First-principles investigation of mechanical properties and fracture toughness in doped FeNbSb half-Heusler alloys

The ductile-to-brittle behavior of thermoelectric half-Heusler alloys is critical for their mechanical reliability. This study investigates the effect of Co/Ni doping on the fracture toughness of FeNbSb using first-principles calculations. All doped derivatives considered in this study, except Fe₄NiNb₃Sb₄, exhibit dynamic and thermodynamic stability. While doping generally improves ductility, the fracture toughness, derived from Griffith's theory, reveals a strong site dependence. Co substitution at Fe or Nb sites raises the toughness to 2.043 MPa m^1/2, exceeding the intrinsic value of 1.964 MPa m^1/2. Conversely, Ni doping and Co at Sb sites reduce the crack propagation resistance. This variation is governed by an atomic-size-effect strategy, where dopants with larger radii enhance toughness, and is correlated with electronic structure evolution near the Fermi level. The findings provide a rationale for designing damage-tolerant half-Heusler materials for thermoelectric and high-temperature applications.