The influence of solute distribution on the high nucleation density of Al crystals in amorphous aluminum alloys

An extension of a previous model that describes the role of solute atoms on glass formability leads to the conclusion that solute distribution plays an important role in the formation and stability of amorphous metals. A random distribution of solutes is shown to produce local solute-depleted regions (on the size scale of the mean inter-solute spacing) that provide preferred sites for the formation of crystalline nuclei. The possibility that these solute-depleted regions are responsible for the exceptionally high number density of critical Al nuclei is explored for three Al–Y binary alloys using a computer simulation. Up to 107 Y atoms were placed at random locations in the system, and the number of solute-free regions were counted as a function of the size of these regions. The experimentally observed number density of critical nuclei (∼3 × 1021 m−3) is reproduced for a critical nucleus about 5 Al atoms in diameter, containing ∼60 Al atoms in an fcc array. Good agreement with previous suggestions of the size of a critical nucleus (about 6 atoms in diameter, containing about 100 atoms) support the conclusion that the current model provides a reasonable physical explanation for the quenched-in features responsible for the exceptionally high nucleation density in some amorphous Al alloys.