  Extensive atomistic simulations based on the quasiparticle (QA) approach are
performed to determine the momentous aspects of the displacive fcc/bcc phase
transformation in a binary system. We demonstrate that the QA is able to
predict the major structural characteristics of fcc/bcc phase transformations,
including the growth of a bcc nuclei in a fcc matrix, and eventually the
formation of an internally twinned structure consisting in two variants with
Kurdjumov-Sachs orientation relationship. At atomic level, we determine the
defect structure of twinning boundaries and fcc/bcc interfaces, and identify
the main mechanism for their propagation. In details, it is shown that twin
boundaries are propagated by the propagation of screw dislocations in fcc along
the <-1-11>_{\alpha} direction, while the propagation of fcc screw dislocations
along coherent terrace edges is the pivotal vector of the fcc/bcc
transformation. The simulation results are compared with our TEM and HRTEM
observations of Fe-rich bcc twinned particle embedded in the fcc Cu-rich matrix
in the Cu-Fe-Co system.
