  Nucleation during solidification in multi-component alloys is a complex
process that comprises the competition between different crystalline phases as
well as chemical composition and ordering. Here, we combine transition
interface sampling with an extensive committor analysis to investigate the
atomistic mechanisms during the initial stages of nucleation in
$\text{Ni}_3\text{Al}$. The formation and growth of crystalline clusters from
the melt are strongly influenced by the interplay between three descriptors:
the size, crystallinity, and chemical short-range order of the emerging nuclei.
We demonstrate that it is essential to include all three features in a
multi-dimensional reaction coordinate to correctly describe the nucleation
mechanism, where in particular the chemical short-range order plays a crucial
role in the stability of small clusters. The necessity of identifying
multi-dimensional reaction coordinates is expected to be of key importance for
the atomistic characterization of nucleation processes in complex,
multi-component systems.
