## 3 Results and Discussion  
Figure 2 shows the optical micrograph of the typical V-4 Cr-4 Ti/Ti bonded interface. It presents a smooth interface without any observable cracks. The range of the diffusion interface is approximately 100 \(\upmu\)m wide and can be divided into two distinct zones. Zone I lies close to the Ti side and is about 70 \(\upmu\)m in width. It features a needle-cluster structure. At the same time, zone II is about 30 \(\upmu\)m wide and is located close to the V-alloy side, which features a uniform band structure. This interface microstructure resembles the pure Ti/steel diffusion-bonded joint [23; 24; 25], which is mainly attributed to the varied concentration of \(\beta\)-Ti stabilizers (e.g., Fe, V, or Cr) across the interface [26; 27]. According to [23; 24; 25; 26; 27], zone II should be the uniform \(\beta\)-Ti phase band structure because of the high concentration of V (see Figure 3). Zone II is adjacent to the V-alloy substrate and is rich in V content, which favors \(\beta\)-Ti solid solution phase stability up to room temperature [27]. Since the \(\beta\)-Ti phase is difficult to etch [23], zone II appears clean without visible metallurgical structure [25]. When the concentration of the stabilizers is relatively low, it may lead to partial \(\beta\)\(\rightarrow\)\(\alpha\) phase transformation and form the Widmanstatten structure [26]. Zone I is located far from the V-alloy substrate and its characteristic Widmanstatten structure (\(\alpha+\beta\) phases) should be attributed to the lower concentration of V and Cr (\(\beta\)-Ti stabilizers). \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Material** & **V** & **Ti** & **Cr** & **Fe** & **C** & **N** & **O** \\ \hline V–4 Cr–4 Ti & Bal. & 3.36 & 3.45 & - & 0.052 & 0.001 & 0.033 \\ Pure Ti & - & Bal. & - & 0.2 & 0.1 & 0.05 & 0.2 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Chemical composition of the parent materials (wt %). Figure 2: Optical micrograph of a typical V–4 Cr–4 Ti/Ti bonded interface. The diffusional behavior of V, Cr, and Ti across the joint was determined by EDS, which is shown in Figure 3. It is shown that the concentration of these elements change gradually across zone I while drastic changes are observed in zone II, which should act as a boundary of the interface. The EDS analysis shows that the V concentration in zone I is lower than \(\sim\)16 wt % while it ranges from \(\sim\)17 to 93 wt % in zone II. The Cr concentration across the interface is very low even in zone II. Since the diffusion coefficient for Cr (D\({}_{\rm Cr}\)) in V is so small (e.g., D\({}_{\rm Cr}\) = 5.84 \(\times\) 10\({}^{-17}\) m\({}^{2}\)/s at 1383 K [28]), the diffusion distance of Cr should be limited in the V-4 Cr-4 Ti alloy. The mutual diffusion of the elements in the parent alloys promotes the formation of the V-4 Cr-4 Ti/Ti joint, which changed the microstructure and also the hardness. Vickers hardness measurements were performed across the substrates and the joint. According to the hardness distribution profile, six characteristic parts (A, B, C, D, E, and F) were defined (see Figure 4). Part A and F refer to the regions in Ti and V-4 Cr-4 Ti alloy substrates far from the bonded interface, which measures 190 HV and 258 HV, respectively. Part C corresponds to zone I and part D to zone II. Part B and E are the intermediate layers shown in Figure 4. It can be found that the highest hardness (332 HV) appeared at the boundary of part C adjacent to the Ti substrate because of the stress concentration induced by the Ti phase transformation during the cooling after the forging process. It has been shown that the appropriate post weld heat treatment (PWHT) can be effective for decreasing the hardness and increasing the toughness of some welded joints [29; 30]. Therefore, the PWHT could also be adopted to recover the hardness of the V-4 Cr-4 Ti/Ti joint. Part E lays in the V-alloy substrate adjacent to the joint and exhibits the lowest hardness at only 180 HV. [PERSON] et al.