Thermodynamic stabilization of precipitates through interface segregation: Chemical effects

Abstract

Precipitation hardening, which relies on a high density of intermetallic precipitates, is a commonly utilized technique for strengthening structural alloys. At high temperatures, however, the precipitates often coarsen to reduce the excess energy of the interface, resulting in a significant reduction in the strengthening achieved. In certain ternary alloys, secondary solute segregation to the interface has been observed to result in the formation of a high density of nanosized precipitates that provide enhanced strength and are resistant to coarsening. To understand the chemical effects involved, and to identify such segregating systems, we develop a thermodynamic model using the framework of the regular nanocrystalline solution model. For various global compositions, temperatures, and thermodynamic parameters, we evaluate equilibrium configurations of a Mg-Sn-Zn alloy by minimizing the Gibbs free energy function with respect to region-specific (bulk solid solution, interface, and precipitate) concentrations and sizes. The results show that Mg2Sn precipitates can be stabilized to nanoscale sizes through Zn segregation to the Mg/Mg2Sn interface, and the precipitates can be stabilized against coarsening at high temperatures through strong Mg-Zn interface interaction. Together with the inclusion of elastic strain energy effects, kinetic contributions, and the input of computationally informed interface 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.