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The Quest for Nanometric Clouds and Clusters in Modern Engineered Materials
Stephan Gerstl1,2,Robin Schäublin1,2,Lijuan Cui3,Yong Dai3,Severin Küchler2,Vladimir Vojtech2,Leonardo Pierobon2,Jörg Löffler2
ETH Zurich1,ETH Zürich2,Paul Scherrer Institut3
In this contribution, we review and demonstrate various methods by which we push our characterization limits to better understand nanometric features in the smallest – possibly the most influential – features in nanostructurally engineered materials. Oxide dispersion strengthening of steels for example and, more generally, materials relying on multi-phase strengthening, precipitation hardening, and cluster-strengthening all require an initial catalytic core for the formation of the desired precipitates or mesoscale phases. There is thus a thrust in looking for these pre-precipitation clusters and nano-clouds, which occur in a wide variety of materials research projects. For their analysis we take advantage of correlative techniques between Atom Probe Tomography (APT), Transmission Electron Microscopy, and electron backscatter diffraction, some in combination with modeling & simulation and others utilizing our original cryo-transfer enabling developments.
We will first focus on FeCr alloys, which model ferritic steels relevant for the fusion community. Here we find intriguing details in the cluster and interfacial chemistry in its α/α’ decomposed high-purity binary system . We applied multiple analysis methods to determine their precipitate core compositions, as their structurally coherent boundaries have often shown to be relatively broad compared to other matrix/precipitate systems.
In the field of ferritic steels for advanced nuclear facilities, the nascent irradiation induced clustering and precipitation of nuclear transmutation byproducts is generally investigated to better understand the alloys’ evolution of mechanical properties over extended lifetimes and thus irradiation doses. In our near-atomic-scale study we are able to identify the isotopic specificity and thus the particular ions of interest that influence the steel’s mechanical properties.
Focusing on SmCo high-temperature supermagnets, we are able to determine the geometry, packing density and local chemistry between Zr-rich nano-platelets and Cu-rich cells, which prove to strongly influence the magnetic properties, such as coercivity . Among these phases, even the finest clustering features may have a strong impact on the macroscopic magnetic properties. With respect to light metals, we show results of room-temperature (‘natural’) aging of Al alloys, where we have also investigated further alloy classes in Al and Mg that require specimens to remain at cryogenic temperatures from after their creation – to focused ion beam sharpening of the APT tips – to transferring them to the atom probe and enable interrogating their arrested nanostructures [3,4,5].
These examples demonstrate the impact of near-atomic to nano-scale analyses not only to better understand the initial ‘gatherings’ of atoms (nano-clouds) within a structure, but to stress the continued need for pushing boundaries of highest-resolution characterization methods, the correlation amongst each other, and deploying cryo-vacuum transfer methods.
 S. Küchler, V. Vojtech, S.S.A. Gerstl, R.E. Schäublin, J.F. Löffler, submitted to Acta Mater.
 L. Pierobon, András Kovács, R.E. Schäublin, S. S. A. Gerstl, J. Caron, U. Wyss, R.E. Dunin-Borkowski, J.F. Löffler, M. Charilaou, arXiv:1901.01922, 2019
 S. Pogatscher,1,2 H. Antrekowitsch,2 M. Werinos,2 F. Moszner,1 S. S. A. Gerstl,3 M. F. Francis,4 W. A. Curtin,4 J. F. Loffler,1 and P. J. Uggowitzer, Phys. Rev. Letters 112 (2014) 225701
 M. Cihova, R.E. Schäublin, L.B. Hauser, S.S.A. Gerstl, C. Simson, P.J. Uggowitzer, J.F. Loffler, Acta Mater. 158 (2018) 214-229.
 R.E. Schäublin, M. Becker, M. Cihova, S.S.A. Gerstl, D. Deiana, C. Hébert, S. Pogatscher, P.J. Uggowitzer and J.F. Löffler, submitted to Acta Mater.