# Interdiffusion in Zr-Mo/W Intermetallics
## 3 Results and Discussion
### Elemental Distribution State of the Diffused Region
#### 3.1.2 Zr-W Binary System  
Figure 3a shows the BSE image and EDX-mapping of the entire diffused region tested by EDX quantitative analysis for the Zr-W diffusion system after annealing at 1300 \({}^{\circ}\)C for 5 h. Different from the Zr-Mo system, the solid solution phase layer was not observed in the Zr-W system. This might be due to the higher activation energy of the solid solution phase in the Zr-W system, which required a higher temperature to be formed. The quantitative analysis of the diffused region by EDX indicated that there were three regions in the Zr-W system. Sequentially, they were pure Zr, W\({}_{2}\)Zr phase, and pure W. The diffused regions of the Zr-W system annealed at 1400 \({}^{\circ}\)C and 1500 \({}^{\circ}\)C for 5 h are shown in Figure 3b and c, respectively. The elemental distribution state was similar to those at 1300 \({}^{\circ}\)C, i.e., the W\({}_{2}\)Zr were formed in the diffused region. When the diffused time was the same, the thickness of the W\({}_{2}\)Zr phase layer increased with the temperature, as shown in Table 2. At 1300 \({}^{\circ}\)C, 1400 \({}^{\circ}\)C, and 1500 \({}^{\circ}\)C, the thickness of the W\({}_{2}\)Zr phase layer was 4.15 \(\upmu\)m, 7.52 \(\upmu\)m, and 14.22 \(\upmu\)m, respectively. The thickness of the W\({}_{2}\)Zr layer was very thin and the increment with temperature was only a few microns. The thickness increased by \(\sim\)7 \(\upmu\)m when the diffusion temperature was increased from 1400 \({}^{\circ}\)C to 1500 \({}^{\circ}\)C, while it increased by only \(\sim\)3.5 \(\upmu\)m when the temperature increased from 1300 \({}^{\circ}\)C to 1400 \({}^{\circ}\)C. This was due to the high melting points of W (3410 \({}^{\circ}\)C) and Zr (1842 \({}^{\circ}\)C), which resulted in the weak ability of atomic migration under the heat activation effect. While the melting point of Mo (2610 \({}^{\circ}\)C) was lower than that of W, the Zr-Mo system required lower activation energy for diffusion. Moreover, the diffusion in the Zr-W system became multiphase diffusion of W-W\({}_{2}\)Zr-Zr as diffusion proceeded. The W\({}_{2}\)Zr phase could be observed on the Zr side at all temperatures in this work, e.g., the image of the Zr side at 1500 \({}^{\circ}\)C shown in Figure 4. This was dominated by two processes: (i) the initial diffusion was also controlled by vacancy mechanism, the Zr \begin{table}
\begin{tabular}{c c c} \hline \multirow{2}{*}{**T\({}^{\prime}\)C**} & \multicolumn{2}{c}{**The Thickness of Each Diffusion Layer/\(\upmu\)m**} \\ \cline{2-3}  & **(Zr, Mo) Solid Solution Phase** & **Mo\({}_{2}\)Zr Phase** \\ \hline
1300 & 297 \(\pm\) 21 & 4.48 \(\pm\) 0.11 \\
1400 & 443 \(\pm\) 30 & 5.62 \(\pm\) 0.23 \\
1500 & 753 \(\pm\) 40 & 7.20 \(\pm\) 0.47 \\ \hline \end{tabular}
\end{table} 
Table 1: The thickness of the (Zr, Mo) solid solution phase and Mo\({}_{2}\)Zr phase in the Zr-Mo binary system after diffusion for 5 h at 1300 \({}^{\circ}\)C, 1400 \({}^{\circ}\)C, and 1500 \({}^{\circ}\)C. \begin{table}
\begin{tabular}{c c c} \hline \multirow{2}{*}{**1300 \({}^{\circ}\)C**} & **1400 \({}^{\circ}\)C** & **1500 \({}^{\circ}\)C** \\ \hline Thickness & 4.15 \(\pm\) 0.23 \(\upmu\)m & 7.52 \(\pm\) 0.46 \(\upmu\)m & 14.22 \(\pm\) 0.69 \(\upmu\)m \\ \hline \end{tabular}
\end{table} 
Table 2: The thickness of W\({}_{2}\)Zr after diffusion for 5 h at 1300–1500 \({}^{\circ}\)C.  
side could easily provide the vacancy for W atomic migration due to a lower melting point of Zr; and (ii) simultaneously, W\({}_{2}\)Zr formed by the assistance of the diffusion and reaction under thermal driving effect. As mentioned above, the interdiffusion region included the Mo\({}_{2}\)Zr phase of only a few microns and the (Zr, Mo) solid solution phase of several hundred microns in the Zr-Mo system after diffusion at 1300-1500 \({}^{\circ}\)C as shown in Figure 5a, and the Mo content in the (Zr, Mo) solid solution was between 0 and 14 at. %, while the interdiffusion region only included the W\({}_{2}\)Zr phase in the Zr-W system, as shown in Figure 5b.