Sulfur segregation at α-iron grain boundaries—II

Abstract

Alpha iron grain boundaries containing large excesses of adsorbed sulfur were examined using transmission electron microscopy. The study revealed that the grain boundaries of the sulfurized iron were commonly associated with very high density dislocation networks that could extend several microns into the bodies of the grains. The dense networks were not observed in pure, sulfur-free iron. The networks are thought to form when the sulfur, having first entered the specimens by preferential diffusion along grain boundaries, diffuses from the grain boundaries laterally into the grains. The sulfur, which diffuses in iron as a substitutional element about 30 times faster than iron self-diffuses, establishes a diverging vacancy flux toward the boundaries. The result is a vacancy undersaturation in the neighborhood of the boundaries that accounts, it is believed, for the high dislocation density; the vacancy undersaturation causes dislocation multiplication by “down-climb”. The abnormal segregation of sulfur in the vicinity of α-iron grain boundaries is explained if it is assumed that sulfur is adsorbed by dislocations in the boundary networks. The possible effects of these grain boundary networks on certain other metallurgical phenomena are considered.