  The atomic structure of the supercooled liquid has often been discussed as a
key source of glass formation in metals. The presence of
icosahedrally-coordinated clusters and their tendency to form networks have
been identified as one possible structural trait leading to glass forming
ability in the Cu-Zr binary system. In this work, we show that this theory is
insufficient to explain glass formation at all compositions in that binary
system. Instead, we propose that the formation of ideally-packed clusters at
the expense of atomic arrangements with excess or deficient free volume can
explain glass-forming by a similar mechanism. We show that this behavior is
reflected in the structural relaxation of a metallic glass during constant
pressure cooling and the time evolution of structure at a constant volume. We
then demonstrate that this theory is sufficient to explain slowed diffusivity
in compositions across the range of Cu-Zr metallic glasses.
