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.
