nanotechRecent experimental and theoretical results suggest that nanocrystalline metals can be thermodynamically stabilized by the free energy gain associated with the segregation of solute atoms to the grain boundary which will reduce grain boundary energy and so the driving force for nanocrystalline coarsening. A detail understanding of this proposed mechanism requires detangling the thermodynamics of the coupled bulk and interfacial regions of the metal. We have developed a novel diffuse-interface model of grain boundary segregation in binary metallic alloys that is capable of accounting for both bulk and interfacial energetics and the interaction of alloying elements with the grain boundary. The model presented here extends prior treatments by independently treating solute-solute interactions within both the bulk grains and the grain boundary regions. This alloys for the simultaneous treatment of both grain boundary segregation and phase separation processes.
Through analytic analysis of 1-D systems, the dependence of grain boundary energy on the alloy segregation energy parameters is examined and the results demonstrate that in order to obtain substantial reduction in the grain boundary energy and so thermodynamic stabilization of nanocrystalline metals that high concentrations of solute atoms is required. These analysis also show that this model can recover classic segregation isotherms such as the McLean isotherm and the Fowler-Guggenheim isotherm. Simulation of 2-D polycrystalline systems demonstrate that substantial retardation of grain growth can be obtained for appropriate alloy and segregation energetics.
In broader terms, this modeling approach provides an avenue to explore gran boundary solute segregation and its competition with phase separation in the understanding and design of hopefully stable nanocrystalline metals.
Future work will extend this model in three critical ways: (1) the influence of solute segregation on the grain boundary mobility, (2) a system with heterogeneous grain boundary properties will be incorporated, (3) the addition of a stress term in the energy functional to account for mechanically-induced instability. These model predictions could help guide alloy development for the purpose of boundary stabilization and aid in the mechanistic interpretation of binary alloy boundary evolution observations.