# Interdiffusion in Zr-Mo/W Intermetallics
## 3 Results and Discussion
### Elemental Distribution State of the Diffused Region
#### 3.1.1 Zr-Mo Binary System  
Figure 2(a1) shows the BSE image and EDX-mapping of the entire diffused region determined by EDX quantitative analysis for the Zr-Mo diffusion system after annealing at 1300 \({}^{\circ}\)C for 5 h.  
The quantitative analysis of the diffused region by EDX indicated that there were four regions in the Zr-Mo system. Sequentially, they were pure Zr, (Zr, Mo) solid solution phase, Mo\({}_{2}\)Zr phase, and pure Mo. The thickness of the solid solution region was about 297 \(\upmu\)m. Between the solid solution phase and the pure Mo phase, there was a very thin layer of Mo\({}_{2}\)Zr with a thickness of about 4.48 \(\upmu\)m. The wavy interface appeared at the interface between Mo\({}_{2}\)Zr and (Zr, Mo) solid solution, which may be due to the anisotropy of diffusion leading to an uneven interface. Moreover, the existence of a small number of impurities in the raw material sample resulted in different energy barriers for the diffusion of atoms at different positions, which may also cause the formation of undulating phase interfaces.  
The curves of concentration distribution of the solid solution phase (region 1) and Mo\({}_{2}\)Zr phase (region 2), as shown in Figure 2(a2), were obtained by fitting and smoothing data from EDX quantitative analysis. The concentration of Mo elements in the solid solution varied continuously from 0 to 10 at. %, and the Mo content in the Mo\({}_{2}\)Zr phase was about 66 at. %.  
Figure 1: (a) Design drawing and (b) physical photograph of the diffusion couple.  
The diffused regions of the Zr-Mo system annealed at 1400 \({}^{\circ}\)C and 1500 \({}^{\circ}\)C for 5 h are shown in Figure 2(b1,c1), respectively. The elemental distribution state was similar to those at 1300 \({}^{\circ}\)C, i.e., the (Zr, Mo) solid solution and Mo\({}_{2}\)Zr were formed in the diffused region. The curves of concentration distribution of the solid solution phase and Mo\({}_{2}\)Zr phase are shown in Figure 2(b2,c2), respectively. The solid solubility of Mo in Zr at (Zr, Mo) solid  
Figure 2: BSE images and EDX mapping of the entire diffused regions in Zr-Mo systems and the concentration distribution curve in the solid solution region after annealing at 1300–1500 \({}^{\circ}\)C for 5 h: (**a1,a2**), 1300 \({}^{\circ}\)C; (**b1,b2**), 1400 \({}^{\circ}\)C; (**c1,c2**), 1500 \({}^{\circ}\)C. The dashed square region 2 in (**a1,b1,c1**) represents the local enlarged images.  
solution increased with temperature. The maximum content of Mo in the solid solution phase was 12 at. % and 14 at. % at 1400 \({}^{\circ}\)C and 1500 \({}^{\circ}\)C, respectively. Furthermore, the thickness of each diffusion layer increased with the temperature. The statistical thickness of the diffusion layer at each temperature is listed in Table 1, and the thickness of each phase was measured 20 times at different locations to obtain the average thickness and standard deviation. When the diffused temperature remained at, 1300 \({}^{\circ}\)C, 1400 \({}^{\circ}\)C and 1500 \({}^{\circ}\)C, the thickness of (Zr, Mo) solid solution phase reached 297 \(\upmu\)m, 443 \(\upmu\)m, and 753 \(\upmu\)m, respectively, and the thickness of Mo\({}_{2}\)Zr phase layer was 4.48 \(\upmu\)m, 5.62 \(\upmu\)m and 7.20 \(\upmu\)m, respectively. From the increment of the diffused distance with temperature, the increment from 1400 \({}^{\circ}\)C to 1500 \({}^{\circ}\)C was larger than that from 1300 \({}^{\circ}\)C to 1400 \({}^{\circ}\)C. The thickness of the (Zr, Mo) solid solution increased by \(\sim\)300 \(\upmu\)m when the diffusion temperature was increased from 1400 \({}^{\circ}\)C to 1500 \({}^{\circ}\)C, while the thickness increased by only \(\sim\)150 \(\upmu\)m when the temperature was increased from 1300 \({}^{\circ}\)C to 1400 \({}^{\circ}\)C. This indicates that the temperature had a significant effect on diffusion promotion.