Grain boundaries and interface congruency in high entropy alloys

Markus Heidelmann, ICAN, University of Duisburg-Essen, Duisburg, Germany
Blazej Grabowski, Max Plank Institute for Iron Research, Dusseldorf, Germany
D. Ma, Max Plank Institute for Iron Research, Dusseldorf, Germany
Michael Feuerbacher, Ernst Ruska-Centre for Microscopy and Spectroscopy with Electrons, Peter Grünberg Institute, Jülich Resea, Jülich, Germany

High entropy alloys (HEAs) [1] which by definition consist of at least five principal elements in a concentration range between 5 to 35 at. % have recently attracted considerable attention due to their promising properties. Despite their compositional complexity HEAs solidify in a simply average crystal structure, usually body-centered or face-centered cubic. This means that ideally five or more different atomic species are randomly distributed across two to four crystallographically distinct lattice sites, i.e. forming an ideal solid solution. High-angle annular dark-field (HAADF) imaging in a scanning transmission electron microscope (STEM) is an exceptional tool for the characterization of HEAs on the atomic level since the image contrast depends on the atomic number Z. Therefore any divergency from the expected random distribution of elements can directly be revealed.

Here we present state-of-the-art aberration-corrected high resolution STEM investigations of two different HEAs consisting of Al-Co-Cr-Fe-Ni (ACCFN) and Zr-Ni-Ti-Ta-Hf (ZNTTH). ACCFN forms two phase separated regions one being rich in Al and Ni while the other shows enrichment in Cr and Fe (see also [2]). Our studies focus on the atomic structure of the interface region between those two constituent phases revealing a congruent structure without lattice mismatch or dislocations. ZNTTH forms a single phase and exhibits a random distribution of the including elements on the bcc lattice sites and in addition structurally and compositionally different intermediate layers at grain boundaries which have been analysed in detail. We kindly acknowledge funding from the DFG under project Pak 36.

[1] J. W. Yeh et al., Adv. Eng. Mat. 6 (2004) 229.
[2] A. Manzoni, H. Daoud, R. Völkl, U. Glatzel, N. Wanderka, Ultramic. (2012).

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