An AEM investigation of the formation of precipitate-free zones in an AI-16 Wt Pct Ag alloy: Determination of equilibriu
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INTRODUCTION
THE concept
of local interfacial equilibrium is often invoked during the study of phase transformations. That the amount of disequilibrium at the interface is small has been proposed by Kirkaldy, ~and has been used as the basis for many investigations concerned with the determination of phase equilibria from two-phase interfaces in nonequilibrium situations. For example, Romig2 and Romig and Goldstein 3 used X-ray microanalysis in a scanning transmission electron microscope at two-phase interfaces to determine the Fe-Ni and Fe-Ni-P phase diagrams at low temperatures. This technique, also known as analytical electron microscopy (AEM), has recently been used by Chen, Notis, and Williams 4 to determine the solubility of Cr in NiO during the precipitation of NiCr204. Similarly, it has been asserted in the literature that in systems with equilibrium precipitates present at grain boundaries, the solute concentration at a grain boundary would be equal to that predicted from the equilibrium phase diagram. 5 Following the 'collector-plate mechanism' of Aaron and Aaronson, 6 this presumes that diffusion along the boundary and across the precipitate/matrix interface is rapid in comparison with diffusion within the matrix. Doig and Edington 5 have used this approach to calculate solute diffusivities in aged Al-based alloys. They applied the Grnbe analysis 7 to electron energy loss (EELS) microanalytical data obtained from regions across grain boundaries, between grain boundary allotriomorphs and in the absence of precipitation within the matrix. In a subsequent paper Doig et al. a obtained solute concentration profiles across grain boundaries in aged A1-Cu and A1-Zn-Mg alloys using AEM. In AI-Cu, the grain boundary solute concentration was observed to be significantly higher than the composition given by the equilibrium phase diagram at the aging temperature. Clark 9 has examined the formation of precipitate-free zones (PFZ) adjacent to grain boundaries in aged A1-Ag alloys. He postulated that the narrow precipitate-free zone (termed the 'white precipitate-free zone' (WPFZ) because of its appearance in micrographs) immediately adjacent to the grain boundary is caused by solute depletion due to grain SAILESH M. MERCHANT, MICHAEL R. NOTIS, and DAVID B. WILLIAMS are all with the Department of Metallurgy and Materials Engineering, Whitaker Lab #5, Lehigh University, Bethlehem, PA 18015. Manuscript submitted February 23, 1983. METALLURGICAL TRANSACTIONS A
boundary precipitation of the equilibrium phase. He also proposed that the width of the solute depleted WPFZ would be governed by the limits of Guinier-Preston zone formation, i.e., the metastable miscibility gap. Because of the limited spatial resolution of electron microprobe analysis, Clark was unable to measure the solute distribution within the WPFZ. In another more recent study concerned with the formation of PFZ, Raghavan 1~suggested that AEM could be used to determine the matrix phase concentrations in equilibrium with both the stable and metastable pre
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