It was noted that the stacking-fault energy of Al (about 150mJ∕m2) inhibited stacking-fault formation and dislocation motion. However, Ag-rich hexagonal close-packed precipitates formed rapidly in Al-rich face-centered cubic Al-Ag, even though the energy difference, ΔEhcp-fcc, between hexagonal close-packed and face-centered cubic homogeneous solid solutions increased with Ag content. By using electronic density functional theory, the stacking fault energy, γSF, versus the distance of Ag(111) planes from intrinsic, extrinsic and twin stacking fault was calculated. It was found that an inhomogeneous distribution of Ag solute, segregated in layers adjacent to stacking faults, led to a favorable stacking-fault energy (the Suzuki effect). It was shown that the relationship, γisf ≈ γesf ≈ 2γtsf  ΔEhcp-fcc, held only for situations that maintained the symmetry of the underlying Bravais lattice (elemental metals, homogeneous solid solutions). It held only roughly for certain layered configurations. It was shown that this defect/solute-mediated low-energy pathway provided a local mechanism, in inhomogeneous solid solutions, for the rapid hexagonal close-packed precipitation observed in Al-rich face-centered cubic Al-Ag.

Solute/Defect-Mediated Pathway for Rapid Nano-Precipitation in Solid Solutions - γ Surface Analysis in FCC Al-Ag. D.Finkenstadt, D.D.Johnson: Physical Review B, 2006, 73[2], 024101