Theory of transformation-mediated twinning

TL;DR

This paper introduces a novel transformation-mediated twinning mechanism in metastable fcc materials, involving a displacive phase transformation to hcp followed by twin formation, which enhances the material's mechanical properties.

Contribution

It reveals a new twinning mechanism in metastable fcc alloys, expanding understanding of deformation processes and guiding the design of stronger, more ductile materials.

Findings

Displacive transformation from fcc to hcp precedes twinning.

Hcp phase nucleation driven by thermodynamic instability.

Enhanced twin boundary formation improves mechanical properties.

Abstract

High-density and nanosized deformation twins in face-centered cubic (fcc)materials can effectively improve the combination of strength and ductility. However, the microscopic dislocation mechanisms enabling a high twinnability remain elusive. Twinning usually occurs via continuous nucleation and gliding of twinning partial dislocations on consecutive close-packed atomic planes. Here we unveil a completely different twinning mechanism being active in metastable fcc materials. The transformation-mediated twinning (TMT) is featured by a preceding displacive transformation from the fcc phase to the hexagonal close-packed (hcp) one, followed by a second-step transformation from the hcp phase to the fcc twin. The nucleation of the intermediate hcp phase is driven by the thermodynamic instability and the negative stacking fault energy of the metastable fcc phase. The intermediate hcp structure is characterized by the easy slips of Shockley partial dislocations on the basal planes, which leads to both fcc and fcc twin platelets during deformation, creating more twin boundaries and further enhancing the prosperity of twins. The disclosed fundamental understanding of the complex dislocation mechanism of deformation twinning in metastable alloys paves the road to design novel materials with outstanding mechanical properties.