New structures in Pd-rich ordered alloys*

An intriguing intermetallic structure with 8:1 stoichiometry was discovered in the 1950s in the Pt-Ti system. Since then, a handful of other Pt/Pd/Ni binary systems have been observed to exhibit this curious structure (e.g., Pt$_8$Zr, Pd$_8$Mo, Ni$_8$Nb, etc). This ordered structure can significantly increase the hardness of an alloy by forming precipitates. Recent calculations and experiments suggest that the 8:1 structure may form in about 20 previously unsuspected Pt/Pd binary systems. Using first-principles calculations, cluster expansion, Monte Carlo modeling, we have explored possible precipitate hardening (via the 8:1 structure) in Pd-Nb, Pd-Mg and Pd-Cu.