Atomistic study of the fcc→bcc transformation in a binary system: Insights from the Quasi-particle Approach

In this work, the Quasi-particle Approach (QA) is applied to qualitatively reproduce the underlying mechanisms of the displacive fcc (γ) → bcc (α) transformation. At the microstructural scale, we demonstrate that the QA is able to predict the growth of a bcc nucleus in a fcc matrix, and the eventual formation of an internally twinned structure consisting in two variants with Kurdjumov-Sachs orientation relationship. At the atomic level, the defect structure of twinning boundaries and fcc/bcc interfaces is identified, and the main mechanism for the propagation of the fcc/bcc interface is analyzed. In detail, it is confirmed that twin boundaries are propagated by the glide of pairs of partial twin dislocations, 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 qualitatively 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.