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Microstructural evolution mechanisms of Ti600 and Ni-25%Si joint brazed with Ti-Zr-Ni-Cu amorphous filler foil
Journal of Materials Processing Technology 240 (2017) 414–419
Contents lists available at ScienceDirect
Journal of Materials Processing Technology journal homepage: www.elsevier.com/locate/jmatprotec
Microstructural evolution mechanisms of Ti600 and Ni-25%Si joint brazed with Ti-Zr-Ni-Cu amorphous filler foil Xiaopeng Li a , Houqin Wang a , Ting Wang b,∗ , Binggang Zhang a,∗∗ , Tao Yu a , Ruishan Li a a b
State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin 150001, China Harbin Institute of Technology at Weihai, Shandong Provincial Key Laboratory of Special Welding Technology, Weihai 264209, China
a r t i c l e
i n f o
Article history: Received 18 July 2016 Received in revised form 22 October 2016 Accepted 24 October 2016 Available online 26 October 2016 Keywords: Ti600 alloy Ni-25at%Si alloy Ti-Zr-Ni-Cu amorphous filler foil Brazing Interfacial microstructure Evolution mechanism
a b s t r a c t A commercial Ti-Zr-Ni-Cu amorphous filler foil was applied to braze the high-temperature Ti600 and Ni-25at%Si, resulting in the good joint between both of alloys. The interfacial microstructure of Ti600/TiZr-Ni-Cu/Ni-25at%Si brazed joint at 1213 K for 10 min is mainly comprised of the continuous Ti2 Ni phase. Most of Ti-rich phases such as ␣-Ti, -Ti, Ti3 Al, Ti2 Ni, (Ti,Zr)2 (Ni,Cu) and (Ti,Zr)2 Si formed in the vicinity of the Ti600 substrate except that Ti5 Si3 precipitates near the Ni-25at%Si region, resulting in the formation of six distinct layers. The corresponding formation mechanism were clarified. The Ti5 Si3 and Ni31 Si12 were firstly formed during the initial stage followed by diffusion reactions at Ni-25at%Si side. The continuous Ti2 Ni phase formed through the reaction of Ti(L) + Ni(L) → Ti2 Ni. ␣-Ti, Ti3 Al, (Ti,Zr)2 (Ni,Cu) and (Ti,Zr)2 Si in the brazing joint precipitated through a solid-solid phase transformation. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Recently, much attention has been paid to the fabrication and deformation of high-temperature Ti600 titanium alloy with a nominal composition of Ti-6Al-2.8Sn-4Zr-0.5Mo-0.4Si-0.1Y (wt.%) due to its good tensile strength, excellent creep performance and superior fatigue resistance at the servicing temperatures of at least 873 K, which could promote further development of the aviation and aerospace industries in reducing the weight of the spacecraft as described by Niu et al. (2008). However, the poor oxidation resistance of such a high-temperature titanium alloy severally restricts its further application at elevated temperatures. Previous reports have demonstrated the Ni-25at%Si alloy possesses excellent high-temperature oxidation resistance as well as high strength at elevated temperature (Cao et al., 2014). In order to make full use of the advantages of these two materials simultaneously, the reliable joining of Ti600 titanium alloy to the Ni-25%Si alloy could be one of good choices.
∗ Corresponding author at: No. 2, Wenhuaxi Road, Weihai City, Shandong 264209, China. ∗∗ Corresponding author at: No. 92, Xi-Da-Zhi Street, Harbin, Heilongjiang 150001, China. E-mail addresses: [email protected] (T. Wang), [email protected] (B. Zhang). http://dx.doi.org/10.1016/j.jmatprotec.2016.10.021 0924-0136/© 2016 Elsevier B.V. All rights reserved.
However, the Ni-Ti-Si phase diagram (Tokunaga et al., 2004) showed at least 20 kinds of intermetallics compounds could form in the ternary Ni-Ti-Si system. Such complex brittle phase species would lead to a poor weldability of Ti600 titanium alloy and the Ni-25%Si alloy. As suggested by Yue et al. (2008) and Liu et al. (2005), both diffusion bonding and various types of brazing are suitable methods to join dissimilar materials, among which the latter is relatively more effective due to its convenience and costeffectiveness as considered by Cao et al. (2013). It may be feasible to adopt the brazing method to join the Ti600 titanium alloy and Ni-25at%Si alloy. The reaction products of Ni-Ti-Si ternary system varied greatly with the composition and process parameters during the brazing process. The study of Yang et al. (2013) shows that silicide is absent in the joints of SiC and TiAl brazed with TiNi fillers, but the study of Li et al. (2002) shows that it appeared in the joints of SiC and Ni with Ti fillers. The brazing of Ti600 titanium alloy to the Ni-25at%Si alloy has not been well studied yet, especially on the crystalline phase species and the corresponding microstructural features after welding. What’s more, an understanding of the crystal structure, chemistry and morphology of these various types of phase species during brazing is of primary importance as they have been known to affect the mechanical properties of joints. Hence, it is necessary to figure out the phase species and their forming mechanisms. In this work, the commercial Ti-Zr-Ni-Cu amorphous alloys are chosen as the filler metal based on the operating temperature
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3. Results and discussion
Fig. 1. The typical morphology of the electron beam floating zone melted Ni-25%Si alloys.
(i.e. 800 K) of Ti600 and the phase transformation temperature of Ni-25%Si. Ti600 titanium alloy and Ni-25%Si alloy were brazed using this filler metal at 1213 K for 10 min. The typical interfacial microstructures of the brazed joints were characterized. Furthermore, the corresponding forming mechanisms of different phases were deduced.
2. Experimental procedure The Ni-25at%Si alloy used in this study was obtained by the electron beam floating zone melting method. The fabricated Ni25at%Si alloy was comprised of Niss, Ni3 Si and Ni31 Si12 as shown in Fig. 1. The ductile Niss lamellar was produced in Ni-25%Si alloy to improve its toughness. The raw Ni-25%Si alloy was cut to blocks with the dimension of 5 mm × 5 mm × 5 mm using a wire electrical discharge machining (WEDM). The dimension of Ti600 alloy specimens for the metallographic observation were 10 mm × 20 mm × 2 mm. The thickness of Ti-Zr-Ni-Cu amorphous filler foil (Ti-38.65 Zr-10.1 Ni-15.7 Cu wt.%) was 50 m. Prior to brazing, the surfaces to be brazed were carefully ground and polished. The polished Ni-25at%Si alloy and Ti600 alloy as well as Ti-Zr-Ni-Cu amorphous filler foils were cleaned ultrasonically for 20 min in acetone and dried by air blowing. A single layer of filler foil was placed between Ni-25at%Si alloy and Ti600 alloy to form a sandwich type joint and then the brazing assemblies were placed into the vacuum furnace. At the beginning, the furnace was heated to 1073 K at a heating rate of 20 K/min, then to the 1213 K at a heating rate of 10 K/min. Subsequently, at the brazing temperature, the brazing couples were held for 10 min, then cooled down to 473 K at a cooling rate of 20 K/min. Finally, the joints were spontaneously cooled down to room temperature in the furnace. The interfacial microstructures of brazed joint were characterized employing a scanning electronic microscopy (SEM, Quanta 200FEG). Componential analysis of reaction phases in joints was performed using EDS (EDS, Genesis Apollo X/XL). Furthermore, the brazing seam was investigated by a transmission electron microscopy (TEM, Talos F200x) with selected area electron diffraction (SAED) in order to accurately identify the interfacial phases. Samples for cross-sectional TEM from specific areas in the brazed seam were obtained using a focused ion beam (FIB, Helios Nanolab600i) instrument with a Cross-beamTM configuration. The X-ray diffraction (XRD, D8 ADVANCE) was used to analyze the phases of the fractured surface.
Reliable brazing of Ti600 alloy and Ni-25at%Si alloy was achieved using Ti-Zr-Ni-Cu amorphous filler foil in the present study. Fig. 2(a) shows the typical interfacial microstructures together with corresponding EDS results for the Ti600/Ni-25at%Si joint brazed at 1213 K for 10 min. It is clearly seen that no pores or micro-cracks existed in the brazed joint and the width of the brazing seam was about 200 m. A 150% increase of the brazing seam width vs. the amorphous filler foil implies the intensive interactions, including dissolution, diffusion, and reaction occurs between the two base metals and the molten brazing alloy during brazing process. The EDS results shown in Fig. 2(b–g) confirm the occurrence of the strong dissolutions of Ti600 and Ni-25%Si substrate into the molten filler metal during brazing, resulting in the existence of alloying elements Si, Al, and Ti within the brazing seam. Meanwhile, the diffusions of elements Ni and Cu towards the Ti600 alloy lead to an increase of Ni content in the front of the Ti600 substrate (Fig. 2(e, g)). However, the element Ti is absent within the reaction layer of Ni-25at%Si substrate side (Fig. 2(d)), which illustrates the full reaction of the element Ti in the brazing seam. The element Zr (seen in Fig. 2(f)) mainly distributing in middle part of the brazing seam suggests its low diffusion rate and the original location of Ti-Zr-Ni-Cu amorphous filler foils. It is also can be deduced the strong dissolution of the two base metals. More detailed images of the various regions of the brazed seam in backscattered electron mode were shown in Fig. 3. The whole interface presents a multilayered structure. Based on the difference in microscopic morphology, the joints was divided into six zones (marked as Zones I, II, III, IV, V and VI). Fig. 3(b–d) shows the high-magnification backscattered electron images (BSE images) of these six characteristic zones respectively, and major elements at each spot in Fig. 3 detected by EDS are listed in Table 1. Fig. 3(b) shows that a diffusion layer and a reaction layer was formed near the Ti600 side. Intensive interactions including dissolution, diffusion and reaction simultaneously occurred at the interface during brazing, which results in the formation of a multi-phase zone. Particularly, the research of Lee et al. (2010) indicated that the diffusion of -Ti stabilizers, such as Ni and Cu atoms, towards Ti600 substrate promoted the phase transformation of ␣-Ti to -Ti. So a continuous diffusion zone (Zone I) was formed containing -Ti phase, as shown in Fig. 3(b). According to the elemental contents in Table 1 and Ti-Ni binary phase diagram, the continuous dark gray region in Zone II is Ti2 Ni intermetallic phase. The following shear tests of the specimens obtained by 1213 K/10 min show that the average shear strength is 43.5 MPa. The XRD of the typical fractured surface in Fig. 4 indicates that Ti2 Ni phase dominates most of the fractured surfaces. Thus, the continuous Ti2 Ni should be avoided. Further observation shows that some black regions and light gray regions with eutectic reaction characterization distributed around the Ti2 Ni phase. The EDS results of the black regions revealed the similar composition of this part with Zone I except slightly increase of Cu. Thus, the black regions in Zone II are mainly composed of ␣-Ti. What’s more, the light gray regions (Point E) around black regions are proved to be (Ti, Zr)2 (Cu, Ni) as the molar ratio of Ti+ Zr and Cu + Ni is approximately 2:1. The analysis of the light gray regions (Point B) located in front of Zone I shows that only three elements, namely Ti (71.84%), Al (15.2%) and Sn (8.5%), accumulated in these blocks. Since the appropriate Al composition can be calculated by Al-equivalent =CAl + 1/3CSn as depicted by Ma and Wang (1998), the actual composition of Al is 21.9 at%, which indicating these blocks are Ti3 Al. It can be seen in Fig. 3(c) that three different regions of different contrast exist in Zone III. The dark gray region has been proved to be Ti2 Ni above. The composition analysis of the white phase marked as Point F indicating a compound comprised of 30.10% Ti, 29.41%
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Fig. 2. Interfacial microstructure and elemental distribution of Ti600 and Ni-25at%Si joint brazed at 1213 K for 10 min using Ti-Zr-Ni-Cu amorphous filler foil. (a) BSE image of the joint and EDS maps of (b) Al, (c) Si, (d) Ti, (e) Ni, (f) Zr and (g) Cu.
Fig. 3. (a) Typical morphology of Ti600 and Ni-25at%Si joint brazed at 1213 K for 10 min using Ti-Zr-Ni-Cu amorphous filler foil and the high magnification BSE images of (b) Zone I and II, (c) Zone III and (d) Zone IV, Zone V and Zone VI.
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Table 1 Chemical compositions and possible phases of each spot marked in Fig. 3 (at.%). Spot
Al
Si
Zr
Sn
Ti
Ni
Cu
Possible phase
A B C D E F G H I J
11.00 15.21 5.06 6.71 9.74 2.26 14.45 1.64 0.63 10.98
1.58 1.35 0.70 1.08 0.69 29.41 1.34 34.99 27.16 2.06
1.84 0.99 2.52 7.58 2.75 27.79 1.44 3.09 0.07 1.85
1.57 8.53 0.22 0.72 1.48 1.89 1.48 0.33 0.29 1.95
76.38 71.87 56.65 55.84 72.91 30.10 73.37 56.98 0.31 72.51
6.55 1.42 31.46 18.11 6.49 5.73 4.19 2.41 69.68 4.88
1.07 0.63 3.39 9.96 5.94 2.82 3.73 0.55 1.85 5.78
␣-Ti + ˇ-Ti Ti3 Al Ti2 Ni (Ti,Zr)2 (Cu,Ni) ␣-Ti + ˇ-Ti (Ti, Zr)2 Si ␣-Ti + ˇ-Ti Ti5 Si3 Ni31 Si12 ␣-Ti + ˇ-Ti
Fig. 4. The XRD of the typical fractured surface of the joint obtained by 1213 K/10 min.
Zr and 27.79% Si was obtained. According to the Ti-Zr-Si ternary phase diagram (Bulanova et al., 2004), the white phase is (Ti, Zr)2 Si. Besides, the black phase marked as Point G was proved as ␣-Ti since it owns the same composition and morphology with Phase E. Fig. 3d displays the three reaction zone, namely Zone IV, Zone V and Zone VI, at the Ni-25at%Si substrate side. According to the contrast of different phase, it seems that the Zone IV mainly consist of continuous Ti2 Ni and ␣-Ti + ˇ-Ti (Phase J). Besides, some black phases marked as Point H also can be found in Zone IV. Further EDS analysis confirmed that the black phase is Ti5 Si3 . As shown in Fig. 3d, the Ti5 Si3 phase can be divided into two categories based on its distribution. Some of the Ti5 Si3 particle dispersed in Ti2 Ni phase and others gathered around the Ti2 Ni phase. The single phase in Zone V was confirmed as Ni31 Si12 according to EDS analysis and Ni-Si phase diagram. It is worth noting that the concentration of Si in Zone V increased by 2.16% compared to the composition of base metal. The following reason was responsible for the phenomenon. As mentioned by Lee et al. (2008), Ni atoms are known to be the dominant diffusing species in the Ni-Si system. The Ni-Ni metal bonds existing in the base metal are more easily to break than the Ni-Si covalent bonds, which will result in the preferentially diffusing of Ni atoms. Therefore, the element Si was much more reluctant to diffuse and accumulated in Zone V. A thin layer (Zone VI) with different contrast was formed between Zone IV and Zone V. Fig. 5 shows the bright field image and selected area electron diffraction patterns from two TEM samples extracted from Zone VI of brazing seam. The TEM sample for Fig. 5(a) was extracted adjacent to Zone IV and the TEM sample for Fig. 5d was extracted adjacent to Zone V. According to the results of TEM and SAED, the continuous light gray phase in Zone VI is TiNi (Fig. 5(b)) and there were some tiny Ti5 Si3 (Fig. 5(c)) particles dispersed at the grain boundary, which appeared solid reaction
character. The white line next to TiNi in Fig. 5(d) was proved as TiNi3 by SAED (Fig. 5(e)). Further observation of Fig. 3(d) displayed that the TiNi layer and TiNi3 layer show diffusion-controlled morphology. As analyzed above, Ni diffused easily to the brazing seam due to the strong affinity of Ti for Ni, which would lead to the formation of Ni-rich Ni-Ti intermetallics. According to the research of He and Liu (2006), the TiNi3 and TiNi are the first two preferential formed phase at Ni-rich side during diffusion bonding process as the Gibbs free energy of these two phases are −34.626 kJ and −30.791 kJ respectively. Interestingly, a Ti-Ni-Si ternary intermetallic was formed between Ti3 Ni and Ni31 Si12 and it was identified as Ti6 Ni16 Si7 by the SAED in Fig. 5(e). The stoichiometry of Ti6 Ni16 Si7 is the most similar with that of Ni31 Si12 among all the Ti-Ni-Si ternary intermetallics. Thus, the ternary Ti6 Ni16 Si7 was formed by the diffusion of Ti to Ni31 Si12. Based on the observations above, the formation mechanism of the mentioned six distinct layers can be explained as follows. The Ti-Zr-Ni-Cu amorphous filler foils began to melt when brazing temperature exceeded its melting point (i.e. 1123 K). Subsequently, the molten brazing alloy wetted the surfaces of both of the base metals and then partial base metals were dissolved into the molten brazing alloy, enhancing the braze seam width indicated in Fig. 6(a, b). The elements Ti, Ni, and Si dissolved in the molten brazing alloy diffuse easily to opposite side as the strong affinity of Ti for Ni and Si. Meanwhile, the element Zr existing in the initial Ti-Zr-Ni-Cu amorphous diffuses near both sides of the brazing seam. The diffusion of Ni to the brazing seam results in the accumulation of Si at the Ni25at%Si side and then lead to the formation of Zone V. Ti5 Si3 firstly formed by the reaction of Ti(l) + Si(l) → Ti5 Si3 in Zone III and Zone IV as it owns the lowest Gibbs free energy (−564.7 kJ/mol at 1213 K) among all the intermetallics in the joints. Then, L + Ti5 Si3 → (Ti, Zr)2 Si occurs in Zone III due to the relative large amount of element Zr as is depicted by Bulanova et al. (2004). Simultaneously, the diffusion zones, namely Zones I and VI, are formed by the diffusion of the molten brazing alloys to both sides of base metals as shown in Fig. 6(c, d). As a result, the Ti6 Ni16 Si7 in Zone VI precipitates by the diffusion of Ti into Ni31 Si12 . As TiNi3 was an ideal stoichiometry intermetallic, it is impossible to induce the formation of Ti6 Ni16 Si7 through the reaction of Ti(s)+ Ni31 Si12 (s) → Ti6 Ni16 Si7 . Therefore, the Ti6 Ni16 Si7 forms before the precipitation of TiNi3 by the reaction of Ti(l)+ Ni31 Si12 → Ti6 Ni16 Si7 . Based on the Gibbs free energy of TiNi3 and TiNi, the TiNi3 was the preferential formed phase prior to TiNi. Thus, the procedure of reactive phase formation in Zone VI could be schematically depicted as Fig. 6(e–g). During cooling process, the Ti3 Al in front of Ti600 were formed by the transformation of primary -Ti → Ti3 Al indicated by its arcuate shape. The continuous Ti2 Ni was formed by reaction of Ti(L) + Ni(L) → Ti2 Ni. The residual liquid phase transformed to Ti + Ti2 Ni formed the contour of Zone III and Zone IV by the eutectic reaction of L → -Ti + Ti2 Ni, as shown in the phase diagram (Tang et al., 1999). Meanwhile, a eutectic reaction of L → Ti + (Ti,Zr)2 (Ni,Cu) would occurs as reported by Qiu et al. (2016).
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Fig. 5. (a) and (d) are the bright field image from two TEM samples extracted from Zone VI. (b), (c), (e) and (f) are the SAED of point A,B, C and D marked in (a) and (d).
Fig. 6. Schematic of the formation of each reaction layer during the brazing process. (a) and (b) shows the interaction between brazing filler and base metal; (c) and (d) the formation procedure of (Ti,Zr)2 Si; (e), (f) and (g) depict the reaction adjacent to Ni-25at%Si; (h), (i) and (j) shows the final solidification stage of brazing seam.
The formation of Zone II, Zone III and Zone IV could be schematically depicted as Fig. 6(h–j).
4. Conclusions • A 150% increase of the brazing seam width vs. the amorphous filler foil results from the intensive interactions between the base metal and brazing filler occurred during the brazing process.
• The typical interfacial microstructure of Ti600/Ti-Zr-Ni-Cu/Ni25at%Si brazed joint at 1213 K for was mainly comprised continuous Ti2 Ni phase. Most of Ti-rich phase such as Tiss, Ti3 Al, Ti2 Ni, (Ti,Zr)2 (Ni,Cu) and (Ti,Zr)2 Si were formed adjacent to Ti600 alloy except Ti5 Si3 was near Ni-25at%Si. Four diffusion layers including Ni31 Si12 layer, Ti6 Ni16 Si7 layer, TiNi3 layer and TiNi layer were form at Ni-25at%Si side. • The main formation procedure of the brazing seam can be divided into four stages. The Ti5 Si3 and Ni31 Si12 were firstly formed at the
X. Li et al. / Journal of Materials Processing Technology 240 (2017) 414–419
initial stage followed by the diffusion reactions at Ni-25at%Si side. The continuous Ti2 Ni phase was then formed by L → -Ti + Ti2 Ni eutectic reaction. Finally, the other phases, such as ␣-Ti, Ti3 Al, (Ti,Zr)2 (Ni,Cu) and (Ti,Zr)2 Si in the brazing joint were formed by solid-solid phase transformation. Acknowledgement This work was supported by National Natural Science Foundation of China (51405098). References Bulanova, M., Firstov, S., Gornaya, I., Miracle, D., 2004. The melting diagram of the Ti-corner of the Ti–Zr–Si system and mechanical properties of as-cast compositions. J. Alloys Compd. 384, 106–114. Cao, J., Song, X., Li, C., Zhao, L., Feng, J., 2013. Brazing ZrO2 ceramic to Ti–6Al–4 V alloy using NiCrSiB amorphous filler foil: interfacial microstructure and joint properties. Mater. Charact. 81, 85–91. Cao, L., Cochrane, R., Mullis, A., 2014. Lamella structure formation in drop-tube processed Ni-25.3at.% Si alloy. J. Alloys Compd. 615, S599–S601. He, P., Liu, D., 2006. Mechanism of forming interfacial intermetallic compounds at interface for solid state diffusion bonding of dissimilar materials. Mater. Sci. Eng. A 437, 430–435. Lee, C., Lu, M., Liao, K., 2008. Vertically well-aligned epitaxial Ni31Si12 nanowire arrays with excellent field emission properties. Appl. Phys. Lett. 93, 1–3.
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Lee, J., Choi, Y., Lee, J., Lee, G., Lee, M., Rhee, C., 2010. Low-temperature brazing of titanium by the application of a Zr–Ti–Ni–Cu–Be bulk metallic glass (BMG) alloy as a filler. Intermetallics 18, 70–73. Li, S., Zhou, Y., Duan, H., 2002. Wettability and interfacial reaction in SiC/Ni plus Ti system. J. Mater. Sci. 37, 2575–2579. Liu, P., Li, Y., Geng, H., Wang, J., 2005. A study of phase constitution near the interface of Mg/Al vacuum diffusion bonding. Mater. Lett. 59, 2001–2005. Ma, J., Wang, Q., 1998. Aging characterization and application of Ti–15–3 alloy. Mater. Sci. Eng. A 243, 150–154. Niu, Y., Hou, H., Li, M., Li, Z., 2008. High temperature deformation behavior of a near alpha Ti600 titanium alloy. Mater. Sci. Eng. A 492, 24–28. Qiu, Q., Wang, Y., Yang, Z., Hu, X., Wang, D., 2016. Microstructure and mechanical properties of TiAl alloy joints vacuum brazed with Ti–Zr–Ni–Cu brazing powder without and with Mo additive. Mater. Des. 90, 650–659. Tang, W., Sundman, B., Sandström, R., Qiu, C., 1999. New modelling of the B2 phase and its associated martensitic transformation in the Ti–Ni system. Acta Mater. 47, 3457–3468. Tokunaga, T., Hashima, K., Ohtani, H., Hasebe, M., 2004. Thermodynamic analysis of the Ni-Si-Ti system using thermochemical properties determined from Ab Initio calculations. Mater. Trans. 45, 1507–1514. Yang, Z., Zhang, L., Tian, X., Liu, Y., He, P., Feng, J., 2013. Interfacial microstructure and mechanical properties of TiAl and C/SiC joint brazed with TiH 2–Ni–B brazing powder. Mater. Charact. 79, 52–59. Yue, X., He, P., Feng, J., Zhang, J., Zhou, F., 2008. Microstructure and interfacial reactions of vacuum brazing titanium alloy to stainless steel using an AgCuTi filler metal. Mater. Charact. 59, 1721–1727.
Contents lists available at ScienceDirect
Journal of Materials Processing Technology journal homepage: www.elsevier.com/locate/jmatprotec
Microstructural evolution mechanisms of Ti600 and Ni-25%Si joint brazed with Ti-Zr-Ni-Cu amorphous filler foil Xiaopeng Li a , Houqin Wang a , Ting Wang b,∗ , Binggang Zhang a,∗∗ , Tao Yu a , Ruishan Li a a b
State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin 150001, China Harbin Institute of Technology at Weihai, Shandong Provincial Key Laboratory of Special Welding Technology, Weihai 264209, China
a r t i c l e
i n f o
Article history: Received 18 July 2016 Received in revised form 22 October 2016 Accepted 24 October 2016 Available online 26 October 2016 Keywords: Ti600 alloy Ni-25at%Si alloy Ti-Zr-Ni-Cu amorphous filler foil Brazing Interfacial microstructure Evolution mechanism
a b s t r a c t A commercial Ti-Zr-Ni-Cu amorphous filler foil was applied to braze the high-temperature Ti600 and Ni-25at%Si, resulting in the good joint between both of alloys. The interfacial microstructure of Ti600/TiZr-Ni-Cu/Ni-25at%Si brazed joint at 1213 K for 10 min is mainly comprised of the continuous Ti2 Ni phase. Most of Ti-rich phases such as ␣-Ti, -Ti, Ti3 Al, Ti2 Ni, (Ti,Zr)2 (Ni,Cu) and (Ti,Zr)2 Si formed in the vicinity of the Ti600 substrate except that Ti5 Si3 precipitates near the Ni-25at%Si region, resulting in the formation of six distinct layers. The corresponding formation mechanism were clarified. The Ti5 Si3 and Ni31 Si12 were firstly formed during the initial stage followed by diffusion reactions at Ni-25at%Si side. The continuous Ti2 Ni phase formed through the reaction of Ti(L) + Ni(L) → Ti2 Ni. ␣-Ti, Ti3 Al, (Ti,Zr)2 (Ni,Cu) and (Ti,Zr)2 Si in the brazing joint precipitated through a solid-solid phase transformation. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Recently, much attention has been paid to the fabrication and deformation of high-temperature Ti600 titanium alloy with a nominal composition of Ti-6Al-2.8Sn-4Zr-0.5Mo-0.4Si-0.1Y (wt.%) due to its good tensile strength, excellent creep performance and superior fatigue resistance at the servicing temperatures of at least 873 K, which could promote further development of the aviation and aerospace industries in reducing the weight of the spacecraft as described by Niu et al. (2008). However, the poor oxidation resistance of such a high-temperature titanium alloy severally restricts its further application at elevated temperatures. Previous reports have demonstrated the Ni-25at%Si alloy possesses excellent high-temperature oxidation resistance as well as high strength at elevated temperature (Cao et al., 2014). In order to make full use of the advantages of these two materials simultaneously, the reliable joining of Ti600 titanium alloy to the Ni-25%Si alloy could be one of good choices.
∗ Corresponding author at: No. 2, Wenhuaxi Road, Weihai City, Shandong 264209, China. ∗∗ Corresponding author at: No. 92, Xi-Da-Zhi Street, Harbin, Heilongjiang 150001, China. E-mail addresses: [email protected] (T. Wang), [email protected] (B. Zhang). http://dx.doi.org/10.1016/j.jmatprotec.2016.10.021 0924-0136/© 2016 Elsevier B.V. All rights reserved.
However, the Ni-Ti-Si phase diagram (Tokunaga et al., 2004) showed at least 20 kinds of intermetallics compounds could form in the ternary Ni-Ti-Si system. Such complex brittle phase species would lead to a poor weldability of Ti600 titanium alloy and the Ni-25%Si alloy. As suggested by Yue et al. (2008) and Liu et al. (2005), both diffusion bonding and various types of brazing are suitable methods to join dissimilar materials, among which the latter is relatively more effective due to its convenience and costeffectiveness as considered by Cao et al. (2013). It may be feasible to adopt the brazing method to join the Ti600 titanium alloy and Ni-25at%Si alloy. The reaction products of Ni-Ti-Si ternary system varied greatly with the composition and process parameters during the brazing process. The study of Yang et al. (2013) shows that silicide is absent in the joints of SiC and TiAl brazed with TiNi fillers, but the study of Li et al. (2002) shows that it appeared in the joints of SiC and Ni with Ti fillers. The brazing of Ti600 titanium alloy to the Ni-25at%Si alloy has not been well studied yet, especially on the crystalline phase species and the corresponding microstructural features after welding. What’s more, an understanding of the crystal structure, chemistry and morphology of these various types of phase species during brazing is of primary importance as they have been known to affect the mechanical properties of joints. Hence, it is necessary to figure out the phase species and their forming mechanisms. In this work, the commercial Ti-Zr-Ni-Cu amorphous alloys are chosen as the filler metal based on the operating temperature
X. Li et al. / Journal of Materials Processing Technology 240 (2017) 414–419
415
3. Results and discussion
Fig. 1. The typical morphology of the electron beam floating zone melted Ni-25%Si alloys.
(i.e. 800 K) of Ti600 and the phase transformation temperature of Ni-25%Si. Ti600 titanium alloy and Ni-25%Si alloy were brazed using this filler metal at 1213 K for 10 min. The typical interfacial microstructures of the brazed joints were characterized. Furthermore, the corresponding forming mechanisms of different phases were deduced.
2. Experimental procedure The Ni-25at%Si alloy used in this study was obtained by the electron beam floating zone melting method. The fabricated Ni25at%Si alloy was comprised of Niss, Ni3 Si and Ni31 Si12 as shown in Fig. 1. The ductile Niss lamellar was produced in Ni-25%Si alloy to improve its toughness. The raw Ni-25%Si alloy was cut to blocks with the dimension of 5 mm × 5 mm × 5 mm using a wire electrical discharge machining (WEDM). The dimension of Ti600 alloy specimens for the metallographic observation were 10 mm × 20 mm × 2 mm. The thickness of Ti-Zr-Ni-Cu amorphous filler foil (Ti-38.65 Zr-10.1 Ni-15.7 Cu wt.%) was 50 m. Prior to brazing, the surfaces to be brazed were carefully ground and polished. The polished Ni-25at%Si alloy and Ti600 alloy as well as Ti-Zr-Ni-Cu amorphous filler foils were cleaned ultrasonically for 20 min in acetone and dried by air blowing. A single layer of filler foil was placed between Ni-25at%Si alloy and Ti600 alloy to form a sandwich type joint and then the brazing assemblies were placed into the vacuum furnace. At the beginning, the furnace was heated to 1073 K at a heating rate of 20 K/min, then to the 1213 K at a heating rate of 10 K/min. Subsequently, at the brazing temperature, the brazing couples were held for 10 min, then cooled down to 473 K at a cooling rate of 20 K/min. Finally, the joints were spontaneously cooled down to room temperature in the furnace. The interfacial microstructures of brazed joint were characterized employing a scanning electronic microscopy (SEM, Quanta 200FEG). Componential analysis of reaction phases in joints was performed using EDS (EDS, Genesis Apollo X/XL). Furthermore, the brazing seam was investigated by a transmission electron microscopy (TEM, Talos F200x) with selected area electron diffraction (SAED) in order to accurately identify the interfacial phases. Samples for cross-sectional TEM from specific areas in the brazed seam were obtained using a focused ion beam (FIB, Helios Nanolab600i) instrument with a Cross-beamTM configuration. The X-ray diffraction (XRD, D8 ADVANCE) was used to analyze the phases of the fractured surface.
Reliable brazing of Ti600 alloy and Ni-25at%Si alloy was achieved using Ti-Zr-Ni-Cu amorphous filler foil in the present study. Fig. 2(a) shows the typical interfacial microstructures together with corresponding EDS results for the Ti600/Ni-25at%Si joint brazed at 1213 K for 10 min. It is clearly seen that no pores or micro-cracks existed in the brazed joint and the width of the brazing seam was about 200 m. A 150% increase of the brazing seam width vs. the amorphous filler foil implies the intensive interactions, including dissolution, diffusion, and reaction occurs between the two base metals and the molten brazing alloy during brazing process. The EDS results shown in Fig. 2(b–g) confirm the occurrence of the strong dissolutions of Ti600 and Ni-25%Si substrate into the molten filler metal during brazing, resulting in the existence of alloying elements Si, Al, and Ti within the brazing seam. Meanwhile, the diffusions of elements Ni and Cu towards the Ti600 alloy lead to an increase of Ni content in the front of the Ti600 substrate (Fig. 2(e, g)). However, the element Ti is absent within the reaction layer of Ni-25at%Si substrate side (Fig. 2(d)), which illustrates the full reaction of the element Ti in the brazing seam. The element Zr (seen in Fig. 2(f)) mainly distributing in middle part of the brazing seam suggests its low diffusion rate and the original location of Ti-Zr-Ni-Cu amorphous filler foils. It is also can be deduced the strong dissolution of the two base metals. More detailed images of the various regions of the brazed seam in backscattered electron mode were shown in Fig. 3. The whole interface presents a multilayered structure. Based on the difference in microscopic morphology, the joints was divided into six zones (marked as Zones I, II, III, IV, V and VI). Fig. 3(b–d) shows the high-magnification backscattered electron images (BSE images) of these six characteristic zones respectively, and major elements at each spot in Fig. 3 detected by EDS are listed in Table 1. Fig. 3(b) shows that a diffusion layer and a reaction layer was formed near the Ti600 side. Intensive interactions including dissolution, diffusion and reaction simultaneously occurred at the interface during brazing, which results in the formation of a multi-phase zone. Particularly, the research of Lee et al. (2010) indicated that the diffusion of -Ti stabilizers, such as Ni and Cu atoms, towards Ti600 substrate promoted the phase transformation of ␣-Ti to -Ti. So a continuous diffusion zone (Zone I) was formed containing -Ti phase, as shown in Fig. 3(b). According to the elemental contents in Table 1 and Ti-Ni binary phase diagram, the continuous dark gray region in Zone II is Ti2 Ni intermetallic phase. The following shear tests of the specimens obtained by 1213 K/10 min show that the average shear strength is 43.5 MPa. The XRD of the typical fractured surface in Fig. 4 indicates that Ti2 Ni phase dominates most of the fractured surfaces. Thus, the continuous Ti2 Ni should be avoided. Further observation shows that some black regions and light gray regions with eutectic reaction characterization distributed around the Ti2 Ni phase. The EDS results of the black regions revealed the similar composition of this part with Zone I except slightly increase of Cu. Thus, the black regions in Zone II are mainly composed of ␣-Ti. What’s more, the light gray regions (Point E) around black regions are proved to be (Ti, Zr)2 (Cu, Ni) as the molar ratio of Ti+ Zr and Cu + Ni is approximately 2:1. The analysis of the light gray regions (Point B) located in front of Zone I shows that only three elements, namely Ti (71.84%), Al (15.2%) and Sn (8.5%), accumulated in these blocks. Since the appropriate Al composition can be calculated by Al-equivalent =CAl + 1/3CSn as depicted by Ma and Wang (1998), the actual composition of Al is 21.9 at%, which indicating these blocks are Ti3 Al. It can be seen in Fig. 3(c) that three different regions of different contrast exist in Zone III. The dark gray region has been proved to be Ti2 Ni above. The composition analysis of the white phase marked as Point F indicating a compound comprised of 30.10% Ti, 29.41%
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Fig. 2. Interfacial microstructure and elemental distribution of Ti600 and Ni-25at%Si joint brazed at 1213 K for 10 min using Ti-Zr-Ni-Cu amorphous filler foil. (a) BSE image of the joint and EDS maps of (b) Al, (c) Si, (d) Ti, (e) Ni, (f) Zr and (g) Cu.
Fig. 3. (a) Typical morphology of Ti600 and Ni-25at%Si joint brazed at 1213 K for 10 min using Ti-Zr-Ni-Cu amorphous filler foil and the high magnification BSE images of (b) Zone I and II, (c) Zone III and (d) Zone IV, Zone V and Zone VI.
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Table 1 Chemical compositions and possible phases of each spot marked in Fig. 3 (at.%). Spot
Al
Si
Zr
Sn
Ti
Ni
Cu
Possible phase
A B C D E F G H I J
11.00 15.21 5.06 6.71 9.74 2.26 14.45 1.64 0.63 10.98
1.58 1.35 0.70 1.08 0.69 29.41 1.34 34.99 27.16 2.06
1.84 0.99 2.52 7.58 2.75 27.79 1.44 3.09 0.07 1.85
1.57 8.53 0.22 0.72 1.48 1.89 1.48 0.33 0.29 1.95
76.38 71.87 56.65 55.84 72.91 30.10 73.37 56.98 0.31 72.51
6.55 1.42 31.46 18.11 6.49 5.73 4.19 2.41 69.68 4.88
1.07 0.63 3.39 9.96 5.94 2.82 3.73 0.55 1.85 5.78
␣-Ti + ˇ-Ti Ti3 Al Ti2 Ni (Ti,Zr)2 (Cu,Ni) ␣-Ti + ˇ-Ti (Ti, Zr)2 Si ␣-Ti + ˇ-Ti Ti5 Si3 Ni31 Si12 ␣-Ti + ˇ-Ti
Fig. 4. The XRD of the typical fractured surface of the joint obtained by 1213 K/10 min.
Zr and 27.79% Si was obtained. According to the Ti-Zr-Si ternary phase diagram (Bulanova et al., 2004), the white phase is (Ti, Zr)2 Si. Besides, the black phase marked as Point G was proved as ␣-Ti since it owns the same composition and morphology with Phase E. Fig. 3d displays the three reaction zone, namely Zone IV, Zone V and Zone VI, at the Ni-25at%Si substrate side. According to the contrast of different phase, it seems that the Zone IV mainly consist of continuous Ti2 Ni and ␣-Ti + ˇ-Ti (Phase J). Besides, some black phases marked as Point H also can be found in Zone IV. Further EDS analysis confirmed that the black phase is Ti5 Si3 . As shown in Fig. 3d, the Ti5 Si3 phase can be divided into two categories based on its distribution. Some of the Ti5 Si3 particle dispersed in Ti2 Ni phase and others gathered around the Ti2 Ni phase. The single phase in Zone V was confirmed as Ni31 Si12 according to EDS analysis and Ni-Si phase diagram. It is worth noting that the concentration of Si in Zone V increased by 2.16% compared to the composition of base metal. The following reason was responsible for the phenomenon. As mentioned by Lee et al. (2008), Ni atoms are known to be the dominant diffusing species in the Ni-Si system. The Ni-Ni metal bonds existing in the base metal are more easily to break than the Ni-Si covalent bonds, which will result in the preferentially diffusing of Ni atoms. Therefore, the element Si was much more reluctant to diffuse and accumulated in Zone V. A thin layer (Zone VI) with different contrast was formed between Zone IV and Zone V. Fig. 5 shows the bright field image and selected area electron diffraction patterns from two TEM samples extracted from Zone VI of brazing seam. The TEM sample for Fig. 5(a) was extracted adjacent to Zone IV and the TEM sample for Fig. 5d was extracted adjacent to Zone V. According to the results of TEM and SAED, the continuous light gray phase in Zone VI is TiNi (Fig. 5(b)) and there were some tiny Ti5 Si3 (Fig. 5(c)) particles dispersed at the grain boundary, which appeared solid reaction
character. The white line next to TiNi in Fig. 5(d) was proved as TiNi3 by SAED (Fig. 5(e)). Further observation of Fig. 3(d) displayed that the TiNi layer and TiNi3 layer show diffusion-controlled morphology. As analyzed above, Ni diffused easily to the brazing seam due to the strong affinity of Ti for Ni, which would lead to the formation of Ni-rich Ni-Ti intermetallics. According to the research of He and Liu (2006), the TiNi3 and TiNi are the first two preferential formed phase at Ni-rich side during diffusion bonding process as the Gibbs free energy of these two phases are −34.626 kJ and −30.791 kJ respectively. Interestingly, a Ti-Ni-Si ternary intermetallic was formed between Ti3 Ni and Ni31 Si12 and it was identified as Ti6 Ni16 Si7 by the SAED in Fig. 5(e). The stoichiometry of Ti6 Ni16 Si7 is the most similar with that of Ni31 Si12 among all the Ti-Ni-Si ternary intermetallics. Thus, the ternary Ti6 Ni16 Si7 was formed by the diffusion of Ti to Ni31 Si12. Based on the observations above, the formation mechanism of the mentioned six distinct layers can be explained as follows. The Ti-Zr-Ni-Cu amorphous filler foils began to melt when brazing temperature exceeded its melting point (i.e. 1123 K). Subsequently, the molten brazing alloy wetted the surfaces of both of the base metals and then partial base metals were dissolved into the molten brazing alloy, enhancing the braze seam width indicated in Fig. 6(a, b). The elements Ti, Ni, and Si dissolved in the molten brazing alloy diffuse easily to opposite side as the strong affinity of Ti for Ni and Si. Meanwhile, the element Zr existing in the initial Ti-Zr-Ni-Cu amorphous diffuses near both sides of the brazing seam. The diffusion of Ni to the brazing seam results in the accumulation of Si at the Ni25at%Si side and then lead to the formation of Zone V. Ti5 Si3 firstly formed by the reaction of Ti(l) + Si(l) → Ti5 Si3 in Zone III and Zone IV as it owns the lowest Gibbs free energy (−564.7 kJ/mol at 1213 K) among all the intermetallics in the joints. Then, L + Ti5 Si3 → (Ti, Zr)2 Si occurs in Zone III due to the relative large amount of element Zr as is depicted by Bulanova et al. (2004). Simultaneously, the diffusion zones, namely Zones I and VI, are formed by the diffusion of the molten brazing alloys to both sides of base metals as shown in Fig. 6(c, d). As a result, the Ti6 Ni16 Si7 in Zone VI precipitates by the diffusion of Ti into Ni31 Si12 . As TiNi3 was an ideal stoichiometry intermetallic, it is impossible to induce the formation of Ti6 Ni16 Si7 through the reaction of Ti(s)+ Ni31 Si12 (s) → Ti6 Ni16 Si7 . Therefore, the Ti6 Ni16 Si7 forms before the precipitation of TiNi3 by the reaction of Ti(l)+ Ni31 Si12 → Ti6 Ni16 Si7 . Based on the Gibbs free energy of TiNi3 and TiNi, the TiNi3 was the preferential formed phase prior to TiNi. Thus, the procedure of reactive phase formation in Zone VI could be schematically depicted as Fig. 6(e–g). During cooling process, the Ti3 Al in front of Ti600 were formed by the transformation of primary -Ti → Ti3 Al indicated by its arcuate shape. The continuous Ti2 Ni was formed by reaction of Ti(L) + Ni(L) → Ti2 Ni. The residual liquid phase transformed to Ti + Ti2 Ni formed the contour of Zone III and Zone IV by the eutectic reaction of L → -Ti + Ti2 Ni, as shown in the phase diagram (Tang et al., 1999). Meanwhile, a eutectic reaction of L → Ti + (Ti,Zr)2 (Ni,Cu) would occurs as reported by Qiu et al. (2016).
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Fig. 5. (a) and (d) are the bright field image from two TEM samples extracted from Zone VI. (b), (c), (e) and (f) are the SAED of point A,B, C and D marked in (a) and (d).
Fig. 6. Schematic of the formation of each reaction layer during the brazing process. (a) and (b) shows the interaction between brazing filler and base metal; (c) and (d) the formation procedure of (Ti,Zr)2 Si; (e), (f) and (g) depict the reaction adjacent to Ni-25at%Si; (h), (i) and (j) shows the final solidification stage of brazing seam.
The formation of Zone II, Zone III and Zone IV could be schematically depicted as Fig. 6(h–j).
4. Conclusions • A 150% increase of the brazing seam width vs. the amorphous filler foil results from the intensive interactions between the base metal and brazing filler occurred during the brazing process.
• The typical interfacial microstructure of Ti600/Ti-Zr-Ni-Cu/Ni25at%Si brazed joint at 1213 K for was mainly comprised continuous Ti2 Ni phase. Most of Ti-rich phase such as Tiss, Ti3 Al, Ti2 Ni, (Ti,Zr)2 (Ni,Cu) and (Ti,Zr)2 Si were formed adjacent to Ti600 alloy except Ti5 Si3 was near Ni-25at%Si. Four diffusion layers including Ni31 Si12 layer, Ti6 Ni16 Si7 layer, TiNi3 layer and TiNi layer were form at Ni-25at%Si side. • The main formation procedure of the brazing seam can be divided into four stages. The Ti5 Si3 and Ni31 Si12 were firstly formed at the
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initial stage followed by the diffusion reactions at Ni-25at%Si side. The continuous Ti2 Ni phase was then formed by L → -Ti + Ti2 Ni eutectic reaction. Finally, the other phases, such as ␣-Ti, Ti3 Al, (Ti,Zr)2 (Ni,Cu) and (Ti,Zr)2 Si in the brazing joint were formed by solid-solid phase transformation. Acknowledgement This work was supported by National Natural Science Foundation of China (51405098). References Bulanova, M., Firstov, S., Gornaya, I., Miracle, D., 2004. The melting diagram of the Ti-corner of the Ti–Zr–Si system and mechanical properties of as-cast compositions. J. Alloys Compd. 384, 106–114. Cao, J., Song, X., Li, C., Zhao, L., Feng, J., 2013. Brazing ZrO2 ceramic to Ti–6Al–4 V alloy using NiCrSiB amorphous filler foil: interfacial microstructure and joint properties. Mater. Charact. 81, 85–91. Cao, L., Cochrane, R., Mullis, A., 2014. Lamella structure formation in drop-tube processed Ni-25.3at.% Si alloy. J. Alloys Compd. 615, S599–S601. He, P., Liu, D., 2006. Mechanism of forming interfacial intermetallic compounds at interface for solid state diffusion bonding of dissimilar materials. Mater. Sci. Eng. A 437, 430–435. Lee, C., Lu, M., Liao, K., 2008. Vertically well-aligned epitaxial Ni31Si12 nanowire arrays with excellent field emission properties. Appl. Phys. Lett. 93, 1–3.
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