The alloying mechanism of Fe-Ni nanoparticles formed under high impact collisions have been investigated using molecular dynamics simulations. The energetics and dynamics of the coalescence and structural evolution of Fe-Ni alloys during the collision process and subsequent quenching has been studied. The extent of alloy segregation and mixing is established to be strongly correlated to the initial momentum, particle sizes and the alloy composition. The resulting Fe-Ni alloy nanostructures have Fe-rich surface and depending on the alloy composition, show the presence of amorphous or well-defined crystalline phases that are absent in their bulk counterparts. In-line with this observation, the nanoscale phase diagram shows a non-monotonic variation in the melting transition. (C) 2011 Elsevier B. V. All rights reserved.