Fcc -> bcc phase transition kinetics in an immiscible binary system: atomistic evidence of the twinning mechanism of transformation

TL;DR

This study uses atomistic simulations to reveal the twinning mechanism during the fcc to bcc phase transition in an immiscible binary system, highlighting dislocation propagation at the atomic level.

Contribution

It provides atomistic evidence of the twinning mechanism and dislocation dynamics involved in the fcc/bcc phase transformation, validated by experimental observations.

Findings

Twin boundaries propagate via screw dislocations in fcc.

Fcc screw dislocations along coherent terrace edges drive the transformation.

Simulation results align with TEM and HRTEM observations.

Abstract

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.