Nucleation instability in supercooled Cu-Zr-Al glass-forming liquids

Few general models representing certain classes of real glass-forming systems play a special role in computer simulations of supercooled liquid and glasses. Recently, it was shown that one of the most widely used model glassformers-the Kob-Andersen binary mixture-crystalizes in quite lengthy molecular dynamics simulations, and moreover, it is in fact a very poor glassformer at large system sizes. Thus, our understanding of crystallization stability of model glassformers is far from complete due to the fact that relatively small system sizes and short time scales have been considered so far. Here we address this issue for two embedded atom models intensively used last years in numerical studies of Cu-Zr-(Al) bulk metallic glasses. Exploring the structural evolution of Cu_64.5Zr_35.5 and Cu₄₆Zr₄₆Al₈ alloys at continuous cooling and isothermal annealing, we observe that both systems nucleate in sufficiently lengthy simulations, although critical nucleation time for the latter is an order of magnitude higher than that for the former. We show that Cu_64.5Zr_35.5 is actually unstable to crystallization for large system sizes (N > 20 000). Both systems crystallize with the formation of tetrahedrally close packed Laves phases of different types. We argue that nucleation instability of the simulated Cu_64.5Zr_35.5 alloy is due to the fact that its composition is very close to that for the stable Cu₂Zr compound with a C15 Laves phase structure.