Nucleation-suppressed phase stabilization in Fe-Au nanoparticles

Four nanoparticle compositions, Fe-21, 35, 47, and 67 at. % Au, have been prepared to study the phase stability and solid-state transformation in confined Fe-Au nanoalloys. The formation of two phases, predicted from bulk thermodynamics, has been suppressed in all compositions. Instead, a single phase solid solution forms after heat treatment at 600 C and slow cooling. However, bulk phase relationships, signified by the precipitation of α-Fe upon cooling, was observed in larger particles (>20 nm) with composition Fe-35 at. % Au. The suppression of the phase transformation/precipitation in small particles is explained thermodynamically, as the free energy decrease associated with the phase transformation does not exceed the increase in energy due to the introduction of an interphase interface (grain boundary) within the cluster. A general equation has been derived to predict the critical cluster size below which transformations are inhibited, which agrees well with the observed experimental results.