Triple junction solute segregation and the stability of nanocrystalline alloys

Thermal stability of nanocrystalline materials is often a barrier to their engineering application, and their stability can be improved by engineering the solute chemistry at grain boundaries to reduce defect free energy. Not often included in such considerations is the fact that in the nanocrystalline regime, the triple junction fraction can be significant, altering the thermodynamic landscape for solute atoms in the intergranular network. This work theoretically establishes that triple junctions can enhance—and in some cases uniquely enable—thermodynamic stabilization, due to their high tendency for solute segregation and their intrinsically favorable defect energetics. Moreover, the present analysis shows that a full spectral analysis of intergranular segregation is preferred for rigorous analysis of these problems, because stabilization effects can be overshadowed by numerical artefacts that arise from the use of average segregation energies. Specifically, the negative tail of the preferential segregation sites is critical to the energetics of stabilization, and contrast between boundaries and triple junctions is most critical at those tails. A survey of several systems show that junction effects can be quite significant in alloys like Ag(Ni) and Al(Zr), but negligible in others like Al(Y) where the grain boundary-triple junction segregation contrast is low.