Magnetism effects on structural properties in Al- and Si- substituted Laves phases Fe$_{2}$Nb and Fe$_{2}$W

Laves phases are promising candidates for the design of new steels with superior mechanical strength. Here we study systematically thermodynamic, structural and magnetic properties in Al- and Si- substituted Laves phase compounds Fe$_{2}$Nb and Fe$_{2}$W using first-principles density functional methods. Spin polarized calculations predict that substituting Al atoms for Fe atoms in Fe$_{2}$Nb is energetically more favorable than substituting Al atoms for Nb atoms. Furthermore, they show that this leads to a significant increase of the lattice constant along the \emph{a} axis. To investigate the effects of magnetism, non-magnetic calculations are carried out for the same Fe-Nb-Al compounds. Theoretical predictions for the other alloys series, Fe-Nb-Si, Fe-W-Al and Fe-W-Si are presented. This work emphasizes the necessity of taking into account magnetism to understand the structural properties of the ternary alloys.