Solute/defect-mediated pathway for rapid nanoprecipitation in solid solutions: γ surface analysis in fcc Al-Ag

In face-centered-cubic (fcc) Al, stacking fault energy (SFE) is high at ca. 1500.3em0exmJ∕m², inhibiting stacking fault (SF) formation and dislocation motion. Yet Ag-rich hcp precipitates form rapidly in Al-rich fcc Al-Ag, even as the energy difference ΔE_hcp-fcc between hcp and fcc homogeneous solid solution increases with Ag content. Using electronic density functional theory methods, we calculate the SFE γSF versus distance of Ag (111) planes from intrinsic (isf), extrinsic (esf) and twin (tsf) SFs. We find that an inhomogeneous distribution of Ag solute segregated in layers adjacent to SFs leads to favorable SFE, a manifestation of the well-known Suzuki effect. We revisit the oft-quoted relation derived at fixed composition of ${{γ}}isf{}{{γ}}esf{}2{{γ}}tsf{∝}{Δ}{E}_{hcp{{-}}fcc}$, and we show that it holds only for cases that maintain symmetry of the underlying Bravais lattice, e.g., elemental metals and homogeneous solid solutions, and only roughly for certain layered configurations. We show that this defect/solute-mediated, low-energy pathway provides a local mechanism in inhomogeneous solid solution for the rapid hcp precipitation observed in Al-rich fcc Al-Ag.