Direct Observation of Zonal Dislocations in σ-FeCr by Atomic-Resolution Scanning Transmission Electron Microscopy

Dislocations are the lattice defects that are mainly responsible for plastic deformation of crystalline materials through their glide motion. The glide motion of dislocations on a slip system generally takes place on a crystallographic plane between two adjacent atomic layers in the case of most metallic materials with relatively simple crystal structures such as face-centered cubic (FCC) and body-centered cubic (BCC) metals. However, the dislocation glide process in materials with complex crystal structures has not been fully clarified yet, because it can be very complicated often involving multiple atomic planes during dislocation glide. One of the dislocation glide models predicted to operate in complex materials is the zonal dislocation model, in which non-uniform atom shuffling is predicted to occur in multiple atomic planes called shear zone. The zonal dislocation model was introduced by M.L. Kronberg to describe possible glide motion of dislocations of the {110}<001> slip in β-U with the tetragonal Ab structure (identical to D8_b structure of σ-FeCr). However, it has never been experimentally verified to occur. This is because β-U is one of the typical radioisotopes, which prevents from further detailed experimental study of its dislocations. Another reason is that the brittleness of the sigma-phase arising from the complex crystal structure has made impossible for plastic flow to occur by dislocation glide. Recently, we have systematically investigated fundamental deformation behavior of hard and brittle materials by utilizing micropillar compression method and have successfully identified the activation of {110}<001> slip in σ-FeCr at room temperature. In the present study, we investigated dislocation core structures of {110}<001> slip in σ-FeCr by atomic-resolution scanning transmission electron microscopy and confirmed that <001> dislocations glide on {110} are of the zonal-type but the thickness of the shear zone was different from that predicted by Kronberg. New atomic shuffling model in the shear zone was proposed based on the experimentally observed dislocation core structure.