Mechanical stability and fracture behavior of the WOs sigma phase: effects of composition and lattice-site occupancy

Precipitates of the topologically close-packed (TCP) phases can significantly impact a material’s mechanical and fracture properties. This theoretical study uses ab initio modeling to investigate the σ phase of WOs, focusing on composition-dependent mechanical and fracture-related properties. We find the effect of Os on WOs σ to be stronger than that of Re in WRe σ phase, with the WOs phase exhibiting peak hardness at the ideal concentration. This difference is attributed to variations in -orbital filling, causing Os to favor low-coordination sites more strongly than Re. While ductility may improve at high Os contents, brittle fracture of the σ phase is still predicted. For Os concentrations above 78 at.%, the phase becomes mechanically unstable. These results indicate that subtle changes in d-orbital filling between Os and Re significantly affect the stability and mechanical properties of the σ phase and may apply to similar TCP phases.