THE EFFECT OF CARBON AND SULFUR ON THE CHARACTER OF THE GRAIN-BOUNDARY POPULATION IN ALPHA-IRON

The grain boundary misorientations and CSL frequencies of a population of about 250 boundaries in ultra high purity alpha-iron doped with sm all quantities of carbon and sulphur have been determined by a SEM mic rodiffraction BKD (Backscattered Kikuchi Diffraction) technique. The a im 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 degrees C after cold hydrostatic extrusion to epsilon = 1.1 It was fou nd that the addition of sulphur and carbon modifies the CGBP of alpha- iron. Sulphur clearly increases the density of low angle grain boundar ies (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 co uld 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 selectio n as a consequence of an impurity-specific dragging effect or a precip itation pinning effect on the migration of boundaries during recrystal lization and subsequent grain growth.