## 3 Results and Discussion
[31] reported that the typical precipitates such as Ti-CON in the V-4 Cr-4 Ti alloy could be decomposed at the forging temperature. Therefore, it is expected that the solid-solution hardening agents, C, O, and N, might be transferred from the V-4 Cr-4 Ti alloy to the Ti substrate during the forging process due to their chemical affinity. During the cooling after forging, the Ti-CON substance should be re-produced. However, its amount in part E should be less than the original one because of the loss of the interstitial impurities. Therefore, the region with less precipitate exhibits less hardness than the V-alloy substrate. In addition, the average hardness of Ti and V-alloy substrates were 190 HV and 258 HV, respectively. The bonding strength of the V-4 Cr-4 Ti/Ti joint was studied using the push-out method [32] and the schematic diagram is shown in Figure 5a. The bonding strength (\(\tau\)) of the joint can be approximately calculated using the formula below. \[\tau=\frac{F}{\pi Dh}, \tag{1}\]  
where \(F\) is the applied maximum load, \(D\) is the diameter of the central V-4 Cr-4 Ti rod, and \(h\) is the disc thickness. Figure 3: Concentration profiles of Ti, V, and Cr across the chemically etched V–4 Cr–4 Ti/Ti joint. Two types of failure of the joint were observed during the shear tests: type one occurred around the joint (e.g., specimen 1# in Figure 5b) and type two occurred in the central V-4 Cr-4 Ti alloy (e.g., specimen 2# in Figure 5b). For the first type, the failure appears at a narrow region between the Ti substrate and the joint, which was confirmed by SEM (see Figure 6). This is probably ascribed to the stress concentration produced by phase transformation of Ti during the cooling process. When Ti metal is cooled from a high forging temperature (950 \({}^{\circ}\)C or higher) to room temperature, it transforms from the bcc structure (\(\beta\)-Ti) to the hcp structure (\(\alpha\)-Ti) [33]. The dilational stress produced at the interface results in the extreme hardness. In addition, since the push-out test is terminated once the specimen has partly failed during the test, the bonding strength of the joint should be higher than the calculated value of 165 MPa (see Figure 5b). The fracture surface on the side of the V-4 Cr-4 Ti alloy was analyzed by using SEM and EDS. A relatively smooth fracture surface morphology was observed at low magnification, which is shown in Figure 6a-1,b-1. At higher magnification, the fracture surface had a plastic deformation during the shear punch test according to the surface microstructure, which is shown in Figure 6a-2,b-2. EDS analysis revealed that the Ti concentration of the typical fracture surfaces on the V-4 Cr-4 Ti alloy side was nearly 100% (see Figure 6a-3,b-3), which indicates the failure of the joint took place in a narrow region in the joint close to the Ti substrate. It was observed that the phase transformation of the Ti substrate gave rise to the extreme hardness as well as the weakness in the transitional region. Figure 4: Vickers-hardness distribution across the V–4 Cr–4 Ti/Ti joint. Figure 5: Bonding strength tests: (**a**) schematic diagram of the apparatus; (**b**) load-displacement curves of the V–4 Cr–4 Ti/Ti joint.