoms from brazing filler begin to dissolve into the liquid system. Because of the high affinity of Ti and O, a number of Ti atoms accumulate nearby Al 2 O 3. Cu atoms begin to spread to Al 2 O 3 and Tie6Ale4V alloy side, respectively. Subsequently, Ti 3 (Cu,Al) 3 O phases nucleate and grow up based on Al 2 O 3 substrate by the reaction among Ti, Cu, O and Al atoms. As its melting point is 2300 C (much higher than the brazing temperature), B powders contacting the liquid system will react with Ti element as well. With reaction of Ti þ B / TiB 2 or Ti þ TiB 2 / TiB, TiB forms with a needle-like shape or TiB 2 forms with a block shape by TieB phase diagram and thermodynamics calculations [9]. Although TiB 2 is easier to be produced than TiB, the growth rate of TiB whisker along the needle directions is six times higher than that of the TiB 2 particle, and the estimated diffusion coefficient for B in TiB along the needle directions is about 45 times larger than that in TiB 2 during brazing process. The activation energies for B diffusion in both TiB and TiB 2 are effectively the same [27]. Thus TiB 2 is just a transition phase, which will give way to TiB before the consumption of the Ti. Stage three (Fig. 10(c)): when the temperature further increases, Ti and Al atoms from Tie6Ale4Valloy diffuse into brazing filler, and Cu atoms invade Tie6Ale4Valloy continually. Ti 3 (Cu,Al) 3 O compounds increase in size and form a continuous layer near Al 2 O 3.TiB 2 continues to translate into TiB by the reaction of Ti þ TiB 2 / TiB. Ti 2 Cu starts to nucleate and grow up to the center of brazing filler by the atoms diffusion and isothermal solidification, based on the Ti 3 (Cu,Al) 3 O/liquid interface and liquid/ Tie6Ale4Valloy interface. Residual liquid is pushed to the center of brazing seam and interdendritic region of Ti 2 Cu. Ti and Al concentrate in residual liquid with the aid of positive segregation. Stage four (Fig. 10(d)): when the temperature reaches the brazing temperature, the diffusion of Ti, Al and Cu atoms between brazing filler and Tie6Ale4V alloy continues with increasing holding time till atoms diffusion reaches equilibrium. Ti 2 Cu continues to nucleate and grow up with the diffusing of Ti from Tie6Ale4Valloy to Al 2 O 3.Ti 3 (Cu,Al) 3 O layer stops thickening. TiB 2, not completely reacted, still reacts with Ti to synthesize TiB. Stage five (Fig. 10(e)): when the temperature declines at a rate of 5 C/min, Ti 2 (Cu,Al) appears in interdendritic region of Ti 2 Cu and the interface of Ti 2 Cu/residual liquid. TiB forms with the interaction of TiB 2 and Ti in brazing filler. TiCu, Ti(Cu,Al) and Ti 2 Cu form in the liquid of brazing filler and TiB whiskers distribute in them by the temperature and composition concentration. The hypereutectoid Ti(s.s) þ Ti 2 Cu organizations precipitate near Tie6Ale4Valloy as temperature drops to the critical temperature of hypereutectoid transformation. Ag(s.s) forms when temperature is below the liquidus temperature of AgeCue Ti alloy. TiB whiskers also can distribute in it. Eventually, Al 2 O 3 / Tie6Ale4Valloy joint reinforced by TiB whiskers is well formed Room-temperature shear strength of Al 2 O 3 /Tie6Ale4V alloy joint Fig. 12 shows the room-temperature shear strength of joints using AgCuTi þ B alloy powders at different brazing temperature for a holding time of 10 min. It shows that the shear strength of joints increased first when the brazing temperature was from 830 to 850 C and then decreased at the temperature from 850 to Fig. 11 Free energy of reaction phases at different temperatures.

9 M. Yang et al.: J. Mater. Sci. Technol., 2013, 29(10), 961e Fig. 12 Shear strength of Al 2 O 3 /Tie6Ale4V alloy joint brazed with AgeCueTi þ B filler at different temperatures for 10 min. Fig. 14 Shear strength of Al 2 O 3 /Tie6Ale4V alloy joint brazed with filler containing different mass fraction Ti at 900 C for 10 min. 950 C. When the brazing temperature was 850 C, joint had maximum shear strength (78 MPa). Fig. 13 shows the roomtemperature shear strength of joints using AgCuTi þ B alloy powders at 900 C for different holding time. The maximum shear strength (55 MPa) was obtained as the holding time was 30 min. Fig. 14 provides the shear strength of joints brazed with brazing fillers of various Ti content. The shear strength of joints increased first then decreased as Ti content increased from 0 to 15 wt%. As Ti content was 10 wt%, joint shear strength reached the maximum of 52 MPa. The shear strength of joints brazed at different brazing temperature, holding time and Ti content, is to a large extent dependent on the joints microstructure. Both the quantity of various intermetallics in brazing seam, and the range of every area are essential factors in determining the shear strength of joints. With increasing brazing temperature, holding time and Ti content in brazing filler, the dissolution of Ti increases. Thus the generation amount of Ti 2 Cu and Ti 2 (Cu,Al) with greater brittleness and hardness than that of TiCu and Ti(Cu,Al) increases in brazing seam, while the amount of TiCu and Ti(Cu,Al) reduces. Area II made of Ti 2 Cu and Ti 2 (Cu,Al) thickens. TiCu and Ti(Cu,Al) transform to Ti 2 Cu and Ti 2 (Cu,Al) in Area III. Area III can be seen as metal matrix composites (MMCs) reinforced by TiB. The needle-like TiB whiskers with low CTE, are favor of the promotion of mechanical properties of joint. Thus sufficient Ti content is needed to transform TiB 2 particles to TiB whiskers. In this experiment, when brazing temperature, holding time and Ti content increase over a proper value, the joints shear strength begins to decrease. One reason is that Ti 3 (Cu,Al) 3 O layer thickens, or TiO with greater brittleness and hardness than Ti 3 (Cu,Al) 3 O forms nearby Al 2 O 3, leading to the increase of residual stress concentration in Area I. The other reason is that the more the dissolution of Ti in melted filler alloy, the more the TieCu compounds forming and the less the Ag(s.s) existing in brazing seam, which makes the brazing seam hard to release or absorb residual stress of joint. However, the joint shear strength reaches the maximum value when the holding time is 30 min, although cracking appears in brazing seam as shown in Fig. 8(f). The residual stress can be released and reduced when the crack is produced in the brazing seam. The joint shear strength drops to the minimum when the brazing temperature is 950 C, because of the cracking in the interface of Al 2 O 3 /brazing seam as shown in Fig. 7. It demonstrates that the shear strength of joint when cracking appears in brazing seam is higher than that in the interface of Al 2 O 3 /brazing seam. Therefore, the dissolution of Ti in brazing seam is a key to control over every area range and mechanical properties of joint. On one hand, the formation of TiB whiskers needs sufficient Ti content, but on the other hand, excessive Ti content can make the brittleness and hardness of brazing seam increase, which causes the joint residual stress and deteriorates the mechanical properties of joint. It is significant to control the dissolution of Ti in the brazing seam through controls of brazing temperature, holding time and composition of brazing fillers. Fig. 13 Shear strength of Al 2 O 3 /Tie6Ale4V alloy joint brazed with AgeCueTi þ B filler at 900 C for various holding time. 4. Conclusion Al 2 O 3 /Tie6Ale4V alloy joints can be obtained using Age CueTi þ B brazing filler. With increasing brazing temperature, holding time and additive Ti content, the dissolution of Ti in brazing seam increases. The formation amount of TiCu and Ti(Cu,Al) decreases, but that of Ti 2 Cu and Ti 2 (Cu,Al) increases, and thus Area II consisting of Ti 2 Cu and Ti 2 (Cu,Al) thickens. Area I consisting of Ti 3 (Cu,Al) 3 O and TiO gradually thins out and transforms from continuous to discontinuous distribution when brazing temperature and holding time increase. Area I consisting of Ti 3 (Cu,Al) 3 O thickens, indicating that the reaction between Ti 3 (Cu,Al) 3 O and Al 2 O 3 only occurs at appropriate brazing temperature and holding time, and Ti content has no effect on that, when additive Ti content increases. TiB whiskers are in situ synthesized by Ti and B atoms during brazing process. The effect of brazing temperature, holding time and Ti content

10 970 M. Yang et al.: J. Mater. Sci. Technol., 2013, 29(10), 961e970 on joints microstructure affects the joints shear strength directly. The maximum shear strength of joint is 78 MPa when the brazing temperature is 850 C and holding time is 10 min in this experiment. The dissolution of Ti in brazing seam is a key to control over every area range and mechanical properties of joint. It is significant to control the dissolution of Ti in brazing seam through controls of brazing temperature, holding time and component of brazing fillers. The shear strength of joint when cracking appears in brazing seam is higher than that in the interface of Al 2 O 3 /brazing seam. Acknowledgments The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (Grant Nos , and ), the Natural Science Foundation of Heilongjiang Province, China (Grant No. QC2011C044) and the Specialized Research Fund for the Doctoral Program of Higher Education, China (Grant No ). REFERENCES [1] R. Asthana, M. Singh, J. Eur. Ceram. Soc. 28 (2008) 617e631. [2] R.K. Shiue, S.K. Wu, Y.T. Chen, C.Y. Shiue, Intermetallics 16 (2008) 1083e1089. [3] G.M. Liu, G.S. Zou, A.P. Wu, D.K. Zhang, Mater. Sci. Eng. A 415 (2006) 213e218. [4] M. Paulasto, F.J.J. Van Loo, J.K. Kivilahti, J. Alloy. Compd. 220 (1995) 136e141. [5] P. He, M.X. Yang, T.S. Lin, Z. Jiao, J. Alloy. Compd. 509 (2011) L289eL292. [6] Y.M. He, J. Zhang, Y. Sun, C.F. Liu, J. Eur. Ceram. Soc. 30 (2010) 3245e3251. [7] Z. Zhong, Z. Zhou, C. Ge, J. Mater. Process. Technol. 209 (2009) 2662e2670. [8] G.B. Lin, J.H. Huang, Powder Metall. 49 (2006) 345e348. [9] M.X. Yang, T.S. Lin, P. He, Y.D. Huang, Mater. Sci. Eng. A 528 (2011) 3520e3525. [10] O.A. Idowu, N.L. Richards, M.C. Chaturvedi, Mater. Sci. Eng. A 397 (2005) 98e112. [11] M. Pouranvari, A. Ekrami, A.H. Kokabi, J. Alloy. Compd. 469 (2009) 270e275. [12] H.S. Na, J.K. Kim, B.Y. Jeong, C.Y. Kang, Metals Mater. Int. 13 (2007) 511e515. [13] A. Elrefaey, W. Tillmann, J. Alloy. Compd. 487 (2009) 639e645. [14] W. Jiang, J.M. Gong, S.T. Tu, Mater. Des. 32 (2011) 736e742. [15] W. Jiang, J.M. Gong, S.T. Tu, Mater. Des. 31 (2010) 2157e 2162, [16] T.S. Lin, M.X. Yang, P. He, C. Huang, F. Pan, Y.D. Huang, Mater. Des. 32 (2011) 4553e4558. [17] J.Y. Kim, K.S. Weil, J. Am. Ceram. Soc. 90 (2007) 3830e3837. [18] J.C. Feng, D. Liu, L.X. Zhang, X.C. Lin, P. He, Mater. Sci. Eng. A 527 (2010) 1522e1528. [19] A.M. Saeed, Z. Hussain, A. Badri, T. Ariga, Mater. Des. 31 (2010) 3339e3345. [20] U.E. Klotz, C. Liu, F.A. Khalid, H.R. Elsener, Mater. Sci. Eng. A 495 (2008) 265e270. [21] S. Buhl, C. Leinenbach, R. Spolenak, K. Wegener, J. Mater. Sci. 45 (2010) 4358e4368. [22] Y. Liu, J. Hu, Y. Zhang, Z. Guo, Y. Yang, J. Mater. Sci. Technol. 27 (2011) 653e658. [23] A.H. Carim, C.H. Mohr, Mater. Lett. 33 (1997) 195e199. [24] G.P. Kelkar, A.H. Carim, J. Am. Ceram. Soc. 76 (1993) 1815e [25] M.X. Yang, T.S. Lin, P. He, Ceram. Int. 38 (2012) 289e294. [26] X.P. Zhang, Y.W. Shi, Scripta Mater. 50 (2004) 1003e1006. [27] Z. Fan, Z.X. Guo, B. Cantor, Compos. Pt. A-Appl. Sci. Manuf. 28 (1997) 131e140.