Distinct migration mechanisms of stepped FCC/BCC martensitic interfaces associated with typical orientation relationships: a molecular dynamics study
ZZ Wei and X Ma and CB Ke and XP Zhang, JOURNAL OF MATERIALS SCIENCE, 57, 19857-19871 (2022).
Previous work on studying FCC/BCC martensite transformations using molecular dynamics (MD) was mainly adopting an atomistically flat interface between the two transforming phases as initial structural model, whereas recent HRTEM observations show that martensitic interfaces are comprised of atomic terraces with regularly distributed steps reticulated by interfacial defects. In this work, migration behavior of stepped FCC/BCC martensitic interface aided by disconnections is studied using MD simulation method, and two typical orientation relationships (ORs), i.e., Kurdjumov-Sachs (KS) and Nishiyama-Wassermann (NW) ORs, commonly found in ferrous alloys are considered. For both ORs, the stepped interfaces maintain a macroscopically planar morphology as they advance in constant velocities by the collective motion of disconnections in the absence of homogeneous nucleation. The migration velocities of interfaces as obtained are 550 and 90 m.s(-1) for the NW and KS ORs, respectively, showing good agreement with published experimental measurements. The disconnections under the NW OR exhibit edge defect character and move laterally along the terrace plane at a velocity approaching the speed of sound, while remaining straight along their dislocation lines, whereas the disconnections under the KS OR possess screw defect character and migrate in a zigzag and sluggish manner through cross-slipping on two nonparallel planes of 101(BCC) by a kink pair mechanism. By implementing a thin slab of atoms as tracing marker in the simulation model, the transformation shear accompanying the interface motion can be quantified and the so-determined transformation shear angles show good agreement with experimental measurements. The results provide a physical insight into the interrelations between interface structure and transformation process, which is crucial for understanding the atomic mechanism of martensitic transformations.
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