# 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
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Spots** & **Ti** & **Al** & **Cu** & **Zr** & **Ni** & **Nb** & **Cr** & **Possible Phase** \\ \hline A & 60.05 & 31.21 & 0.50 & 0.44 & 0.03 & 1.51 & 3.27 & Ti\({}_{2}\)Al \\ B & 28.39 & 30.38 & 28.41 & 6.54 & 6.44 & 1.47 & 3.58 & AlTiCu \\ C & 76.94 & 6.83 & 5.00 & 5.11 & 3.49 & 0.56 & 3.08 & \(\alpha\)-Ti \\ D & 40.03 & 15.90 & 24.06 & 14.41 & 4.59 & 1.63 & 4.49 & (Ti\({}_{2}\)Zr)\({}_{2}\)(Zr,Ni) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Chemical composition and possible phase of each spot marked in Figure 3 (at. %). Figure 4: (**a**) SEM image of the joint interface brazed at 1213 K for 600 s; (**b**) magnification image of zone I; (**c**) magnification image of zone II. Figure 5: X-ray diffraction (XRD) pattern of fractured surface of brazed joint. From the above experimental results, we propose a possible formation model to explain the microstructure evolution of the joint, as shown in Figure 7. During brazing, when the temperature was brought to the melting point of a Cu-based amorphous alloy filler, the filler began to melt, and wetted the surface of Ti-48 Al-2 Nb-2 Cr base materials. The base materials were partially dissolved into the molten alloy. The Ti and Al diffused into the molten filler due to the concentration gradient. At the same time, the elements in the molten filler, especially the Cu, diffused into the Ti-48 Al-2 Nb-2 Cr base materials, as shown in Figure 7a. During the heat process, the Ti and Al contents of the \(\gamma\)-TiAl and \(\alpha_{2}\)-Ti\({}_{3}\)Al phases in the Ti-48 Al-2 Nb-2 Cr base materials gradually decreased, and eventually fell below the critical composition required for their existence [40]. As a consequence, the \(\gamma\)-TiAl and \(\alpha_{2}\)-Ti\({}_{3}\)Al phases transformed into the Ti\({}_{2}\)Al phase through chemical reactions of TiAl \(\rightarrow\) Al + Ti\({}_{2}\)Al and Ti\({}_{3}\)Al \(\rightarrow\) Ti + Ti\({}_{2}\)Al, respectively. Meanwhile, an eutectic reaction occurred based on Cu-Ni-Ti (Zr) ternary phase diagram \(L\rightarrow\beta\)-Ti + (Ti,Zr)\({}_{2}\)(Cu, Ni), as shown in Figure 7b. With the further interdiffusion at the interface, the AlCuTi phase appeared according to the Al-Cu-Ti ternary alloy phase diagram. When the brazing temperature was further increased, the thickness of the interfacial reaction layers containing AlCuTi and Ti\({}_{2}\)Al increased. In addition, the (Ti,Zr)\({}_{2}\)(Cu, Ni) phase grew and aggregated, as shown in Figure 7c. At the cooling stage, when the brazing temperature is lower than the \(\beta\rightarrow\alpha\) transformation temperature, the eutectoid decomposition of \(\beta\)-Ti to \(\alpha\)-Ti and (Ti, Zr)\({}_{2}\)(Cu, Ni) phases happens: \(\beta\)-Ti \(\rightarrow\alpha\)-Ti + (Ti, Zr)\({}_{2}\)(Cu, Ni). Therefore, the central reaction layer was finally formed with \(\alpha\)-Ti and (Ti, Zr)\({}_{2}\)(Cu, Ni), as shown in Figure 7d. Figure 6: Interfacial microstructures of the joints brazed for 600 s at brazing temperature of **(a)** 1183 K; **(b)** 1213 K; **(c)** 1243 K; and **(d)** 1273 K.