Grain boundary segregation of sulfur and antimony in iron alloys
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I.
INTRODUCTION
ANTIMONYsegregation to the grain boundaries of iron and steel has been shown by a number of researchers to contribute to temper embrittlement and hydrogen-induced intergranular fracture. Temper embrittlement of Ni + Cr steels was related to grain boundary antimony segregation by Smith and Low, Jr. ,[1] Ohtani et al. ,[21and Kameda and McMahon, Jr. ,[3] while Jones et al. [41found that antimony had a strong effect on the fracture mode transition of iron tested at cathodic potentials. Kameda and McMahon, Jr. also found that antimony segregation in Ni + Cr steels had a significant effect on the subcritical crack growth behavior in gaseous hydrogen. Therefore, the conditions under which antimony segregation occurs in iron and steels constitute an important technological issue. In Ni + Cr steels, the amount of antimony segregation exceeded that observed in binary Fe + Sb alloys; this enhancement was connected by several authors tLz'5] to nickel segregation. G u t t m a n n [6] presented a cosegregation model to explain the enhanced segregation of antimony where an intermediate, attractive interaction between the impurity and an alloying element such as nickel produces enhanced segregation. Increasing the attractive interaction increases the enhancement; on the other hand, large attractive interactions cause precipitation and a reduction in the amount of impurity in solution. Cosegregation of antimony and manganese is also predicted by Guttmann's theory, although Briant and Ritter [5] found that the addition of manganese to Fe + Ni + Cr + Sb + C alloys caused precipitation of an antimonide and the elimination of antimony segregation. They concluded that alloying effects on segregation of antimony
R. H. JONES and M. T. THOMAS, Staff Scientists, D. R. BAER, Senior Research Scientist, and L. A. CHARLOT, Senior Technical Specialist, are with Battelle-Northwest Laboratory, P. O. Box 999, Richland, WA 99352. Manuscript submitted June 16, 1987. METALLURGICALTRANSACTIONS A
in steels could be explained by the effect of alloying on the chemical activity of antimony in steel. It is clear from the studies cited previously that antimony segregation in iron and steels is a complex process in which the controlling mechanisms are still under discussion. The purpose of this paper is to report on the grain boundary chemistry of Fe, Fe + Sb, and Fe + Mn + Sb alloys in which the dominant segregating species are sulfur and antimony. For this work, grain boundary compositions were determined by AES, the chemical state of the grain boundary segregants was determined by XPS, and the chemical composition of grain boundary and matrix particles was determined by STEM-EDS. II.
EXPERIMENTAL PROCEDURE
A. Materials
Iron and iron alloys with nominal compositions (at. pct) of Fe + 0.03Sb, Fe + 0.07Mn, Fe + 0.1Mn + 0.02Sb (Fe + Mn + Sbl), and Fe + 0.1Mn + 0.05Sb (Fe + Mn + Sb2) were prepared by vacuum induction melting electrolytic grade Glidden A104 iron to which manganese and antimony were added. The 10-kg ingot was homogenized,
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