# Brazing Ti-48 Al-2 Nb-2 Cr Alloys with Cu-Based Amorphous Alloy Filler
## 3 Results and Discussion
### Interfacial Microstructure of the Brazed Joint
observed the obvious AlCuTi layer through SEM at the interface between BAg-8 and TiAl substrate for the specimen brazed at 1223 K [34]. Spot C in Figure 4c was mainly composed of 76.94 at. % Ti and 6.83 at. % Al. It has been reported that Al is \(\alpha\)-Ti stabilizer, while Cu and Ni are \(\beta\)-Ti stabilizer elements [36]. Therefore, spot C was inferred to be \(\alpha\)-Ti. Spot D in zone II was mainly composed of Zr, Ti, Cu, and Ni. The atom ratio of Cu + Ni and Ti + Zr was approximately 1:2. Compared with the original contents of Cu in an amorphous alloy, the decrease of Cu contents mainly caused by the consumption of copper from molten brae during brazing results in the formation of AlCuTi, as aforementioned. Both Cu and Ni are completely miscible with each other, as well as Ti and Zr. Cu and Ni have a strong affinity to Ti (Zr), and they are apt to form (Ti,Zr)\({}_{2}\)(Cu,Ni). In fact, both element pairs of Ti and Zr, and Cu and Ni, are not only chemically compatible with each other, they are also fully soluble to each other. Therefore, Zr and Ni can be regarded namely as Ti and Cu in the intermetallics herein. Thus, the Zr\({}_{2}\)Cu and Zr\({}_{2}\)Ni can be regarded as Ti\({}_{2}\)Cu and Ti\({}_{2}\)Ni. Therefore, the four phases can be collectively referred to as (Ti, Zr)\({}_{2}\)(Cu, Ni). So, spot D can be considered as having a phase formula of (Ti, Zr)\({}_{2}\)(Cu, Ni). The formation of (Ti, Zr)\({}_{2}\)(Cu, Ni) phase was also demonstrated in the previous work when amorphous filler metal was used [37; 38]. Figure 5 shows the XRD patterns of the fractured surface of the sample brazed at 1213 K for 600 s. However, AlCuTi has not been detected by XRD due to the amount being too small for detection by the equipment. Based on the micro-XRD results, the chemical composition in Table 2, and the phase diagrams, phases A, B, C, and D can be recognized as Ti\({}_{2}\)Al, AlCuTi, \(\alpha\)-Ti, and (Ti, Zr)\({}_{2}\)(Cu, Ni). Figure 3: Interfacial microstructure and elemental distribution of Ti-48 Al-2 Nb-2 Cr alloy joint brazed at 1213 K for 600 s. (**a**) SEM image and (**b**) Ti, (**c**) Cu, (**d**) Zr, (**e**) Ni, (**f**) Al, (**g**) Nb, (**h**) Cr. Figure 6 shows the SEM images of the brazed joint at different brazing temperatures for 600 s. Obviously, two characteristic zones can be seen in all of the brazed joints, and a continuous lamellar interface reaction layers forms in the interface, which is the same as in Figure 4a. The width of the whole joint remains unchanged with the brazing temperature. However, the width of the interfacial reaction layer zone I increased with the brazing temperature. Correspondingly, the width of the central brazed layer zone II decreased. For 1123 K, the width of interfacial reaction layer was about 5 \(\upmu\)m. When the brazing temperature reached 1273 K, the width of the interfacial reaction layer increased to 15 \(\upmu\)m. Also, the amount of the black phase in the interfacial reaction layer defined as the Ti\({}_{2}\)Al phase increased. However, compared with the Ti\({}_{2}\)Al phase, the amount of gray phase AlCuTi decreased in the interfacial reaction layer. It is reported that once the continuous TiAl intermetallic compounds form in the joint interface, further atomic interdiffusion between the base material and filler metal will be obstructed due to the limited solubility of Cu and Ni in the TiAl alloy [39]. Hence, the existence of Ti\({}_{2}\)Al obstructs the growth of AlCuTi in the interfacial reaction layer. The block-like \(\alpha\)-Ti grows and accumulates with increasing brazing temperature.