Screw dislocation driven martensitic nucleation: A step toward consilience of deformation scenario in fcc materials

Martensitic transformation (MT), constituting an essential part of deformation scenario, plays a key role in plasticity and thermoelasticity of face-centered cubic (fcc) materials. Despite being an area of intense research, discrepancies remain about the essential parameter dictating nucleation of hexagonal closepacked (hcp) martensite and about the dislocation activities governing plasticity. Here, we show that screw dislocation induces the torsional flow of close-packed atomic planes of fcc matrix, characterized by Eshelby twist, and its dissociation provides a self-perpetuating step to bring forth Frank partial dislocation acting as a critical component to accomplish the atomic periodicity for fcc-to-hcp MT. The critical condition to initiate fcc-to-hcp MT was estimated from the Eshelby twist angle measured from highangle annular dark field scanning transmission electron microscopy (HAADF STEM). Once the critical condition is satisfied, the trajectory of screw dislocation can span two atomic planes, its dissociation proceeds by forming both Frank partial sessile to (111) and Shockley partial glissile along OM plane. Based on our dislocation model for MT, we demonstrate how dissociation route of perfect dislocation can be exploited to determine deformation mechanism (MT, twinning, slip). By incorporating dislocation dissociation model into the concept of stacking fault energy, we suggest a synthesized concept of deformation scenario that can provide fundamental and predictive insight into plasticity and transformability of fcc material. (C) 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.