Microstructural analysis by high-voltage electron diffraction of severely drawn iron wires

Abstract

Transverse sections of Fe-0.007 wt pct C wires drawn to true strains up to In(D _o/D)²= 6 were examined by transmission electron microscopy at 800 kV, using bright-field and selected-area diffraction. The size of the area selected (0.1 μm) enabled single-crystal diffraction patterns to be obtained from individual cells in the substructure. Because of the characteristic (110) bec wire texture, nearly all the diffraction patterns were (110), so it was possible to describe completely the relative misorientations of individual cells in groups of up to 50 contiguous cells with an accuracy of about 0.5 deg. In this way, maps were drawn showing the size, shape, crystallographic orientation, and grouping of the cells. These maps illustrate the development of the cellular substructure at large deformations as it evolves from many, slightly misoriented crystallites in the initially large grains (at strains of unity and less) to a much smaller number of strongly misoriented, fibrous subgrains in each severely elongated grain (at strains above 4). The trend is toward pure (110) tilt boundaries of very large misorientation and one cell per grain at the highest strains. It is necessary to consider both the slip distance (which is probably greater than the average cell size) and the forest-dislocation density (represented by the cell walls cut by the glide dislocations) in order to characterize the microstructure Jflow-stress relationship.