## 3 Results and Discussion
### Mo-Re-Ni (\(\sigma\) Phase)  
The phase diagram of this system was studied along two ternary isothermal sections at 1200 \({}^{\circ}\)C and 1600 \({}^{\circ}\)C (Figure 12) [25]. The shape of the homogeneity domain of the \(\sigma\) phase deserves special attention. Contrary to Mo-Re-Cr systems, the \(\sigma\) phase exists in only one binary system (Mo-Re). However, it extends from the binary Mo-Re border deep into the ternary field, not only towards the center of the composition field but also pointing towards Mo\({}_{2}\)Ni binary composition. This latter point can be explained by the fact that the \(\sigma\) phase is close to be stable in the binary Mo-Ni system (see the  
Figure 11: Calculated site occupancies at 1200 \({}^{\circ}\)C along the section Mo\({}_{0.46}\)Re\({}_{0.54}\)–Cr\({}_{0.46}\)Re\({}_{0.54}\) (blue line in Figure 10). The symbols correspond to the experimental occupancies at 1000 \({}^{\circ}\)C (Cr–Re) [20] and 1200 \({}^{\circ}\)C (Mo–Re) [24].  
Figure 10: Isothermal section at 1150 \({}^{\circ}\)C of the Mo–Re–Cr system (redrawn from [23], Copyright 1999, Elsevier Science). The red triangle shows the three ordered stable or metastable compositions of the \(\sigma\) phase, Mo:Re, Mo:Cr, and Re:Cr, as in Figure 2 (solid lines: normal substitution, dashed line: site reversal). The site occupancies of Figure 10 are calculated along the blue line.  
calculated ground state of the Mo-Ni system, [PERSON] (to be published)). It also extends toward the third binary system, Re-Ni.  
Contrary to the Mo-Re-Cr system, this system could be studied experimentally due to particularly favorable diffraction selectivity using both XRD and ND diffraction (see Figure 3) [4]. Three ternary samples whose composition was chosen to be representative of the homogeneity domain were studied. The results demonstrated, for the first time experimentally, the site reversal of Re atoms when the composition is changed from Mo\(\cdot\)Re to the Re-Ni binary. The results are shown along one section in Figure 13. The result is confirmed by DFT calculations and the agreement between the calculated and experimental site occupancies is exceptional.  
Figure 12: Isothermal section at 1600 \({}^{\circ}\)C of the Mo–Re–Ni system (redrawn from [25], Copyright 2013, Elsevier Science). The red triangle shows the three ordered stable or metastable compositions of the \(\sigma\) phase, Mo\(\cdot\)Re, Mo\(\cdot\)Ni, and Re\(\cdot\)Ni, as in Figure 2 (solid line: normal substitution, dashed line: site reversal). The site occupancies of Figure 13 are calculated along the blue line.  
Figure 13: Calculated and experimental site occupancies for the \(\sigma\) phase at 1200 \({}^{\circ}\)C along the section Mo\(\cdot\)Re\(\cdot\)Re\(\cdot\)Ni\({}_{0.3}\) (blue line in Figure 12) [4].  
One should note the difference between this system and the Mo-Re-Cr system. Starting from the Mo-Re system, the \(\sigma\) phase extends mainly towards the \(AC_{x}\) compound (Mo2 Ni), which is metastable while it extends towards the stable \(BC_{x}\) compound in the Re-Cr system. Consequently, in Mo-Re-Cr, the site reversal is in the direction of the homogeneity domain, while in Mo-Re-Ni it is perpendicular. This is simply due to the relative stability of the three binary compounds \(AB_{x}\), \(AC_{x}\), and \(BC_{x}\).