  Precipitation hardening, which relies on a high density of intermetallic
precipitates, is a commonly utilized technique for strengthening structural
alloys. Structural alloys are commonly strengthened through a high density of
small size intermetallic precipitates. At high temperatures, however, the
precipitates coarsen to reduce the excess energy of the interface, resulting in
a significant reduction in the strengthening provided by the precipitates. In
certain ternary alloys, the secondary solute segregates to the interface and
results in the formation of a high density of nanosize precipitates that
provide enhanced strength and are resistant to coarsening. To understand the
chemical effects involved, and to identify such systems, we develop a
thermodynamic model using the framework of the regular nanocrystalline solution
model. For various global compositions, temperatures and thermodynamic
parameters, equilibrium configuration of Mg-Sn-Zn alloy is evaluated by
minimizing the Gibbs free energy function with respect to the region-specific
(bulk solid-solution, interface and precipitate) concentrations and sizes. The
results show that Mg$_2$Sn precipitates can be stabilized to nanoscale sizes
through Zn segregation to Mg/Mg$_2$Sn interface, and the precipitates can be
stabilized against coarsening at high-temperatures by providing a larger Zn
concentration in the system. Together with the inclusion of elastic strain
energy effects and the input of computationally informed interface
thermodynamic parameters in the future, the model is expected to provide a more
realistic prediction of segregation and precipitate stabilization in ternary
alloys of structural importance.
