*3.1. Typical Interfacial Microstructure of the Titanium*/*Au*/*Al2O3 Joint*

Vacuum brazing of titanium alloy and Al2O3 ceramic was achieved using Au filler foil. Figure 2 shows the typical interfacial microstructure and the main element distribution of the titanium/Au/Al2O3 joint brazed at 1115 ◦C for 1 min. As shown in Figure 2a, according to the different contrasts of each phase by SEM in the backscattered electron mode, the joint could be classified into four continuous reaction zones (zone I to IV), and zone V adjacent to the Al2O3 substrate consisted of a white phase dispersed with some light grey granular phases.

Figure 2a also shows variations of elemental concentration of Ti, Au, Al, and O along the white dashed line. The concentration profile of element Ti showed a stepwise decrease from titanium substrate to Al2O3 ceramic, with a noticeable enrichment on the metal/Al2O3 interface. Meanwhile, the main distribution of element Au in the seam exhibited a stepwise increase from titanium to Al2O3 ceramic, and it displayed a minute amount in titanium substrate. The elements of Al and O were mainly distributed in Al2O3 ceramic. These results were consistent with the corresponding elemental distribution in the typical joint displayed in Figure 2b–e.

The interdiffusion of Ti and Au indicated that the active element Ti diffused from the titanium substrate and spread in the brazing seam, and it eventually accumulated adjacent to the surface of Al2O3 ceramic. Meanwhile, Au melted and diffused into the titanium substrate during the brazing process.

**Figure 2.** Microstructure and corresponding element distribution of the titanium/Au/Al2O3 joint brazed at 1115 ◦C for 1 min. (**a**) Microstructure and elemental distribution maps of (**b**) Ti, (**c**) Au, (**d**) Al, and (**e**) O.

In order to investigate microstructure characteristics of the titanium/Au/Al2O3 joint in detail, Figure 3 shows the microstructures of zones I–V under high magnification. EDS data showing elemental concentrations and possible phases at each spot are listed in Table 1. The characteristic areas of I to VI adjacent to titanium are shown in Figure 3a under high magnification. The characteristic areas of VI and V next to Al2O3 ceramic are displayed in Figure 3b under high magnification. According to the EDS results shown in Table 1 and the Ti–Au binary phase diagram illustrated in Figure 4 [41], the reaction layers that formed from the titanium substrate to Al2O3 substrate were a Ti3Au phase (Spot A), a TiAu phase (Spot B), a TiAu2 phase (Spot C), a TiAu4 phase (Spot D), and an Au phase (Spot E) containing TiAu4 particles, respectively.

From the above analysis, it was proposed that during brazing, active element Ti spread and accumulated on the metal/Al2O3 ceramic interface, which could be deduced from the reaction with Al2O3 and the formation of TiOx [25,30,31,42,43]. However, TiOx was hard to observe with SEM and EDS owing to its limited thickness. Apart from reacting with Al2O3, the dissolved Ti in the brazing seam also reacted with molten Au, forming Ti–Au intermetallic compounds (IMCs).

To sum up, the typical interfacial microstructure of the titanium/Au/Al2O3 joint brazed at 1115 ◦C for 1 min consisted of titanium/Ti3Au layer/TiAu layer/TiAu2 layer/TiAu4 layer/Au + granular TiAu4 layer/TiOx phase/Al2O3 ceramic.

**Figure 3.** Microstructure of the titanium/Au/Al2O3 joint at a high magnification: (**a**) the titanium/brazing seam interface; (**b**) the brazing seam/Al2O3 interface.


**Table 1.** Energy dispersive spectroscopy (EDS) results of the spots marked in Figure 3 (at. %).

In order to illuminate the formation mechanism of the typical interfacial microstructure and different Ti–Au IMCs in the titanium/Au/Al2O3 joint, the Ti–Au binary system was studied using the phase diagram shown in Figure 4 [41]. The complex interfacial microstructural morphology and arrangement of various intermetallic compounds (IMCs) generated during the brazing process were strongly dependent on the brazing temperature. The brazing process of titanium to Al2O3 ceramic can be deduced as follows. According to the Ti–Au binary phase diagram (Figure 4), it was observed that element Au melted to the liquid phase when the temperature exceeded 1064 ◦C. The active element Ti diffused from titanium substrate and dissolved into liquid Au gradually. As shown in Figure 4, marked by the red line at 1115 ◦C, with an increasing Ti concentration in the liquid phase, Ti began to react with molten Au to form TiAu4 IMC by the peritectic reaction of Au(L) + Ti → TiAu4. Thus, a continuous TiAu4 layer formed in the brazing seam and inhibited the interdiffusion of Ti and Au. Because of the decreasing concentration gradient of Ti from titanium to the TiAu4 layer, the Ti3Au, TiAu, and TiAu2 layers simultaneously formed between titanium and the TiAu4 layer. During the cooling process, TiAu4 particles and the Au phase directly precipitated in the remnant liquid phase by the eutectic reaction of L → Au + TiAu4 adjacent to the ceramic substrate. Finally, the typical microstructure of titanium/Au/Al2O3 joint with five reaction zones was obtained in the brazing seam, as illustrated in Figure 3.

**Figure 4.** Ti–Au binary phase diagram.

*3.2. E*ff*ects of Processing Parameters on the Microstructure of the Titanium*/*Au*/*Al2O3 Joint*

Figure 5 displays the microstructure evolution of the joints brazed at different temperatures for 1 min. Brazing temperatures had a significant effect on the interfacial microstructure, and the thicknesses of reaction layers were measured and illustrated in Figure 5f. With increasing temperature, the thicknesses of Ti3Au + TiAu + TiAu2 layers (zone I–III) adjacent to the titanium substrate increased gradually (Figure 5a–e). The thickness of the TiAu2 layer (zone III) increased first and then decreased, and the maximum thickness of 17.4 μm was obtained under 1115 ◦C. Meanwhile, as the brazing

temperature increased, the thickness of the Au layer with granular TiAu4 (zone V) next to Al2O3 ceramic notably decreased.

**Figure 5.** Microstructure of the titanium/Au/Al2O3 joint brazed at different brazing temperature for 1 min: (**a**) 1105 ◦C, (**b**) 1110 ◦C, (**c**) 1115 ◦C, (**d**) 1120 ◦C, (**e**) 1125 ◦C, and (**f**) the thicknesses of Ti-Au layers (zone I–III).

Figure 6 shows the microstructure evolution of the joints brazed at 1115 ◦C for different holding times. The microstructure of the joints changed significantly with the prolongation of holding time, and the thickness of the reaction layers were measured and illustrated in Figure 6d. As shown in Figure 6a–c, with the prolongation of holding time from 1 to 5 min, the thicknesses of Ti3Au + TiAu + TiAu2 layers (zone I–III) increased. The thickness of the TiAu2 layer (zone III) increased first and then decreased, and the maximum thickness was obtained for a holding time of 3 min. Meanwhile, as the holding time increased, the thickness of the Au layer with granular TiAu4 (zone V) next to Al2O3 ceramic did not change significantly.

**Figure 6.** Microstructure of the titanium/Au/Al2O3 joint brazed at 1115 ◦C for different holding times: (**a**) 1 min, (**b**) 3 min, (**c**) 5 min, and (**d**) the thicknesses of Ti-Au layers (zone I-III).

Based on the above analyses, brazing temperature and holding time, which affected the dissolution of Ti from titanium substrate, had significant effects on the microstructure evolution of the joints.

A conceptual model was established and illustrated in Figure 7 to show the evolution of the microstructure. The reaction process could be classified into three stages. As shown in Figure 7a, during the brazing process when the temperature was above the melting point of Au, Au foil first converted into liquid. Then, Ti dissolved into molten Au under the driving force of the concentration gradient, and it reacted with Au to form the TiAu4 layer between the Ti substrate and Au (Figure 7b). Finally, the Ti3Au, TiAu, and TiAu2 layers simultaneously formed between titanium and the TiAu4 layer along the concentration gradient of Ti. During the cooling process, TiAu4 particles and the Au phase directly precipitated because the residual element Ti was present in the remnant liquid phase adjacent to the ceramic substrate (Figure 7c). When brazing temperature or holding time increased, the mutual diffusion of Ti and Au became more sufficient. As a result, the thicknesses of Ti3Au + TiAu + TiAu2 layers increased gradually, especially the TiAu2 layer. Meanwhile, the Au phase containing TiAu4 particles reduced, as shown in Figure 7d. With the further increase of brazing temperature or holding time, the diffusion of Au was adequate, and the mount of Ti was sufficient. Ti3Au and TiAu layers increased, resulting in the decreased thickness of the TiAu2 layer. It was notable that the TiAu4 layer almost occupied the brazing seam next to the ceramic (Figure 7e).

It has been widely reported that TiOx could be generated on the interface of Ti containing metal and Al2O3 [25,30,32,35,38–40]. The limited thickness of TiOx and many other compounds in metal–ceramic interfaces led to a decreased accuracy in the identification of titanium oxides [20,44]. As brazing temperature or holding time rose, more TiOx phases formed adjacent to Al2O3 (Figure 7c–e).

**Figure 7.** Schematic of the microstructure evolution for the titanium/Au/Al2O3 joint. (**a**) Mutual diffusion of Ti and Au; (**b**) formation of the TiAu4 layer; (**c**) formation of reaction layers of the joint; and (**d**) and (**e**) growth of reaction layers with the increase of brazing temperature or holding time.
