We developed the calculation method of precipitate nucleation free energy. This
new treatment divides the free energy into cluster energy and entropy terms.
The former, including the internal enthalpy change and the interface energy,
is accurately calculated by ab initio methods. The latter is rigorously
derived from the Bragg-Williams approximation. We applied this method on
a Fe-Cu system in which small Cu clusters possess bcc lattice, and spherical
and coherent interface. The results showed reasonable agreement with the
empirically determined critical radius and activation energy barrier. In this
paper, we extend the newly proposed calculation method to ternary
systems. The effect of Ni addition on the copper precipitation in Fe-Cu
system was examined by this method. It revealed that added Ni prefers
segregating at matrix/cluster interface, and reduces the activation energy
barrier as well as the interface energy. The mechanism of the Ni addition
will be reasonably understood by the simple bond model. Using the same
formulation, vacancies are also treated precisely. Vacancies in Fe-Cu
alloy prefer locating inside of Cu clusters, and the activation energy barrier
decreases due to the reduction of the driving force. A rather highly strained
system of Cu-Co will be also explored.
E-mail: seko@cms.mtl.kyoto-u.ac.jp