  Using molecular simulations, we shed light on the mechanism underlying
crystal nucleation in metal alloys and unravel the interplay between crystal
nucleation and glass transition, as the conditions of crystallization lie close
to this transition. While decreasing the temperature of crystallization usually
results in a lower free energy barrier, we find an unexpected reversal of
behavior for glass-forming alloys as the temperature of crystallization
approaches the glass transition. For this purpose, we simulate the
crystallization process in two glass-forming Copper alloys, $Ag_6Cu_4$, which
has a positive heat of mixing, and in $CuZr$, characterized by a large negative
heat of mixing. Our results allow us to identify that this unusual behavior is
directly correlated with a non-monotonic temperature dependence for the
formation energy of connected icosahedral structures, which are incompatible
with crystalline order and impede the development of the crystal nucleus,
leading to an unexpectedly larger free energy barrier at low temperature. This,
in turn, promotes the formation of a predominantly closed-packed critical
nucleus, with fewer defects, thereby suggesting a new way to control the
structure of the crystal nucleus, which is of key importance in catalysis.
