Investigation of Interfaces by Atom Probe Tomography
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THE characteristics of the internal interfaces, such as grain- and phase boundaries, can have a significant and sometimes even decisive effect on the properties of technical materials.[1,2] Grain boundaries (GBs) are well known to influence electric conductivity[3] and magnetic[4] and mechanical properties.[5,6] They can be weak spots for chemical attacks,[7] reducing the corrosion resistance and the service lifetime of structural materials. Minority atoms often segregate to the GBs which can produce unwanted effects like embrittlement and decreased yield strength[6,8,9]; in other cases, however, GB segregation can lower excess energy of the GBs and help to stabilize the desired grain structure.[10] The more open structure and the weaker bonding in the GBs make them optimal sites for heterogenous nucleation.[11] GBs are also known as diffusion shortcuts, with diffusivities often many million times larger than that of the bulk.[12] Consequently, GB diffusion can be the dominant atomic transport mechanism for materials even with relatively large grain sizes. The importance of GBs is especially true for nanocrystalline materials where the fraction of intergranular material can reach a few percent. Furthermore, as it was pointed out by Palumbo et al., the volume fraction of a
ZOLTA´N BALOGH and PATRICK STENDER, Post Doctoral Research Fellows, MOHAMMED REDA CHELLALI, Doctoral Student, and GUIDO SCHMITZ, University Professor, are with the Institute of Materials Physics, University of Mu¨nster, Mu¨nster, Germany. Contact e-mail: [email protected] Manuscript submitted August 28, 2012. METALLURGICAL AND MATERIALS TRANSACTIONS A
topological substructure of the grain boundaries (GBs) called triple junctions (TJs) rises even faster than the GBs with decreasing grain size.[13] Nowadays, advanced theoretic and computer studies aim to investigate more delicate structural or chemical properties of the grain boundaries. These include a detailed simulation of the GB core,[14] the transition regime between the GB core and the bulk volume[15] or TJs.[16] These tendencies indicate the need for experimental methods which can deliver reliable, atomic-scale information from internal interfaces. Probably the most important challenge in the analysis of buried interfaces is the designation of the region of interest since the interface itself is embedded in a bulk matrix. So, techniques providing data over a larger volume or surface area (e.g., Auger analysis, neutron or X-ray based methods) contain information from both the GBs and other defects. Consequently, the evaluation and the interpretation of the data may contain artifacts. As a further complication, a typical interface is rough, faceted, or even curved. Accordingly, methods based on projection to a 2D plane (like HR-TEM) may be unable to provide a clear characterization of the interface.[17] An ideal instrument to investigate buried interfaces should be local, projection free, and chemically sensitive in subnanometer range. While not without its own issues, atom probe tomogra
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