Compositional tuning of B2 Fe50Co50 alloys with Pt and Ni for enhanced mechanical and magnetic performance: A DFT study

FeCo alloys are widely recognized soft magnetic materials for use in power generators, transformers, and magnetic shields due to their high saturation magnetization and mechanical strength. However, ordered B2 FeCo suffers from severe room-temperature brittleness, posing significant challenges for advanced engineering applications. In this study, spin-polarized density functional theory (DFT) combined with phonon calculations is employed to systematically investigate the effect of ternary alloying (M = Ni, Pt) on the structural, magnetic, mechanical, vibrational, and thermal properties of Fe 50 Co 50-x M x (0 ≤ x ≤ 25 at%) alloys. Quantitatively, elastic stability and phonon dispersion analyses demonstrate that stable ductility enhancement is confined to a narrow concentration window of x = 6.25 at%, where the Pugh ratio increases from 1.9936 for binary FeCo to 2.5144 for Fe 50 Co 43.75 Ni 6.25 and 2.4851 for Fe 50 Co 43.75 Pt 6.25 . Phonon spectra confirm full dynamical stability at 6.25 at% with real vibrational frequencies throughout the Brillouin zone. Conversely, solute contents exceeding 6.25 at% induce mechanical instability via Born criteria C 11 < C 12 ) and dynamical instability characterized by soft transverse acoustic phonon modes with imaginary frequencies. Although solute addition reduces the net magnetic moment through electronic band filling, strengthened magnetic exchange coupling elevates the Curie temperature (Tc) across the composition range. Thermodynamic modelling confirms lattice softening with increasing solute content, reflected by a systematic decrease in Debye temperature ( θ D ) alongside increases in heat capacity (Cv) and thermal expansion (α). Overall, controlled 6.25 at% alloying of Ni or Pt provides an optimal design pathway to achieve balanced mechanical formability, dynamical lattice stability, and soft magnetic performance.