The influence of grain boundary precipitation on the measurement of chromium redistribution and phosphorus segregation i
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I.
INTRODUCTION
SEVERAL Auger electron spectroscopy studies have been performed on the grain boundary segregation behavior of impurities in nickel base and iron-nickel alloys. 1-6 In many cases, investigators report a scatter in the Auger measurements ranging from 5 to 30 pct of the average value. Recently, Briant I found that the variation in the segregation of phosphorus and antimony to grain boundaries in iron alloys is within about +30 pct of the average value, but some boundaries had significantly different compositions both above and below the average. He considered several factors that could contribute to this variability, among them angular variations in the different grain boundary facets with respect to the cylindrical mirror analyzer, variations produced by the fracture process, variations which result from compositional banding in the matrix, and variations caused by nonequilibrium segregation. In this paper we discuss yet another contribution, the influence of grain boundary precipitates. When a precipitate nucleates at the grain boundary, the structural and chemical environment of the segregant becomes greatly complicated. The precipitate distorts the grain boundary locally and gives rise to precipitate-matrix interfaces in addition to intercrystalline boundaries. Precipitation thus results in significant changes in the matrix chemistry near the precipitate-matrix interface. In carbon containing Ni-16Cr-9Fe alloys, the precipitation of chromium carbides of the form Cr7C3 results in a chromium depleted zone where the chromium concentration at the carbide-matrix interface can fall well below 50 pct of the bulk level. 7 Hence, the composition of the alloy within a few thousand Angstroms of the grain boundary can differ significantly from that in the bulk. G. S. WAS is Assistant Professor, Department of Nuclear Engineering, University of Michigan, Ann Arbor, MI 48109. J. R. MARTIN is Supervisor, Surface Analysis Central Facility, Center for Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139. Manuscript submitted July 15, 1984.
METALLURGICALTRANSACTIONS A
In a recent study of phosphorus segregation in lnconel 600, Guttmann 6 noticed considerable scatter in Cr and Fe which is most probably due to the presence of chromium carbides. In fact, as we will show, the varying grain boundary composition and the presence of a precipitate render inadequate the simple technique of ratioing the Auger signal of the segregated specie to a major element Auger peak to describe grain boundary segregation. The purpose of this investigation is three-fold: (1) to determine the influence of grain boundary precipitation on the measurement of the major alloying element Cr and segregated P by Auger electron spectroscopy, (2) to determine whether P segregation is affected by the precipitation of chromium carbides at the grain boundary, and (3) to use the results of (1) and (2) to determine the nature of the fracture path in this alloy and the precipitate coverage of the fracture surface
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