Applications of Atom Probe Microanalysis in Materials Science
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MRS BULLETIN/JULY 1994
analytical capability of the atom probe. In fact, the atom probe may to used to analyze all elements in the periodic table and has had applications ranging from characterizing the distribution of implanted hydrogen to phase transformations in uranium alloys. Unfortunately, not all segregants image as distinctly as the boron atom in this example, and therefore the lack of bright spot decoration is not a good indi-
Figure 1. Field ion micrograph of a pair of grain boundaries in borondoped Ni3AI.4 Two distinct types of regions are apparent in the upper boundary: (a) low coverage where only isolated boron atoms are observed, and (b) significantly higher coverage that is associated with a small facet with a different habit plane. A relatively uniform distribution of boron atoms was found in the lower boundary. (Courtesy J.A. Horton, Oak Ridge National Laboratory.)
cation of a solute-free interface. The atom probe may be used to completely characterize the solute distribution and precipitation at boundaries with the use of its mass spectrometer. Intermetallics One of the primary roles of the atom probe is to assist in the design of new materials with improved properties by performing a detailed characterization of the microstructure. Some types of information obtainable with the atom probe are the size and morphology of the microstructural features and distribution of the alloying elements. One particular example of this type of application is the determination of the role of microalloying elements in the nickel aluminides such as Ni3Al and NiAl. Due to their good elevated temperature properties and low density, these materials are potential replacement alloys for high-temperature applications. Unfortunately, pure stoichiometric Ni3Al and NiAl alloys exhibit poor room temperature ductility, which makes fabrication difficult. In the case of Ni3Al, this behavior can be improved by small additions of suitable microalloying elements such as boron. However, this method has not been as effective in NiAl. The reason for this surprising difference has been determined with the atom probe and is related to the distribution of boron in the microstructures. Since these materials are susceptible to intergranular fracture in the undoped condition but not in the boron-doped condition, the primary microstructural features of interest are the grain boundaries. Grain boundaries in boron-doped Ni3 Al (Figure 1) reveal that the boron distribution varies from boundary to boundary and even along a single boundary.4 A relatively uniform distribution of boron atoms was found in the lower boundary. The thickness of the boron-enriched region appears to be slightly more extensive than simple submonolayer type segregation and is indicative of an ultrathin film. Grain boundary films up to ~4 ran thick have been observed in this material. In the upper boundary, there are two distinct types of regions. The first exhibits low coverage, where only isolated boron atoms are observed. In contrast, the second type, which is associa
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