Deformation Twinning in Octahedron-Based Face-Centered Cubic Metallic Structures: Localized Shear-Force Dipoles Drive Atomic Displacements

Twinning is found to impart favorable mechanical, physical and chemical properties to nanostructured materials. One important twinning mode, deformation twinning, prevails in coarse-grained hexagonal close-packed (HCP) crystalline materials and body-centered cubic (BCC) and face-centered cubic (FCC) nanomaterials under high-stress conditions. In FCC structures, the {111} deformation twinning is traditionally believed to nucleate and grow through layer-by-layer emission of 1/6 Shockley partial dislocations on consecutive {111} planes. Here, we report that by conducting high-resolution transmission electron microscopy (HRTEM) observation, deformation twinning is, for the first time, found to occur in nanocrystalline (Fe, Nb)23Zr6 particles with a Mn23Th6-type FCC structure that is composed of a Zr-octahedron-based FCC network connected by alloying elements Fe and Nb like the large FCC structure such as metal-organic-framework (MOF). Based on direct atomic-scale observations, we discover a new mechanism for the {111} deformation twinning in FCC structures. To form a [112]/(111) twin, for example, short ( (‾1‾11) planes within two adjacent (111) plane layers in the repeated three-layer sequence of (111) planes are shear deformed continuously by a shear-force dipole along the [112] direction like a domino effect, whereas the other (111) plane in the repeated sequence remains intact. Through this route, a small energy for twinning is expected because only 2/3 (111) planes need to be transformed to form a twin. In addition, a loading criterion for deformation twinning of a FCC NP under uniaxial compression is proposed based on our results. Our work here not only provides a fundamental understanding on deformation twinning in FCC structures, but also opens up studies of deformation behaviors in a class of Mn23Th6-type FCC materials.