High-resolution TEM Analysis of Defect Structures in Mechanically Milled, Nanocrystalline Fe

Abstract

Grain interior and grain-boundary structures of mechanically milled ultrafine-grained iron powder were investigated with atomic resolution using high-resolution transmission electron microscopy (HRTEM). Grain boundaries in the powder appear wavy and irregular, with no amorphous or other unusual defect structures present. Diffraction analysis of HRTEM images indicates that elongated grains approximately 100 nm long and 20 nm wide consist of several sub-grains along their length, each rotated by several degrees around a common ‹110› axis. The presence of partial disclination dipoles in the sub-grains was also observed by HRTEM. Such structures indicate that shear, fragmentation and rotation of grains several to tens of nanometers in diameter occurs during severe plastic deformation. A mechanism for the formation of an ultrafine-grained structure in mechanically milled Fe involving partial disclinations is proposed.