The effect of carbon and sulphur on the character of the grain boundary population in α-iron

The grain boundary misorientations and CSL frequencies of a population of about 250 boundaries in ultra high purity α-iron doped with small quantities of carbon and sulphur have been determined by a SEM microdiffraction BKD (Backscattered Kikuchi Diffraction) technique. The aim is to examine the effect of these elements on the character of the grain boundary population (CGBP). Samples containing about 75 wt. ppm of sulphur and/or 150 (200) wt. ppm carbon were recrystallized at 725 °C after cold hydrostatic extrusion to ϵ = 1.1 It was found that the addition of sulphur and carbon modifies the CGBP of α-iron. Sulphur clearly increases the density of low angle grain boundaries (LANGB), but only in the pure FeS binary alloy. The fractions of the LANGB and coincidence site lattice grain boundaries (CSLGB) in the FeCS alloy are rather similar to those of FeC and thus the CGBP could be controlled by carbon. As a consequence, the lowest fraction of high energy random grain boundaries (RANGB) appears in the FeS alloy at about two-thirds of the total population. The effect of carbon and sulphur on the CGBP is interpreted in terms of grain boundary selection as a consequence of an impurity-specific dragging effect or a precipitation pinning effect on the migration of boundaries during recrystallization and subsequent grain growth.