  As approaching the glass transition, particle motion in liquids becomes
highly heterogeneous and regions with virtually no mobility coexist with
liquid-like domains. This complex dynamics is believed to be responsible for
different phenomena including non-exponential relaxation and the breakdown of
Stokes-Einstein relation. Understanding the relationships between dynamical
heterogeneities and local structure in metallic liquids and glasses is a major
scientific challenge. Here we use classical molecular dynamics simulations to
study the atomic dynamics and microscopic structure of
$\mbox{Cu}_{50}\mbox{Zr}_{50}$ alloy in the supercooling regime. Dynamical
heterogeneities are identified via an isoconfigurational analysis. As deeper
supercooling is achieved a transition from isolated to clustering low mobility
atoms is reported. These slow clusters, whose size grow upon cooling, are also
associated to concentration fluctuations, characterized by a Zr-enriched phase,
with a composition $\mbox{CuZr}_2$. In addition, a structural analysis of slow
clusters based on Voronoi tessellation evidences an increase with respect of
the bulk system of the fraction of Cu atoms having a local icosahedral order.
These results are in agreement with the consolidated scenario of the relevant
role played by icosahedral order in the dynamic slowing-down in supercooled
metal alloys.
