# 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  
Figure 3 shows the interfacial microstructures and corresponding EDS results for the joint brazed at 1213 K for 600 s. Clearly, the TiAl substrates are tightly bonded by the Cu-based amorphous filler. No cracks or pores are found in the joint. Obvious reaction layers are detected in the interface, as shown in Figure 3a, which can be attributed to the interdiffusion and nucleation of new phases at the brazing temperature or during cooling. From Figure 3b-e, the main elements of Ti, Cu, Zr, and Ni are widely distributed in the whole joint. Moreover, Ti is aggregated on the location of the blocky black phase with the absence of Cu and Zr. It can be seen that the Al mainly distributes in the Ti-48 Al-2 Nb-2 Cr side of the seam, as shown in Figure 3f. From Figure 3g,h, the existence of Nb and Cr in the brazing beam is attributed to the dissolution and diffusion of elements from the Ti-48 Al-2 Nb-2 Cr alloy into the filler metal during brazing. Figure 4a shows detailed microstructures of the brazed joint at 1123 K for 600 s. The width of the brazed joint is about 90 \(\upmu\)m, which is thicker than the amorphous filler. Two kinds of reaction zones were identified in the brazed joint, which are marked as zone I and II, respectively. Figure 4b,c demonstrates the high magnification images of these two characteristic zones. Based on the contrast differences, zone I mainly consists of two phases marked by A and B. Some B\({}_{2}\) phase can be observed in the TiAl alloy side. Similarly, zone II is also a two-phase mixed region, as marked by C and D. Table 1 lists the chemical composition of the brazed joints through EDS. From Figure 4b, a lamellar structure approximately 10-\(\upmu\)m thick composed of alternate black and grey strips are detected in the brazed joint adjacent to the TiAl base material. According to the EDS results, the black phase (spot A) is mainly composed of Ti and Al at an approximate ratio of 2:1 with few Zr, Cu, Ni, Nb, and Cr. Based on the composition of the selected spots (marked as A-D in Figure 4b,c) and previous studies [31], spot A is inferred as the Ti\({}_{2}\)Al. It is reported that Ti\({}_{2}\)Al was an intermediate phase during the phase transformation between the Ti\({}_{3}\)Al and TiAl phases, and it was hexagonal with lattice parameters a = 0.29 nm and c = 1.39 nm [32; 33]. Furthermore, [PERSON] et al. demonstrated that there was a reaction of Ti + TiAl \(\rightarrow\) Ti\({}_{2}\)Al during hot press sintering [34]. Similarly, the gray phase (spot B) is mainly composed of Al, Cu, and Ti at a ratio of 1:1:1. Based on the composition, spot B is inferred to be AlCuTi. As [PERSON] reported, this kind of intermetallic was common in the reactions between TiAl alloy, Cu, and \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Position** & **Ti** & **Al** & **Nb** & **Cr** & **Possible Phase** \\ \hline
1 & 49.96 & 44.83 & 1.70 & 3.52 & \(\gamma\) \\
2 & 48.12 & 47.60 & 1.06 & 3.22 & \(\gamma+\alpha_{2}\) \\
3 & 54.11 & 33.09 & 1.21 & 11.59 & B\({}_{2}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Chemical composition and possible phase of each spot marked in Figure 1 (at. %). Figure 2: **(a)** Scanning electron microscopy (SEM) image and (b) X-ray diffraction (XRD) pattern of Ti-48 Al-2 Nb-2 Cr base material. Ni in the brazing process [35]. According to the Al-Cu-Ti ternary alloy phase diagram [14], AlCuTi and TiAl share a boundary curve, indicating that AlCuTi is likely to be directly adjacent to the TiAl phase of the base material, which is consistent with our experimental observation, as shown in Figure 4b. Thus, spot B is identified as AlCuTi corresponding to the Al-Cu-Ti ternary alloy phase diagram. Also, the formation of a AlCuTi intermetallic compound in brazed joints has been reported in previous works. [PERSON] et al. reported the presence of a AlCuTi phase in transparent alumina ceramic and TiAl alloy with AgCuTi filler metal [14]. [PERSON] et al.