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I.

INTRODUCTION

G R A I N boundary segregation may occur under nonequilibrium or equilibrium conditions. Nonequilibrium grain boundary segregation occurs up to several microns from the boundary, either as a result of the diffusion of point defect-solute atom pairs to the boundary or preferential diffusion of solute atoms along the boundary. 1 In contrast, equilibrium segregation occurs when the local compositional changes at the grain boundary exist in equilibrium with the surrounding matrix. Equilibrium segregation in metals results in a segregated region only a few atom diameters wide. 2 The segregation of Bi to grain boundaries in Cu is of the equilibrium type. 3'4 Several previous studies of Bi segregation in Cu have utilized Auger Electron Spectroscopy (AES). 3'4'5 One study found that in alloys containing 7.8 wt pct Bi the maximum grain boundary composition was 56.5 wt pct. This same study utilized sputtering to determine that the segregant extended at most a few atomic diameters from the boundary. 5 The narrow width of the segregated region has been observed in other studies. 3 Another study5 found in dilute alloys containing only 0.008 wt pct Bi that the maximum grain boundary composition was 80.7 wt pct. Also it was reported that the segregation was insensitive to the grain size and varied significantly from boundary to boundary] The marked anisotropy was attributed to differences in boundary structure and energy. 4 The study of equilibrium segregation presents a special problem because of the narrow width of the segregated region. AES has been used extensively because of the high depth resolution of the technique. The main disadvantage of the technique is that the sample must be fractured under ultra-high vacuum to expose the grain surfaces. The difficulty with the fracturing of the sample is that boundaries with low segregation levels do not fracture for AES measurement. As a result the fracture path tends to select those boundaries that are more heavily segregated, making boundaries with low segregation difficult to study by AES. Furthermore, only one of the two grain surfaces is usually examined, and the distribution of segregant may be affected by the fracture process. J. R. MICHAEL, Research Assistant, and D. B. WILLIAMS, Professor, are with the Department of Metallurgy and Materials Engineering, Whitaker Laboratory #5, Lehigh University, Bethlehem, PA 18015. Manuscript submitted June 8, 1983.

METALLURGICAL TRANSACTIONS A

The Scanning Transmission Electron Microscope (STEM) equipped with chemical analysis capability such as an Energy Dispersive X-ray Spectrometer (EDS) and otherwise termed the Analytical Electron Microscope (AEM), has been shown to have sufficient spatial resolution to detect equilibrium segregation. 6'v'8 AEM does not require the grain boundary to be disturbed by fracturing and has the ability to image grain boundaries at high magnifications and to obtain simultaneous crystallographic information through electron diffraction. The quantification of the X-ray data obtained from th