  Primary crystallization in high Al-content metallic glasses is driven by
nanometer-diameter regions with internal structure similar to fcc Al.
Comparison of fluctuation electron microscopy (FEM) data to FEM simulations of
fcc Al clusters dispersed in a dense-random packed matrix is used to extract
the diameter and volume fraction of the ordered regions in a Al88Y7Fe5 base
glass and in glasses with 1 at.% Cu substituted for Y or Al. The size and
density of nanocrystals were measured as a function of isothermal annealing
time for the same alloys. The volume fraction of crystalline material grows
under isothermal annealing, so the phase transformation is not purely grain
coarsening, but the crystalline volume fraction is lower than the volume
fraction of ordered regions in the as-quenched samples, so not all of the
ordered regions act as nuclei. Changes in diameter and volume fraction of the
ordered regions with alloying are correlated with changes in the
crystallization temperature, nucleation rate, and nanocrystal density. No
evidence for phase separation is observed, and FEM simulations from a molecular
dynamics quenched structural model of similar composition do not show the
features observed in experiment.
