{202¯1} Twinning in hexagonal close-packed Re nanocrystals mediated by {101¯1}|{101¯0} interfacial facets

Twinning is an essential deformation mode of crystals which is attracting growing attention due to its potential in simultaneously improve the strength and ductility of metals. It is generally believed that twinning is mediated by shear and atomic shuffles on an invariant twinning plane. Here, by using in situ high resolution transmission electron microscopy, we report on an extension twinning mode in rhenium nanocrystals along the $〈10\bar{1}\bar{4}〉$ direction on the $\left\{20\bar{2}1\right\}$ plane, which can be mediated by the formation of interfacial defects that fall between the parent prismatic plane and the twin pyramidal plane which are a pair of corresponding planes of the $\left\{20\overline{2}1\right\}$ twinning mode. The incoherent twin boundary can partly evolve into the coherent twinning plane during detwinning. The findings provide direct evidences to the $\left\{20\overline{2}1\right\}$ twin in hexagonal close-packed metals and corroborate the conjecture that twinning nucleation is essentially a transformation that conforms to and establishes the lattice correspondence which can be mediated by interfacial processes other than homogeneous shear on the twinning plane.