Thermodynamic and First-Principles Design of W–Si-Based Protective Coatings for W-Re/HfC Alloys

W–Re/HfC alloys require effective surface protection against oxidation under severe high-temperature environments. In this work, W–Si-based protective coatings were investigated using calculation of phase diagrams (CALPHAD) thermodynamic calculations and first-principles simulations. Phase equilibria of W–Si–X systems (X = Al, Ti, and B) were evaluated at 800–1300 °C. The calculations indicate that Al, Ti, and B introduce additional Al–W intermetallics, Ti-rich silicides, and W–B/B–Si-related phases, respectively, whereas the binary W–Si system exhibits a simpler calculated phase constitution. The selected WSi2 structure has a calculated formation energy of −0.300 eV atom−1 and exhibits pronounced W-d/Si-s and p orbital hybridization. Climbing-image nudged elastic band (CI-NEB) calculations yield migration barriers of 1.276 eV for Si in W and 1.805 eV for W in Si along the investigated interstitial pathways. Increasing Si content promotes the calculated stability field of WSi2 and suppresses W5Si3, although excessive Si leads to the coexistence of WSi2 with residual Si. These results provide a thermodynamic and atomistic basis for the subsequent experimental development of WSi2-rich protective coatings on W–Re/HfC alloys.