EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

Displacive solid-state phase transformations from the face centered cubic (fcc) austenite phase (γ) to body centered cubic (bcc) ferrite phase (α) play a pivotal role in the physical properties of steels and ferrous alloys. The rapid change in crystal structure inherently alters the mechanical properties of these materials, including fatigue, plasticity and strength [1,2].
In this work, extensive atomistic simulations based on the quasiparticle (QA) approach are performed to determine the main 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 glide of partial twin dislocations, while the glide of fcc screw dislocations along coherent terrace edges is the main 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.

[1] BPJ Sandvik and CM Wayman. Characteristics of lath martensite: Part ii. the martensite-austenite interface. Metallurgical Transactions A, 14(4):823–834, 1983.

[2] BPJ Sandvik and CM Wayman. Characteristics of lath martensite: Part iii. some theoretical considerations. Metallurgical Transactions A, 14(4):835–844, 1983.