Development of the dislocation arrangement in Fe-0·9 wt% Si single crystals deformed in tension

Abstract

The dislocation arrangement in Fe-0·9 wt% Si single crystals after tensile deformation at room temperature to different stages has been studied by TEM. In stage I of the work-hardening curve, primary dislocations are predominantly observed. Most of these are clustered into bundles in the form of braids. Fine and relatively homogeneous slip traces on the surface show that the slip proceeds homogeneously in the whole volume of the crystal. Stage II, typical of f.c.c. crystals, is miming. Instead, the increase of the work-hardening coefficient at the end of stage I is followed immediately by an inflexion point on the work-hardening curve and by stage III. In the vicinity of the inflexion point, the density of secondary dislocations increases to a value comparable to the density of primary dislocations. The primary and secondary dislocations hinder each other in gliding. As a consequence, the primary braids are dissolved and crossed grids of primary and secondary dislocations are created parallel to the primary slip plane. The cross-grids occur in pairs and form the substructure of the dislocation sheets which are the most striking component of the dislocation arrangement in stage III. At the beginning of stage III the whole volume of the crystal is divided by the sheets into layers in which high and low densities of secondary dislocations alternate. The distribution of the sheets does not change significantly in stage III. The macroscopic deformation in stage III proceeds by glide of primary dislocations in the layers free of secondary dislocations. This glide manifests itself by discrete wavy glide bands observed on the surface in stage III.