Site preferences and ordering in Nb(Al_1-xM_x)₂ (M = Ni or Cu) ternary Laves phases

The properties of binary Laves phases can often be improved by incorporation of additional elements into the crystal structure. In some cases, only a ternary Laves phase exists in an alloy system but with a substantial homogeneity range. Knowledge of the site preference in these compositionally complex intermetallics is necessary to tune the properties for improved performance. The site preferences and substitutional ordering behavior of two ternary C14 Laves phases Nb(Al_1-xNi_x)₂ and Nb(Al_1-xCu_x)₂ are studied with first-principles cluster expansion and Monte Carlo simulations. The ternary Laves phases show extended solubility ranges with constant Nb concentration from experiments, but do not have binary Laves analogues AB₂ in their respective systems. At low alloying concentrations the alloying elements Ni and Cu have preferences for the 6h B-sites in the respective Nb(Al_1-xNi_x)₂ and Nb(Al_1-xCu_x)₂ Laves phases. At higher alloying concentrations (x>0.25), Cu atoms are distributed with a slight preference for the 6h B-sites, but the Ni atoms show higher preferences for the 2a B-sites. Pair correlation analysis from the Monte Carlo simulations show that both Laves phases prefer heteroatomic 1st and 2nd nearest neighbor pairs. Drop-synthesis calorimetry experiments and first-principles calculations of formation enthalpies as a function of composition are found to be in good agreement. The Monte Carlo simulation also show that the site preferences of the alloying element depend on the preference of homoatomic bond pair-types on the Kagome layers formed by 6h atoms.