  A method based on the Gibbs' adsorption isotherm is developed to calculate
the decrease in interfacial free energy resulting from solute segregation at an
internal interface, built on measured concentration profiles. Utilizing
atom-probe tomography (APT), we first measure a concentration profile of the
relative interfacial excess of solute atoms across an interface. To accomplish
this we utilize a new method based on J. W. Cahn's formalism for the
calculation of the Gibbs interfacial excess. We also introduce a method to
calculate the decrease in interfacial free energy that is caused by the
segregating solute atoms. This method yields a discrete profile of the decrease
in interfacial free energies, which takes into account the measured
concentration profile and calculated Gibbsian excess profile. We demonstrate
that this method can be used for both homo- and hetero-phase interfaces and
takes into account the actual distribution of solute atoms across an interface
as determined by APT. It is applied to the case of the semiconducting system
PbTe-PbS 12 mol.%-Na 1 mol.%, where Na segregation at the PbS/PbTe interface is
anticipated to reduce the interfacial free energy of the {100} facets. We also
consider the case of the nickel-based Alloy 600, where B and Si segregation are
suspected to impede inter-granular stress corrosion cracking (IGSCC) at homo-
(GB) and hetero-phase metal carbide (M7C3) interfaces. The concentration
profiles associated with internal interfaces are measured by APT using an
ultraviolet (wavelength = 355 nm) laser to dissect nanotips on an atom-by-atom
and atomic plane-by-plane basis.
