Identification of 2D Boundaries from 3D Atom Probe Data, and Spatial Correlation of Atomic Distributions with Interfaces
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ABSTRACT The Three Dimensional Atom Probe produces a real space map of the elemental identities and positions of atoms field-evaporated from a sharply pointed specimen. The analyzed volume is on the order of 20 nm x 20 nm x 100 nm. This is large enough to enclose microstructural features such as grain- or heterophase boundaries. Correlation of the measured atomic positions with such features results in an atom-by-atom description of the chemical environment of these crystallographic defects. We describe here a method for identifying these interfaces and profiling the composition in the vicinity of the interfaces without any assumptions about the interface geometry. This approach is applied to quantitative determination of interfacial segregation of Ag at a MgO/Cu(Ag) heterophase interface. We discuss the implications of our technique with respect to classical treatments of segregation at interfaces.
INTRODUCTION Experimental techniques such as Auger Electron Spectroscopy (AES), Scanning Transmission Electron Microscopy (STEM) with Electron Energy Loss Spectroscopy (EELS), or onedimensional Atom-Probe Field-Ion Microscopy (APFIM) can be used to measure the interfacial excess of a solute at an interface, for example grain boundaries [I] or heterophase boundaries [2]. However, these techniques are not straightforward to apply when the interface is not planar, because composition measurements are taken as a one-dimensional profile, and the interface is assumed to be planar and uniform. In contrast, Three Dimensional Atom Probe (3DAP) [3,4] analysis results in a three-dimensional map of positions and chemical identities of the detected atoms, and therefore the three dimensional structure of interfaces can be quantified [5]. In some cases, a planar interface within this volume can be identified, and a one dimensional profile can be generated such that the data can be treated like that from a one-dimensional atom-probe analysis. However, analyses of segregation near small precipitates or of segregation to interfaces in most microstructures resulting from a phase separation are not likely to have uniform, planar interfaces. Indeed, even a basic identification of the interfaces in these structures can be complicated. This paper outlines the data analysis procedure we use to extract a quantitative value for segregation of a solute at an interface from 3DAP data. Starting with a set of positions and identities of atoms in the sample, we first perform a sampling, which generates a regularly spaced three-dimensional grid of composition values. Then the grid concentrations are interpolatedto identify the interface corresponding to a chosen composition of a given species. A correlationis then performed between the original set of atomic positions and this interface, wherein the distance from each atom to the interface is calculated, and a histogram of the populations of all the species with respect to proximity to the interface is tabulated. The result is that the three-dimensional atomic positions have been transformed into a s
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